HIV Human Immunodeficiency Virus

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)

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

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

HIV Life Cycle

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

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)

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

Function of HIV Protease

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.

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

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.

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

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

Remember Proteins are Amino Acids linked together

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”

Gathering Information about HIV Protease

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.

Gathering Information about HIV Protease

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

Process of Cleaving the Protein

Stage 1:

Stage 2:

Stage 3:

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)

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

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

How does an Enzyme Recognize its Substrate Lock and Key Model

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

Induced-Fit Model Remember baseball…

Induced-Fit Model