Enzymology 2 Flash Cards

Suicide Inhibitors 2

1. Aspirin Inactivates cyclooxygenase (COX) a enzyme for making prostaglandins from fatty arachidonic acid in the inflammatory response. 2. Nerve gases irreversible inhibitors of acetylcholine esterase (the enzyme that inactivates acetylcholine) 3. Pump Inhibitors (PPIs). decrease acid secretion in stomach and inhibiting the enzymes that synthesize bacterial cell walls.

Regulation of Enzymes

A - no inhibitor B - Competitive Inhibition C - Noncompetitive inhibition or suicide inhibitor D - Uncompetitive inhibition

Un-competitive Inhibition:

An inhibitor binds only to the Enzyme-substrate complex (ES) It binds an allosteric site (ie, not the active site) Vmax decreases Km decreases 20,21

Competitive inhibition:

An inhibitor, I, binds the active site. The enzyme cannot act on I, so it does not produce a product. If both I and S are present, they "compete" for binding to the active site Vmax unchanged Km increased 14,15

Competitive inhibition: Example 1

Cholesterol is made in liver from acetate and HMG-CoA reductase is the rate-limiting step in cholesterol synthesis. Pravastatin competes with HMGCoA for the active site of HMG-CoA reductase and reduces plasma cholesterol

Michaelis-Menten Equation

How fast the enzyme functions depends on the substrate concentration Key Points: 1. high affinity implies lower Km. 2. Faster catalytic rate implies higher Km. 3. If the catalytic rate is low compared to the rate of binding it is @ binding constant. 4. If the catalytic rate is higher than binding, then the Km will be higher than the binding constant. 6,7

Noncompetitive inhibition:

Occurs by binding of an inhibitor, I, outside the active site. The binding of I does not interfere directly with substrate The binding of I likely influences the conformation of the enzyme to reduce (or enhance) activity. change is in Vmax. Km unchanged 18,19

Competitive inhibition: Example 2

Since ethanol has a higher affinity for ADH than methanol, the best treatment for methanol poisoning is to drink vodka or whiskey as an antidote.

Regulation of HexoKinase: Competition and Allostery

The enzyme has regulatory sites for ADP (lower left), and glucose-6-phosphate (upper left site) The regulatory sites can activate (ADP) or inhibit (G6P) enzyme activity uses allosteric inhibition

Enzyme velocity

The maximal velocity at the high substrate concentrations is referred to as Vmax. The substrate concentration that causes an activity of ½ Vmax is the KM. so at ½ Vmax, then [S] = Km 4,5

Km and Vmax

e.g. Liver and muscle take up glucose. Muscle is unresponsive, and liver is highly responsive. Hexokinase for glycolysis in the muscle and most other tissues has LOW Km Glucokinase for glycolysis in the liver and pancreas. Both catalyze has HIGH Km

Lineweaver Burk Plot

s to extract the Km and Vmax from the data, a simple linearization is used.

Suicide Inhibitors 1

suicide inhibitors destroy enzymes and often have long-term effects. Vmax decreases Km is unchanged