CH4710 RM22

turnover number (k_cat)

the number of substrate molecules converted to product in a given unit of time on a single enzyme molecule when the enzyme is saturated

simpler def: how many substrate molecules are transformed into products per unit time

What is the equation for the catalytic efficiency of an enzyme?

k_cat/K_m (units of M^-1s^-1)

Interpretation of k_cat/K_m value

k_cat/K_m indicates how effective the enzyme is on a particular substrate

as ratio increases, rate of catalysis increases

diffusion-controlled reactions

reaction rate = the rate of transport of the reactants through the solution/medium where they are

2 classes of enzyme inhibitors

1. reversible

2. irreversible

4 types of reversible inhibition

1. competitive

2. uncompetitive

3. mixed

4. noncompetitive

competitive inhibitor

competes with the substrate for the active site of an enzyme

uncompetitive inhibitor

binds at a site that’s not the active site

binds only to the ES complex

What is the difference between competitive and uncompetitive inhibition?

In competitive inhibition, an inhibitor will bind to the enzyme’s active site so that the substrate can’t.

In uncompetitive inhibition, the substrate will bind to the enzyme’s active site, forming the ES complex. Then, an inhibitor will bind to a separate site.

non-competitive inhibitor

binds to site that’s not an active site

has equal affinity to E and ES forms

What is the difference between non-competitive and uncompetitive inhibition?

Non-competitive inhibition has equal affinity to E and ES forms.

Uncompetitive inhibition will only have the ES form.

K_I equation

K_I = [E][I]/[EI]

([I] is the concentration of inhibitor)

Values of alpha (⍺) equation

⍺ = 1+ [I]/K_I

How does ⍺ conclude where an inhibitor will bind/what type of inhibition will be carried out?

• ⍺ > 1.0 means binding is on the enzyme E (competitive)

• ⍺ < 1.0 means binding is on the ES complex (uncompetitive)

• ⍺ = 1.0 means inhibitor has equal affinity to E and ES forms (non-competitive)

irreversible inhibition

the irreversible inhibitor will bind covalently with or destroy a functional group on an enzyme that is essential for the enzyme’s activity

OR the irreversible inhibitor can also form a highly stable noncovalent association with the functional group

2 types of irreversible inhibitors

1. suicide substrates (SS)

2. transition-state analogs

Describe the process of suicide substrate inhibition.

1. the SS binds to the active site of the enzyme

2. the SS is converted to a product

3. a part of the product forms a covalent bond with the enzyme, while the other part leaves

4. enzyme becomes inactive

transition-state analogs

stable molecules designed to resemble transition states

• bind to enzyme active site more tightly than the substrate can

• blocks the substrate permanently from active site

• ex. HIV Protease inhibitor (HIV protease makes small viral proteins that form viruses)

Describe/explain

HIV Protease cleaves the large Gag-Pol precursor protein to make smaller viral proteins, which are important for the survival of the virus.

Describe/explain

mechanism action of HIV protease

• HIV protease is dimer

• water attacks carbonyl carbon

• tetrahedral intermediate is formed

• intermediate collapses

• amino acid leaving group is expelled