|Mutation Codon||Ser -> Ile|
|Mutation Triplet||AGT -> ATT|
First of all, we explored the amino acid properties and compared them for the original and the mutated amino acid. Therefore we created the possible effect that the mutation could have on the protein.
|polar, tiny, hydrophilic, neutral||aliphatic, hydrophobic, neutra||Ile is much bigger than Ser and also is branched, because it is an aliphatic amino acid. Therefore the structure of both amino acids is really different and Ile is to big for the position where Ser was. Therefore, there has to be a big change in the 3D structure of the protein and the protein probably will loose its function.|
Visualisation of the Mutation
In the next step, we created the visualization of the muation with PyMol. Therefore we created a picture for the original amino acid, for the new mutated amino acid and finally for both together in one picture whereas the mutation is white colored. The following pictures display that the original amino acid Serine looks different to Isoleucine. Serine is very small whereas Isoleucine is bigger and has two spreadin chains. The first part of the rest agrees in both amino acids. In this case the difference is not so heavy, but can also cause some structural changes which can have affects on the protein function. All in all, the mutation will probably have no structural or functional changes.
|picture original aa||picture mutated aa||combined picture|
Subsitution Matrices Values
Afterwards, we looked at the values of the substitution matrices PAM1, PAM250 and BLOSSUM62. Therefore we looked detailed at the three values: the value for accoding amino acid substitution, the most frequent value for the substitution of the examined amino acid and the rarest substitution.
In this case, the substitution of Serine to Isoleucine acid has very low value that is nearer to the values for the rarest subsitution for PAM1. Contrary, for PAM250 the value for the amino acid subsitution Histidine to Aspartic acid is average. This means the most frequent subsitution value is almost as far as the rarerest subsitution from the the underlying value. The difference between the two PAMs can be ascribed to the different preparations of these two kind of substitutions matrices. For the PAM1-matrice the substitution rate is 1% which means the prabability that one amino acid changes is 1% and that there is 99% similarity. Contrary, PAM250 means that 250 mutations have been fixed per 100 residues which has as result only similarity about 20%. A possible reason that PAM250 has a better value for the amino acid substitution is that the similarity is low and the amino acids are probably dissimilar. BLOSOUM62 has like PAM1 for this substitution a very low value that is nearer to the values for the rarest subsitution. Therefore, according to PAM1 and BLOSOUM62 a mutation at this position will almost certainly cause structural changes which can affect functional changes. The value from PAM250 is not realy significant and therefore we are not able to determine effects on the protein.
|PAM 1||Pam 250||BLOSOUM 62|
|value aa||most frequent substitution||rarest substitution||value aa||most frequent substitution||rarest substitution||value aa||most frequent substitution||rarest substitution|
|2||38 (Thr)||1 (Leu)||5||9 (Ala, Gly, Pro, Thr)||3 (Phe)||-2||1 (Ala, Asn, Thr)||-3 (Trp)|
Besides, we looked additional at the position specific scoring matrix (PSSM) for ouer sequence. In contrast to PAM and BLOSOUM, the PSSM contains a specific substitution rate for each position in the sequence. Therefore, the PSSM is more position specific than PAM or BLOSOUM. We extracted the substitution value for the underlying mutation, the value for the most frequent substitution and the rarest substitution.
In this case the substitution rate for Serine to Isoleucine at this position is very low and near the value for the rarest substitution. This means this substitution at this position is likely very uncommon which indicates that this substitution has bad effects as a consequence. Therefore, we conclude that this mutation will probably cause protein structure changes as well as functional changes.
|value aa||most frequent substitution||rarest substitution|
Conservation Analysis with Multiple Alignments
As a next step we created a multiple alignment which contains the HEXA sequence and 9 other mammalian homologous sequences from uniprot. Afterwards we looked at the position of the different mutations and looked at the conservation level on this position. The regarded mutation is presented by the colored column. Here we can see, that the many other mammalians have on this position another amino acid. Only four other mammalian agrees and have at this position a Serine. Therefore, the mutation on this position is not highly conserved and a mutation there will cause probably no structural and functional changes for the protein.
