Enzymes, Part 2

• study of chemical reactions that are catalyzed by enzymes • enzyme’s catalytic mechanism • control of enzyme activity • role in metabolism • possible inhibition by drugs or agonists

• describes how reaction velocity varies with substrate concentration • enzyme reversibly combines with its substrate to form an ES complex that yields a product P, releasing the enzyme

• characteristic of an enzyme and its particular substrate and reflects the affinity of the enzyme for that substrate • Km is equal to the substrate concentration at which the reaction velocity is 1/2Vmax • Km does not vary with enzyme concentration

• michaelis-menten kinetics - myoglobin oxygen binding

• sigmoidal curve - hemoglobin binding to O2

• maximal velocity -> number of substrate molecules converted to product per unit time (umol/min) • rate of enzyme/catalyzed reaction increases with substrate concentration until a max velocity is reached -> saturation (substrate are bound to all available binding sites in enzyme)

• reaction velocity increases with temp until peak is reached • increase is result of increased number of molecules having sufficient energy to pass over the energy barrier • further elevation of temp results in decrease in reaction velocity (denaturation) • optimal temp for most mammalian enzymes is 35-40C

• extreme pH (concentration of H+) conditions can affect reaction velocity • pH can affect ionization state of active site • extreme pH conditions can also denature enzymes • pH optimum of enzymes may vary

• any substance that can diminish velocity of an enzyme-catalyzed reaction

• can be irreversible (covalent bonds)

• bind to enzyme through non-covalent bonds • ex: dilution of enzyme-inhibitor complex leads to separation of reversibly bound inhibitor -> enzyme activity can be restored

• competitive inhibition - noncompetitive inhibition

• binds reversibly to the same site that the substrate would normally attach to thus competes with substrate for that site • reduce affinity • increase Km • LB plot Km moves closer to zero, Vmax unchanged

• inhibitor binds at a site different from substrate • can also bind free enzyme (not only ES complex) -> preventing reaction from occurring • decrease the Vmax • Km remains unchanged

• B-lactam antibiotics (penicillin, amoxicillin): act by inhibiting enzymes that are important for bacterial cell wall synthesis • Angiotensin-converting enzyme (ACE) inhibitors: block enzyme that cleaves angiotensin I to the potent vasoconstrictor angiotensin II -> cause vasodilation -> lower blood pressure • aspirin: irreversibly inhibits prostaglandin and thromboxane synthesis