Enzymes Large molecules made of various amino acids Act as catalysts to speed up reactions w/out...
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Transcript of Enzymes Large molecules made of various amino acids Act as catalysts to speed up reactions w/out...
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
• Vmax occurs when enzyme active sites are saturated with substrate
• Km (Michaelis-Menten constant) reflects affinity of enzyme for its substrate
• smaller the Km, the greater the affinity an enzyme has for its substrate
Enzyme Kinetics
• [S] generally < than its Km
– 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 Km and fourth has a high Km
Hexokinase can phosphorylate glucose even with a low blood [glucose]; a high Km prevents liver from taking up blood glucose when [glucose] is low
Regulation of Enzymes
• Enzymes concentration– Will increase Vmax but not Km
– Vmax 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 Vmax but will Km
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 Vmax but not Km
Regulation of Enzyme Kinetics
• Phosphorylation– cAMP activates protein kinase– activates catabolic enzymes– inactivates anabolic (synthetic) enzymes
• Effect of temperature, pH
Metabolic Pathway
E1 E2 E3
A ------> B ------> C ------> Dinitial substrate final substrate
First step is usually irreversible
and controlled by an allosteric enzyme