Secondary Structure Mutation Analysis
As a next step we compared the different results of the secondary structure prediction tools JPred and PsiPred. Afterwards we can examine in which secondary structure element and where therein the mutation takes place. This can give an overview of how drastical the mutation can be. In this case both tools agree and predict at the position of the mutation a coil. This has a result, that the mutation at this position would not destroy or split a secondary structure element. It will probably only changes the coil between two secondary structure elements, but this can sometimes also cause a change of the the following secondary structure. We think that a drastical change of the protein structure and its function is unlikly because the mutation does not affect a secondary struture element. The change of the coil will probably only take places between two secondary structure elements which will probably not changes the protein.
JPred: ...HHHHHHHHCCCEEEECCCCCHHHHHHHHHCCCCCCCCCCCCCCCCCCCCCCCCCC... PsiPred: ...HHHHHHHHCCCEEEECCCCCHHHHHHHHCCCCCCCCCCCCCCCCCCCCCCCCCCH...
Comparison with the real Structure:
Afterwards we also visualize the position of the muation (red) in the real 3D-structure of PDB and compare it with the predicted secondary structure. The visualisation can therefore like above the predicted secondary structure display if the mutation is in a secondary structure element or in some other regions.
Here in this case the mutationposition almost agree with the position of the predicted secondary structure and is within a coil. Like explained above this means a mutation will probably not destroy a secondary structure element which affects no drastical structural change. Otherwise it can cause a change of the position of the two nearest secondary structure element which can has a functional loose as a consequence. We think that a structural change is unlikely, because it is not within a secondary structure element and will therefore not cause extrem changes of the protein.
Next, we looked at the result of the SNAP prediction. For this prediction we took the amino acid of the certain position and checked every possible amino acid mutation. Afterwards we extract the result for Isoleucine which is the real mutation in this case. SNAP has a result that the exhange from Serine to Isoleucine at this position is neutral with a relative high accuracy. This means that this certain mutation on this position cause very likely no structural and functional changes of the protein.
|Substitution||Prediction||Reliability Index||Expected Accuracy|
A detailed list of all possible substitutions can be found [here]
Next, we used SIFT Prediction which displays if a mutation is neutral or not. Therefore, it first shows a row which contains a score for the particular mutationposition to a certain amino acid. The amino acid which are not tolerated at this position are colored red. Besides, it also constructs a table which lists the amino acids that are predicted as tolerated and not-tolerated.
In this case, there are seven substitutions that are tolerated: Methionine, Isoleucine, Alanine, Serine, Valine, Threonine and Leucine. The substitution to Isoleucine is tolerated at this position. This means that this mutation at this position is probably neutral and will not cause any structural and function changes of the protein.
Each entry contains the score at a particular position (row) for an amino acid substitution (column). Substitutions predicted to be intolerant are highlighted in red.
Threshold for intolerance is 0.05.
Amino acid color code: nonpolar, uncharged polar, basic, acidic.
Capital letters indicate amino acids appearing in the alignment, lower case letters result from prediction.
Finally, we also regarded the PolyPhen2 prediction for this muation. This prediction visualizes have strongly demaging the mutation probably will be. Therefore it gives the result for two possible cases: HumDiv and HumVar. HumDiv is a prefered model for evaluation rare allels, dense mapping of regions identified by genome-wide assiociation studies and analysis of neutral selection. In contrast, HumVar is a prefered model for diagnostic of Mendelian diseases which require distinguishing mutations with drastic effects from all remaining human variations including abundant mildly deleterious allels. We decided to look at both possible models, which are agrees in the most cases.
In this case both models predict that the mutation is benign. This means that the mutation is neutral and will probably not damage the structure and the function of the protein.