Difference between revisions of "Homology modelling TSD"

Revision as of 17:04, 4 June 2012

There will be no curiosity, no enjoyment of the process of life. All competing pleasures will be destroyed. But always — do not forget this, Winston — always there will be the intoxication of power, constantly increasing and constantly growing subtler. Always, at every moment, there will be the thrill of victory, the sensation of trampling on an enemy who is helpless. If you want a picture of the future, imagine a boot stamping on a human face — forever.

1984

Homology modelling protocol.

Templates

Since similar sets were already collected for Task 2, the information was reused. In addition searches with HHpred on pdb70 and with COMA on pdb40 were performed. If two structures were mapped to the same Uniprot entry, only one, the 'most native' one, was used. The set of chosen templates is displayed in <xr id="tab:templates" />.
None of the searches revealed any structure with >80% sequence identity, other than the two already known structures 2gjx and 2gk1 which share 100% sequence identity. To still perform the task, 2gjx, which is the native structure <ref name="2gjxref">Lemieux,M. et al. (2006) Crystallographic Structure of Human beta-Hexosaminidase A: Interpretation of Tay-Sachs Mutations and Loss of GM2 Ganglioside Hydrolysis. Journal of molecular biology, 359, 913-29.</ref>, was chosen as reference and 2gk1, which has the inhibitor NGT bound, will be used as template. In the range between 40/80% sequence identity only one entry could be added from COMA. Most of the hits found by either COMA or HHpred turned out to have a sequence identity of lower than 25%.

	PDB id	Sequence identity	Method
> 80% identity	2gk1 chain A	100%	Task2/HHpred/COMA

40% - 80% identity	1o7a chain D	56.6%	Task 2
40% - 80% identity	3lmy chain A	54%	COMA

< 30% identity	3nsm chain A	27.5%	Task 2
	3gh5 chain A	20.7%	Task 2

Table TODO: </figtable>

Important residues

Based on previous analyses in the introduction to Tay-Sachs disease the following residues were deemed important and are therefore specifically observed in the subsequent alignments and structures: R178, D207, E323, N423, E462. The numbers are based om the Uniprot entry P06865. The naming of the residues in the reference structure 2gjx also corresponds to this.

Modeller

Automatic alignments

2gk1

The alignments produced by the simple and 2D alignments methods within modeller are very similar. One differing position is located at the and of the first alpha helix: Since the preceding loop region is not part of the structure a large gap is needed and found by both alignment methods. However the first residue after the helix, therefore belonging to the loop region is positioned after the gap in the normal alignment and before it in the structure based one. Clearly this is a borderline case and the choice should not make a difference. Placing the threonine after the gap however aligns it with another threonine and is therefore the slightly more convincing choice.

The models produced by Modeller, given the two alignments are shown in <xr id="fig:modeller_2gk1_overall"/>. As can be seen the two models are very similar among each other and are also very well aligned to the reference structure 2gjx. The looped regions with the important residues in particular show only marginal deviation to the reference. (TODO check 6A distance stuff and incorporate here.) The only difference is found at the loop region that is not part of the reference structure. The region for the two models is shown in <xr id="fig:modeller_2gk1_loop_compare"/>. Placing the single threonine before or after the gap in the alignment has a larger effect than expected. Placing it after the gap (simple alignment method) elongates the alpha-helix by one residue, leading to a loop running along the surface of the protein, while placing it before the gap (2D-Aligner) leads to a slightly shorter helix and a loop that partly runs buried in the protein and ends with a short beta-strand. The structure based on the simple alignment approximates the reference structure slightly better and there seems to be no apparent reason for the placement of a beta-strand at this position, as given by the structural alignment based model. Since the loop is not resolved in the reference structure there is no clear answer as to which model is better, however the loop running inside the protein creates atomic clashes with another loop region, therefore that specific model is not possible (see <xr id="fig:modeller_2gk1_2d_clashes"/>). In any case it is interesting to see what large effects the placement of a single, comparably neutral, residue like threonine can have.

