enzymes large molecules made of various amino acids act as catalysts to speed up reactions w/out...

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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

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Page 1: 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

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

Page 2: 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

Enzymes lower the energy of activation for a reaction

Page 3: 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

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

Page 4: 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

• 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

Page 5: 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

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

Page 6: 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

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

Page 7: 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

Regulation of Enzymes

• Enzymes concentration– Will increase Vmax but not Km

– Vmax proportional to [E]

• Competitive inhibition

Page 8: 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

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

Page 9: 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
Page 10: 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

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

Page 11: 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

Regulation of Enzyme Kinetics

• Phosphorylation– cAMP activates protein kinase– activates catabolic enzymes– inactivates anabolic (synthetic) enzymes

• Effect of temperature, pH

Page 12: 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

Metabolic Pathway

E1 E2 E3

A ------> B ------> C ------> Dinitial substrate final substrate

First step is usually irreversible

and controlled by an allosteric enzyme