Enzymes as Biological Catalysts

Enzymes

• Enzymes speed up the rate at which a reaction reaches equilibrium
• Bind and stabilize the transition state
• Reduce activation energy by providing an alternative reaction pathway

• Specific - have a limited range of substrates

• Potent - can convert many substrate molecules into product per second

Isozymes

• Isoforms of enzymes - catalyze the same reaction but have different properties and structure

Phosphorylation reactions

• Carried out by protein kinases

Co-factors and co-enzymes

• Cofactors: metal ions required for enzymatic activity

• Coenzymes: organic molecules required for enzymatic activity

• Prosthetic groups: tightly bound coenzymes

• Apoenzyme: enzyme without cofactor

• Holoenzyme: enzyme with cofactor

Substrate binding

• Substrate binds to active site

• Active site contains amino acids for catalytic activity

Lock and key model

• Active site of unbound enzyme is complementary to the shape of the substrate

Induced fit model

• Binding of a substrate induces a conformational change in the enzyme

• Results in a complementary fit

Temperature and pH

• Each enzyme will have an optimum - at either side of the optimum, enzyme activity will decrease

• Active site can be altered by extreme temperature/pH