</figure>

Structural alignment of models created with Modeller using *2gk1* as template. The reference structure is denoted in green, the models created using the simple and 2D-Aligner are coloured in magenta and cyan respectively. Important residues are highlighted in yellow and the essential residue E323 in orange.
<figure id="fig:modeller_2gk1_overall">
<figure id="fig:modeller_2gk1_loop_compare">
<figure id="fig:modeller_2gk1_2d_clashes">

1o7a

The two alignments (simple, 2D) of this structure with intermediate sequence identity exhibit the same difference with the placement of threonine before or after the gap. Two gaps in the middle of the sequence, both of length three, do not differ between the two alignments. A second difference is found at the end of the alignment where the 2D-Aligner inserts a five residue gap before the last two residues, while the normal alignment simply ends in the gap. Since this is the loop region at the end of the structure, there is nothing to support the shift of the last two residues and the simple alignment is the more convincing one. It also aligns asparagine with glutamine instead of the more distant glutamic acid. Whatever the choice, one would not expect a difference in the produced models.

Structural alignment of models created with Modeller using 1o7a as template. Color coding is equal to the one used in <xr id="fig:modeller_2gk1_overall" />.

</figure>

The two models are shown in <xr id="fig:modeller_1o7a_overall"/>. Again, there is generally a large agreement between the models and the reference structure. However compared to the high sequence similarity template stronger deviations in loop regions can be observed for both models. The loops containing the important residues however are very well aligned and undistorted. The largest differences are again found at the region of the loop that has not been resolved in the reference. Again, placing the threonine after the gap in the alignment leads to a slightly longer alpha-helix, however the resulting loop this time runs 'above' the alpha helix at the surface of the protein and towards the end enters the same cavity the loop in the model based on 2gk1 and the 2D-Aligner entered. For the 2D-Aligner based model, the loop begin and ends similarly as for 2gk1 although differing in the paths it takes inside the protein. Atomic clashes seem very likely for the latter model (not shown), so the one based on the simple alignment again seems more convincing.
The loop region at the end of the sequence that showed a one residue difference in the alignments, also differs in the models, however since it is not resolved in the reference structure there is no telling which one is better.

3gh5

Since the sequence similarity is very low in this case which is reflected in the differing alignments. It is easily observable that the simple alignment methods tries to group gaps together instead of creating short breaks. The structure based alignment introduces several short gaps in looped regions for no apparent reason. More importantly the 2D-Aligner introduces a gap in the middle of the first helix, while it is preserved in the simple alignment. This is very unexpected since the whole point of the 2D-Aligner is that such cases are smoothed out instead of created in the first place. Another helix is broken by both methods, however the 2D-Aligner performs worse again, since it introduces two gaps in the helix, instead of only one. The last residue of 3gh5 is separated by a gap in both alignment methods for no apparent reason, however the structural alignment method again introduces two gaps instead of one, with no clear explanation.

</figure>

Structural alignment of models created with Modeller using *3gh5* as template. Color coding is equal to the one used in <xr id="fig:modeller_2gk1_overall" />.
<figure id="fig:modeller_3gh5_overall">
<figure id="fig:modeller_3gh5_loopCompare">
<figure id="fig:modeller_3gh5_e462">

The two models created using the low sequence identity template 3gh5 are shown in <xr id="fig:modeller_3gh5_overall" />. As to be expected there are larger differences to the reference than before. While all large alpha-helices and the central beta-barrel are correctly aligned, four smaller helices and several loop regions are not aligned correctly. The important residues are mostly very well placed: E462 is slightly recessed. The increase in distance (c.f. <xr id="fig:modeller_3gh5_e462"/>) could in theory already lead to a deterioration of the hydrogen bond at this position, however this bonding was based on a substitution ligand and the real importance of this residue is not known. The same goes for E423 which is completely misaligned. The single, essential <ref name="2gjxref"/> residue E323 is correctly aligned.
The first helix is bent and too long in direction of the N-terminus. In addition the gap introduced in the structure based aligned leads to a premature end of the helix in C-terminal direction. The other helix that contains only one gap in the simple alignment and two in the structure based one ends prematurely in both models. The loop missing in the reference structure is modelled very comparably in both models by extending the nearby beta-sheet by one strongly bend strand. Given the orientation of the strands and preceding alpha-helix in the reference this is clearly wrong. A detailed view of this part is shown in <xr id="fig:modeller_3gh5_loopCompare"/>.

