Enzymes Large molecules made of various amino acids Act as catalysts to speed up reactions w/out being destroyed –Highly specific –Lowers energy of activation level

Enzymes lower the energy of activation for a reaction

Enzyme Kinetics E + S  ES complex  E + P Effect of substrate concentration –10 test tubes of fixed [E] –Add gradations of [S] –Measure rate of reactions

V max occurs when enzyme active sites are saturated with substrate K m (Michaelis- Menten constant) reflects affinity of enzyme for its substrate smaller the K m, the greater the affinity an enzyme has for its substrate

Enzyme Kinetics [S] generally < than its K m –Only uses fraction of enzyme catalytic ability –Enzyme is able to respond to changes in [S] Isozymes (isoenzymes) are variations of same enzyme –Four isozymes of hexokinase Three have low K m and fourth has a high K m

Hexokinase can phosphorylate glucose even with a low blood [glucose]; a high K m prevents liver from taking up blood glucose when [glucose] is low

Regulation of Enzymes Enzymes concentration –Will increase V max but not K m –V max proportional to [E] Competitive inhibition

Regulation of Enzyme Kinetics Competitive inhibition –have similar geometric shape –Compete with enzyme for substrate –Can be overcome by  [S] –Will not affect V max but will  K m

Regulation of Enzyme Kinetics Allosteric regulation (noncompetive inhibitor/stimulator) –Allosteric enzymes don’t follow Michaelis- Menton kinetics; rather, most follow a sigmoidal model –Does not bind to active site on E Changes shape of E which either  of  ability to bind with S –Will change V max but not K m

Regulation of Enzyme Kinetics Phosphorylation –cAMP activates protein kinase –activates catabolic enzymes –inactivates anabolic (synthetic) enzymes Effect of temperature, pH

Metabolic Pathway E 1 E 2 E 3 A > B > C > D initial substrate final substrate First step is usually irreversible and controlled by an allosteric enzyme