1. HIV Human Immunodeficiency Virus

2. HIV Human Immunodeficiency Virus
What is a virus?
Microorganisms which are capable of surviving and reproducing only when they are in a host cell
HIV places itself in human cells (T-cells)

3. What is the function of T-cells?
Recognize components of an infectious agent
Produce antibody proteins which recognize the “invaders” in the body
Antibodies “bind” to pathogens and make them ineffective
This is the principle Immune system in the body

4. HIV makes T-cell ineffective
AIDS patients are vulnerable to a variety of infections, to which we are normally immuned

5. HIV Life Cycle

6. HIV Life Cycle HIV attaches itself on the surface of T-cell
Viral membrane fuses with the T-cell membrane
Viral RNA and the enzyme Reverse Transcriptase are released
Viral RNA is converted into DNA
Viral DNA is integrated into the genome of the cell

7. What happens if viral DNA is integrated into the cell?
The cell unwittingly starts producing viral DNA, which in a sequence of steps produces a large and inactive protein
(translation)

8. Protein is Inactive So, what’s the problem?
The “polyprotein” is converted into smaller proteins through the function of a key enzyme called HIV Protease

9. Function of HIV Protease

10. Once released, these smaller proteins can assemble into new virus particles within the cell.
As the final stage, the viruses pass through the cell membrane and drag some of its lipids to create the outer membrane of intact virus particles.

11. A single infectious virus can utilize a T-cell to produce hundreds of copies of itself.

12. Key Enzymes for HIV Replication
Reverse transcriptase:
Copies the viral RNA into double stranded DNA.
HIV integrase:
Integrates the viral DNA into the genome of the infected cell.
HIV protease:
Cuts the long polyprotein into smaller functional proteins.

13. How to Treat AIDS If the three enzymes, Reverse Transcriptase
HIV Integrase
HIV Protease
were made ineffective (inhibited)

14. Let’s Characterize HIV Protease
Belongs to a group of enzymes called
Aspartic Acid Protease Enzymes
Remember What a Protease does
Breaks the protein chain into smaller chains
By breaking amide linkages

15. Remember Proteins are Amino Acids linked together

16. HIV Protease has a tough job to do
Why?
It is not easy to cleave the amino acid linkages
HIV Protease cannot cleave the chain randomly; it has to cleave it at specific sites “selectively”

17. Gathering Information about HIV Protease

18. Describing HIV Protease
Consists of two units (a dimer) which are mirror images of each other
Has a large hole (a cavity) referred to as “binding pocket” which accommodates the protein to be cleaved
The cavity is flanked by two aspartic acid side chains.

19. Gathering Information about HIV Protease

20. Process of Cleaving the Protein
Step 1:
Forming the Enzyme-Substrate Complex
Step 2:
Catalysis and the Stabilization of the Transition State
Step 3:
Formation and Release of Products

21. Process of Cleaving the Protein

22. Stage 1:

23. Stage 2:

24. Stage 3:

25. HIV Protease and General Principles of Enzyme Catalysis
Enzyme-Substrate Complex:
Enzymes form a “structural interaction” with the substrate
“Active Pocket” of the enzyme has chemical groups that attach the substrate to the enzyme
The diversity of the chemical groups can achieve highly selective binding
(Unlike metal catalysts)

26. HIV Protease and General Principles of Enzyme Catalysis
2. Active site also promotes chemical reaction
In HIV protease the aspartic acid side chain takes a H ion (H+) from water and makes it more reactive (OH-)
3. Once the two fragments of the product is formed, they are only weakly bound (the product(s) must leave the active site to give way to further molecules

27. The Enzyme Shows its Tightest Binding for the Transition State of the Reaction
These binding interactions stabilize the Transition State

28. How does an Enzyme Recognize its Substrate
Lock and Key Model

29. Non-polar region Non-polar region hydrophobic interactions
Types of Interaction
SUBSTRATE ENZYME INTERACTION
Polar region Polar region hydrogen bonding
Non-polar region Non-polar region hydrophobic interactions
Positive charge Negative charge charge
Negative charge Positive charge charge

31. Induced-Fit Model
Remember baseball…

32. Induced-Fit Model