Evaluation of alignment methods

While the templates with higher sequence similarity result in almost equal alignments with both methods, the low similarity template shows that the 2D-Aligner exhibits exactly the opposite of the expected behaviour and actually makes the alignment worse by introducing more gaps that do not conserve but destroy secondary structure elements. Therefore the simple alignment methods is much more convincing.

This is reflected in the models created from these alignments. While differences are only present in places where correct and incorrect are hard to judge, the models created with the simple alignment always seem slightly more convincing. In any case the ones created with the structure based method are not better than the simple ones.

Manually edited alignments

All the alignments created in the previous step were analysed in respect to the correct alignment of important residues. For the 2gk1 and 1o7a alignments there is nothing to improve upon. The only editing operations that come to mind would turn the simple method based alignment into the 2D based one and vice versa. Since this has already been discussed above no further analysis is performed for these two templates.

The alignment of 3gh5 allowed for some editing operations. Taking the alignment produced by the simple method as a basis all important residues but N423 are already correctly aligned. Before that residue however there is the alpha helix, that is broken by both alignments methods. In the edited alignment this helix has been completely aligned and a gap placed before and after it, thereby only affecting loop regions. N423 is left as is, since there is no asparagine nearby that would result in a better alignment and since it is aligned to an aspartic acid which is a likely substitution that should not affect the formation of the hydrogen bond. Finally, the last residue is not placed separately and the alignment instead ends with gaps. The resulting alignment is shown in <xr id="fig:modeller3gh5edited"/>.

>P1;3GH5 structureX:../../input/3GH5.pdb: -16 :A:+507 :A:MOL_ID 1; MOLECULE BETA-HEXOSAMINIDASE; CHAIN A; [...] HHHHSSGLVPRGSHMASMSQPSILPKPVSYTVGSGQFVLTKNASIFVAGNNVGETDELFNIGQALAKKLNAS TG-------YTISVVKSNQPTAGSIYLTTVGGN---AALGNEGYDLITTSNQVTLTANKPEGVFRGNQTLLQ LLPAGIEKNTVVSGVQWVIPHSNISDKPEYEYRGLMLDVARHFFTVDEVKRQIDLASQYKINKFHMHLSDDQ GWRIEIKSWPDLIEIGSKGQVGGGPGGYYTQEQFKDIVSYAAERYIEVIPEIDMPGHTNAALASYGELNPDG KRKAMRTDTAVGYSTLMPRAEITYQFVEDVISELAAISPSPYIHLGGDESNAT----------------SAA DYDYFFGRVTAIANSY----GKKVVGWDPS-DTSSGATSDSVLQNWTCSASTGTAAKAKGMKVIVSPANAYL DMKYYSDS-PIGLQWRGF-VNTNRAYNWDPTDCIKGANIYGVESTLWTETFVTQDHLDYMLYPKLLSNAEVG WTARGDRNWDDFKERLIEHTPRLQNKGIKFFADPIV---------* >P1;P06865 sequence:: : : : :::-1.00:-1.00 MTSSRLWFSLLLAAAFAGRATALWPWPQNFQTSDQRYVLYPNNFQFQYDVSSAAQPGCSVLDEAFQRYRDLL FGSGSWPRPYLTGKRHTLEKNVLVVSVVTPGCNQLPTLESVENYTLTINDDQCLLLSETVWGALRGLETFSQ LVWKSAEGT-------FFINKTEIEDFPRFPHRGLLLDTSRHYLPLSSILDTLDVMAYNKLNVFHWHLVDDP SFPYESFTFPELMRKGSYNPVTH----IYTAQDVKEVIEYARLRGIRVLAEFDTPGHTLSWGPGIPGLLTPC YSGSEPSGT---FGPVNPSLNNTYEFMSTFFLEVSSVFPDFYLHLGGDEVDFTCWKSNPEIQDFMRKKGFGE DFKQLESFYIQTLLDIVSSYGKGYVVWQEVFDNKVKIQPDTIIQVWREDIPVNYMKELE--LVTKAGFRALL SAPWYLNRISYGPDWKDFYIVEPLAFEGTPE---QKALVIGGEACMWGE-YVDNTNLVPRLWPRAGAVAERL WSNKLTSDLTFAYERLSHFRCELLRRGVQAQPLNVGFCEQEFEQT*

