Homology modeling is a technique used to predict the 3D structure of a protein based on the alignment of its amino acid sequence to known protein structures. It relies on the observation that structure is more conserved than sequence during evolution. The key steps in homology modeling include: 1) identifying a template structure through sequence alignment tools like BLAST, 2) correcting any errors in the initial alignment, 3) generating the protein backbone based on the template structure, 4) modeling any loops or missing regions, 5) adding side chains, 6) optimizing the model structure energetically, and 7) validating that the final model matches the template structure and has correct stereochemistry. Homology modeling is useful for applications like structure-based drug design

1. Homology Modeling

2. Limitations of Experimental Methods
Annotated proteins in the databank: ~ 100,000
Proteins with known structure: ~5,000 !
Total number including ORFs: ~ 700,000

3. Protein modeling
Provide a solid basis for,
 structure-based drug design
 analysis of protein function, interactions,
antigenic behavior
rational design of proteins with increased
stability or novel functions

4. Homology Modeling
Based on two major observations:
1) The structure of a protein is
determined by its amino acid sequence
2) Structure is much more conserved than sequence
during evolution

5. What’s homology modeling?
Predicts the three-dimensional structure of a given protein
sequence (target) based on an alignment to one or more
known protein structures (templates).
If similarity between the target sequence and the template
sequence is detected, structural similarity can be assumed.
In general, 30% sequence identity is required to generate an
useful model.

6. Accuracy and application of protein
structure

7. Does sequence similarity implies
structure similarity?
Safe Zone
Twilight Zone

8. Structure prediction by homology
modeling

9. In summary: homology modeling steps
1) Template recognition & initial alignment
2) Alignment correction
3) Backbone generation
4) Loop modeling
5) Side-chain modeling
6) Model optimization
7) Model validation

10. Steps in Homology Modeling
Step 1: Template Recognition and Initial Alignment:-
In practice, one just feeds the query
sequence to one of the countless BLAST
servers on the web, selects a search of the PDB,
and obtains a list of hits—the modeling templates
and corresponding alignments

11. BLAST
Basic Local Alignment Search Tool
Most widely used bioinformatics program
Emphasizes speed over sensitivity
(speed is vital to making the algorithm practical
on the huge genome databases)

12. Overview - METABOLISM

13. Sites of biotransformation
 where ever appropriate enzymes occur; plasma,
kidney, lung, gut wall and
LIVER
 the liver is ideally placed to intercept natural ingested
toxins (bypassed by injections etc) and has a major
role in biotransformation

14. 1.Residue exchange matrix

16. 2. An alignment matrix
By comparing thousands of sequences and sequence
families, it became clear that the opening of gaps
is about as unlikely as at least a couple of nonidentical
residues in a row.
Opening penalty: for every new gap
Gap extension penalty: for every residue that is skipped
in the alignment

17. Step 2: Alignment Correction
Sometimes it may be difficult to align two
sequences in a region where the percentage
sequence identity is very low.
One can then use other sequences from
homologous proteins to find a solution.
LTLTLTLT
YAYAYAYAVY
TYTYTYTYT
TYTYTYTYT
−LTLTLTLT−
−YAYAYAYAY

18. 2 is correct, because it leads to a small gap, compared
to a huge hole associated with alignment 1.

19. Template
Sequence 1
7
11
5
97.5 Å
1.3 Å

20. Step 3: Backbone Generation:-
One simply copies the coordinates of those template
residues that show up in the alignment with the
model sequence.
If two aligned residues differ, only the backbone
coordinates (N,Cα,C and O) can be copied.
If they are the same, one can also include the side
chain.

21. Step 4: Loop Modeling:-
There are two main approaches to loop modeling:-
1). Knowledge based: one searches the PDB for
known loops with endpoints that match the residues
between which the loop has to be inserted and simply
copies the loop conformation.
2). Energy based: as in true ab initio fold prediction,
an energy function is used to judge the quality of a
loop

22. Step 5: Side-Chain Modeling:-
Comparing the side-chain conformations (rotamers) of
residues that are conserved in structurally similar proteins
Similar torsion angle about the Cα −Cβ bond -
It is therefore possible to simply copy conserved
residues entirely from the template to the model

23. Step 6: Model Optimization:-
Energy = Stretching Energy +Bending Energy
+Torsion Energy +Non-Bonded Interaction Energy

24. Step 7: Model Validation
Model should be evaluated for:
- correctness of the overall fold/structure
- errors over localized regions
- stereochemical parameters: bond lengths, angles, etc

25. Step 7: Model Validation

26. QUESTIONS?????

27. B.Satish Kumar M.Tech (bio-tech)
JNTU
Thank You