Edited Alignment of 3gh5_A with P06865 based on the simple (non-structural) alignment produced by Modeller. The helix region and single residue at the end that were changed are highlighted in red. Bold, blue residues are those deemed important (c.f. above) </figure>

The model created from the edited alignment is very similar to the simple alignment it is based on. The helix that was edited, so that it contains no gaps is indeed correctly modelled and does not end prematurely anymore. Apart from that however none of the errors present in the initial model are corrected.

Score based evaluation

Looking at the structures is a very good way to assess the quality of a model, however automatic rankings are highly desirable. Therefore the models from before are evaluated in the following by the four different scores TM_score, GDT_HA, GDT_TS and C-alpha RMSD. TODO not really, cleanup when clearer

TM-Score

The scores calculated by TM-score are shown in <xr id="tbl:modeller_tmscore"/>. Generally the scores behave as to be expected, given the previous analyses of the model structures, with the high sequence identity template resulting in very good results, the medium identity template in only slightly worse ones and a strong deterioration for the template with low sequence identity.
For the high identity template, there are only small differences between the two models assessed. From the scores alone it is not possible to favour one model over the other, since TM-score and GDT-TS are almost equal, RMSD favours the simple alignment based model and GDT-HA favours the 2D-Aligner based model. The simple alignment based model seemed more convincing in the visual analysis but since the loop region that lead to this conclusion is not part of the reference structure, none of the scores can capture this. In addition the question remains, whether the more convincing loop modelling really is the correct one.
It can be observed that the RMSD is already comparably bad for the medium identity model, although the visual analysis suggested that this model is mostly very good. This is fact is better captured by the TM-Score and GDT-TS, while the criteria for the more rigorous <ref name="gdtha">Read,R. (2007) Assessment of CASP7 predictions in the high accuracy template‐based modeling category. Proteins: Structure, Function, and, 27-37.</ref> GDT-HA seem to be too strong already. In the visual comparison the model based on the simple alignment seemed more convincing, this is, if at all, only captured by the commond residue RMSD.
For the low sequence identity model it can be seen that the TM-score is comparably high. Since it was noted in the visual analysis that the largest deviations are found in loops and this measure is less sensitive to deviations in loops, this is to be expected. However GDT should also be less sensitive to loop deviations and reports a significantly larger drop in modelling performance. RMSD is definitely affected to much by the loops and not a suitable measure for models such as this coming from a lower identity template. The visual analysis of the model based on the manually edited alignment showed no major improvements over the simple alignment model it was based upon. This is reflected in the scores which are very similar between the two models.

	Residues in common	Common residue RMSD	TM	GDT-TS	GDT-HA
2gk1	492	0.446	0.9968	0.9970	0.9472
2gk1 2D	492	0.531	0.9972	0.9970	0.9802
1o7a	492	1.521	0.9784	0.9482	0.8262
1o7a 2D	492	1.642	0.9765	0.9472	0.8293
3gh5	492	8.005	0.7614	0.5762	0.4192
3gh5 2D	492	7.219	0.7792	0.5971	0.4334
3gh5 Edited	492	7.696	0.7551	0.5579	0.4004

Table TODO: Calculated TM scores for models created with Modeller. </figtable>

SAP Scores

The SAP Scores are shown in <xr id="tbl:modeller_sap"/>. The increase in the unweighted RMSD when descending to lower sequence identity templates is comparable to the full atom RMSD shown above in <xr id="TODO"/>. However the absolute values are lower, suggesting, that the backbone is better approximated as it might seem from the full atom RMSD. The weighted RMSD, giving less importance to regions of lower similarity <ref name="sap>TODO</ref>", delivers more useful results for the medium identity template. This already seemed fairly high in the full atom RMSD, which did not correlate with the visual analysis that found these two models to be mostly well aligned to the reference. However for the low sequence identity template the weighted RMSD remains very low as well. Given that there are not only loop regions, but also several small secondary structure elements deviating in these two models, this scores actually seems to be too optimistic.

	Weighted RMSD	Unweighted RMSD (all matched)
2gk1	0.365	0.434
2gk1 2D	0.246	0.304
1o7a	0.509	0.980
1o7a 2D	0.496	0.862
3gh5	1.132	2.671
3gh5 2D	1.060	3.147
3gh5 Edited	1.085	3.261

Table TODO: RMSD scores for Modeller models calculated with SAP and Pymol. </figtable>

SWISS-MODEL

General

The SWISS-MODEL server provides as output a model in pdb format and the corresponding target template alignment. Besides it also offers diverse scores and functions for the model evaluation.
ANOLEA is the atomic empirical mean force potential. Therefore a program performs energy calculations on a protein chain. Negative energy values (in green) represent favourable energy environment whereas positive values (in red) unfavourable energy environment.
There is also a local model reliability score, the residue error computed along the sequence
QMEAN is a composite scoring function for both the estimation of the global quality of the entire model as well as for the local per-residue analysis of different regions within a model.
QMEAN4 is a scoring function to describe the model quality which can be used in order to compare and rank alternative models of the same target.. It is a linear combination of the 4 statistical potential terms C_beta interaction energy, all-atom pairwise energy, solvation energy and torsion angle energy. Hereby the QMEAN raw score ranges from 0 to 1 and indicates the reliability of the model. The QMEAN Z-score represents the absolute quality of the model by describing the likelihood that a given model is of comparable quality to experimental structures. Is calculated by comparison to reference structures and has a range of -4 to 4; the smaller the value the worse the model quality <ref name="swissmodel">Arnold K., Bordoli L., Kopp J., and Schwede T. (2006). The SWISS-MODEL Workspace: A web-based environment for protein structure homology modelling. Bioinformatics, 22,195-201.</ref>. For a more detailed explanation, see [1].

Default Modelling

2gjx chain E gets automatically assigned as template. this is not surprising as the 2gjx_a is the reference and all the chains are virtually identical. The resulting alignment between target and template consists of identical matches. Only in the first region a loop in the template is missing and therefore a gap is inserted (see <xr id="fig:2gjxali"/>).

TARGET 1 LWPWPQNF QTSDQRYVLY PNNFQFQYDV SSAAQPGCSV LDEAFQRYRD 2gjxE 23 lwpwpqnf qtsdqryvly pnnfqfqydv ssaaqpgcsv ldeafqryrd TARGET ss sss sssss hh hhhhhhhhhh 2gjxE ss sss sssss hh hhhhhhhhhh TARGET 49 LLFGSGSWPR PYLTGKRHTL EKNVLVVSVV TPGCNQLPTL ESVENYTLTI 2gjxE 71 llfg------ --------tl eknvlvvsvv tpgcnqlptl esvenytlti TARGET hh ! sssss ssssss 2gjxE hh sssss ssssss

Alignment of 2gjx_E with P06865. The gap is highlighted with a exclamation mark. </figure>

For the whole alignment see Swissmodel 2gjx alignment.
<figure id="fig:2gjxanolea">

ANOLEA score for the first region of the 2gjx model.

</figure> As the 2gjx chain is our chosen reference sequence it is expected to provide a perfect model. ANOLEA displays only one weakness of the model which is found at the beginning between position 70 and 90, see <xr id="fig:2gjxanolea"/>. This is also supported by the QMEAN, which signalises a potential error at this site. This is the only gap region from the alignment and thus it receives low support from the template structure.
The further scores (see <xr id="tab:swissscores2gjx"/>) show the good quality of the model, as the energy values are comparably low which signalises a favourable structure and the QMEAN4 close to 1 gives the model a good rank. With the "perfect match template" it is surprising that the Z-scores are negative as this stands for a decay in quality.

Scoring function term	Raw score	Z-score
C_beta interaction energy	-106	-0.99
All-atom pairwise energy	-13340	-0.33
Solvation energy	-37	-0.62
Torsion angle energy	-93	-1.62
QMEAN4 score	0.68	-1.63

Table TODO: Scores for default modelling of P06865 by SWISS-MODEL. </figtable>

SWISS-MODEL for the Hex A structure computed with the 2gxj template in comparison to the reference.The reference is displayed in green and for the model the error color-coding from SWISSMODEL is adopted meaning that confident residues are colored blue and the higher the estimated error the more red the residues are displayed. The active site is shown in yellow and the important H-bond residue in orange.

</figure> In <xr id="fig:2gjxswissmodel"/> the computed model is display (blue-red, depending on error) together with the reference (green). It is clear that the model fits the reference perfectly which was the expectancy as the the template is the reference just with another chain (template 2gjx_E, reference 2gjx_A). The only region where the target and reference do not is the loop region colored in red belonging to the gap in the template structure, see <xr id="fig:2gjxali"/>, which was correctly detected by the SWISS-MODEL error scores. The active site and the important residues are matched perfectly by the model. As the model is very accurate the negative Z-scores remains difficult to justify.

High sequence identity

The model with 2gk1 as template is very much alike the previous model, which is not surprising as the templates have 100% sequence identity. The alignment of the Hex A subunit and the template expresses the same gap as shown above (for the whole alignment see Swismsmodel 2gk1 alignment) This alignment also correspond to the structural alignment from Modeller. <figure id="fig:2gk1anolea">

ANOLEA score for the first region of the 2gk1 model.

</figure> The ANOLEA and QMEAN are very good for the alignment, again except for the first region, where there are some signs of lower model quality, see <xr id="fig:2gk1anolea"/>. The scores are again very similar to those of the 2gjx template, see <xr id="tab:swissscores2gk1"/> which accredits the quality of the model. The Z-scores are also negative but show a better overall tendency. It could be that individual terms of geometrical features are slightly improves in this model

Scoring function term	Raw score	Z-score
C_beta interaction energy	-122	-0.81
All-atom pairwise energy	-13468	-0.34
Solvation energy	-44	-0.12
Torsion angle energy	-109	-1.06
QMEAN4 score	0.69	-0.96

Table TODO: Scores for the model of P06865 with 2gk1 as template by SWISS-MODEL. </figtable>

SWISS-MODEL derived from 2gk1 template in comparison to the reference.The reference is displayed in green and is shown in error color-coding from SWISSMODEL: confident residues are colored blue and the higher the estimated error the more red the residues are displayed. The active site is shown in yellow and the important H-bond residue in orange.

</figure>

The model with the reference are displayed in <xr id="fig:2gkiswissmodel"/>: There is very little red coloring which shows a low error. The only outlier is the wrongly assigned helix. Besides this the model is very accurate and conserves the active site as well as the important residues.

Medium Sequence identity

The alignment for the modelling with 1o7a as template is as matching as it was previously the case but again the most gaps appear in the beginning of the alignment, see <xr id="fig:1o7aali"/>.

TARGET 1 TALWPWPQ NFQTSDQRYV LYPNNFQFQY DVSSAAQPGC SVLDEAFQRY 1o7aD 54 palwplpl svkmtpnllh lapenfyish spnstagpsc tlleeafr-- TARGET sssss ssss s ssss s hhhhhh 1o7aD sssss ssss s ssss s hhhhhhhh TARGET 49 RDLLFGSGSW PRPYLTGKRH TLEKNVLVVS VVTPGCNQLP TLESVENYTL 1o7aD 100 -----ryhgy ifgtqvq--- q---llvsi- tlqsecdafp nissdesytl TARGET sss hhhh sss ssssss sss 1o7aD hhhhh h s sssss s sss

Alignment of 1o7a with P06865. </figure>

For the whole alignment see Swissmodel 1o7a alignment.

ANOLEA score for the first region of the 1o7a model.

</figure> The ANOLEA signals a high error for the gapped alignment region (<xr id="fig:1o7aanolea"/>). With a look at the Modeller alignments (LIINK) it seems that this region is difficult as it is not aligned in the same way.
The scores are a little worse that for the model with the very high sequence identity, see <xr id="tab:swissscore1o7a"/>. The energies are slightly worse the QMEAN4 lower and the Z-score more negative.

Scoring function term	Raw score	Z-score
C_beta interaction energy	-97	-1.15
All-atom pairwise energy	-11181	-0.99
Solvation energy	-29	-1.24
Torsion angle energy	-72	-2.33
QMEAN4 score	0.594	-2.73

Table TODO: Scores for 1o7a modelling of P06865 by SWISS-MODEL. </figtable>

SWISS-MODEL derived from 1o7a template in comparison to the reference.The reference is displayed in green and is shown in error color-coding from SWISSMODEL: confident residues are colored blue and the higher the estimated error the more red the residues are displayed. The active site is shown in yellow and the important H-bond residue in orange.

</figure>

The model in comparison to the reference is shown in <xr <figure id="fig:1o7aswissmodel"/>. It stand out that there is more error than in the previous models as there is more red coloring in the model structure. Especially the loop regions are pointed in wrong directions. The erroneous helix on the right of the figure, colored red, stands out as the greatest mistake of the model. It correspond to the gap region of the alignment, see <xr id="fig:1o7aali"/>, which is also highlighted in red. It seems that the region and especially the gaps are erroneous and thus the model is not accurate there. Besides the important residues are all in their right place and overall the model fits the reference quite well.

Alignment mode

The model from the 1o7a template is the first one to have a different alignment than the one proposed by Modeller. Thus it is very interesting to find out which alignment leads to a better structure and therefore the SWISS-MODEL was employed with the alignment from Modeller. This alignment is different in the way that there is only one big gap and not several small ones in the first region (see <xr id="fig:1o7a_ali"/> and <xr id="fig:1o7aali"/>).

TARGET 21 TALWPWPQ NFQTSDQRYV LYPNNFQFQY 1o7aD 54 palwplpl svkmtpnllh lapenfyish TARGET sssss ssss s ssss 1o7aD sssss ssss s ssss TARGET 49 DVSSAAQPGC SVLDEAFQRY RDLLFGSGSW PRPYLTGKRH TLEKNVLVVS 1o7aD 82 spnstagpsc tlleeafrry hgyifg---- ---------- tqvqqllvsi TARGET s hhhhhhhhhh hhhh ssssss 1o7aD s hhhhhhhhhh hhhh ssssss

Alignment of 1o7a with P06865. </figure>

Indeed the scores as swon in <xr id="tab:swissscore1o7a_a"/> are better than the scores from the SWISS-MODEL alignment. The energy values are lower and the Z-score higher. The values are more comparable to those received from the models above with templates of high sequence identity.

Scoring function term	Raw score	Z-score
C_beta interaction energy	-126	-0.78
All-atom pairwise energy	-13033	-0.48
Solvation energy	-35	-0.78
Torsion angle energy	-90	-1.67
QMEAN4 score	0.648	-1.87

Table TODO: Scores for default modelling of P06865 by SWISS-MODEL. </figtable>

SWISS-MODEL derived from target-1o7a alignment in comparison to the reference.The reference is displayed in green and is shown in error color-coding from SWISSMODEL: confident residues are colored blue and the higher the estimated error the more red the residues are displayed. The active site is shown in yellow and the important H-bond residue in orange.

</figure>

The <xr id="fig:1o7a_alswissmodel"/> with the superimposed reference seals it, the alignment provided by Modeller suits the structure of the Hex A subunit better than the alignment computed by SWISS-MODEL. There is very little error and no helix or sheets are erroneously modelled. This model is comparable to the first, which was computed with the reference sequence it self.

Low sequence identity

With the 3gh5 as template the automated SWISS-MODEL was not able to calculate a model structure for the Hex A subunit. Two alignments were produced, one with Blast and one with HHsearch. While the Blast alignment quality between target and template was too low to start with, the HHsearch alignment reached the next level and was sent to modelling but the building of a model was not successful.
The same occured for 3nsm which has a sequence identity about 7% higher than 3gh5. A sequence identity lower than 30% seems to be too low for modelling.
As SWISS-MODEL performed better with the Modeller alignment this was also tried for 3gh5 and 3nsm. Unfortunately SIWSS-MODEL dit not yeald any results.

Evaluation

The main scores provided by SWISS-MODEL from all successful models are shown in <xr id="tab:swissOwneval"/>. The QMEAN raw scores are all in the same range, only the model from the 1o7a template in automated mode is slightly worse than the rest. The Z scores are all negative which can be explained by the diverse single atom RMSDs TODO!!!!. The model from 2gk1 has the highest Z-score and should therefore by considered as the most qualitative by the program measures. <figtable id="tab:swissOwneval">

	QMEAN raw score	QMEAN Z-score
2gjx	0.658	-1.63
2gk1	0.698	-0.96
1o7a	0.594	-2.73
Alignment mode 1o7a	0.648	-1.87

Table TODO: Scores provided by SWISS-MODEL. </figtable>

The RMSD as well as the GDT-TS, GDT-HA and the TM score all display the 1o7a model as the worst and the 2gk1 model as the best (see <xr id="tab:swisseval"/> ), which correspond the the scores calculated by SWISS-MODEL. This is a little surprising as the actual reference is 2gjx. However the model model with created with it as template does not outperform the 2hk1-model which seems to be more suitable for the P06865 modelling. The difference has of course only a little extent as the 2gjx, 2gk1 and P06865 share a 100% sequence identity. The RMSD ranks the model from 1o7a with the Modeller alignment better than the model withou alignment but the other scores do not. The RMSD exhibits the greatest deviations which shows its sensitivity to erroneous outlier loop regions. The GDT-HA score also separates the high sequence identity models from the medium sequence identity models by expressing a stronger variation than the GDT-TS score. On the one hand it is justifiable that the 1o7a models are ranked worse as they do not have the same number of shared residues but on the other hand the models from 1o7a are also very exact. Thus the GDT-TS score could be most appropriate in this comparison.

	Residues in common	Common residue RMSD	TM	GDT-TS	GDT-HA
2gjx	492	0.573	0.995	0.983	0.924
2gk1	492	0.213	0.999	1.000	0.999
1o7a	486	2.411	0.952	0.913	0.802
Alignment mode 1o7a	487	1.344	0.973	0.950	0.834

Table TODO: Calculated TM scores. </figtable>

For completion the sap scores and the RMSD of a 6Å radius around the catalytic site are shown in <xr id="tab:swissevalsap"/>. The results correlate with the results from <xr id="tab:swissOwneval"/> as well as <xr id="tab:swisseval"/>. 2gk1 receives the best values and 1o7a the worst. It should be noted here that the model of 2gk1 stands out as the best by all scores, although it has an completely erroneous helix. The weighted RMSD makes no differentiation between the 1o7a model and the rest and is thus comparable to the GDT-TS score except for the high rank it gives to 2gk1. The RMSD around the catalytic site shows a slightly different behaviour as here the 2gjx model is assigned by far the worst RMSD.

	Matched atoms	Weighted RMSD	Unweighted RMSD	RMSD 6Å around active site
2gjx	492	0.414	0.573	0.590
2gk1	491	0.189	0.212	0.131
1o7a	471	0.486	1.455	0.291
Alignment mode 1o7a	473	0.484	0.744	0.205

Table TODO: Calculated RMSD scores with SAP and Pymol. </figtable>


	Table TODO: 6 A around active site from default modelling (left) and modelling with template 2gk1 (right).

</figtable>

The direct comparison of the active site modelling from the 2gjx and the 2gk1 models is displayed in <xr id="tab:activesite"/>. It is clear to see why the model from 2gjx (left) has the worst RMSD in the catalytic site region as the residues point in the same directions but are disrupted otherwise. The modelling from 2gk1 (right) seems to fit the active site very well. Here the residues overlay accurately which leads to an RMSD of merely 0.131.

Overall all the measures correlate and are also very suitable for evaluation. For the performed modelling a not so strict measure seems better so that an exact model as the one from 1o7a is viewed as such.
The performance of SWISS-MODEL was satisfactory, that is iff a model could be created. The high sequence identity SWISS-MODEL requires could make it difficult to resolve structures of new proteins if there are no similar sequences available.