4 enzyme bound homocysteine has a pk a of 6 the zinc

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36.4 Enzyme-bound homocysteine has a pK a of 6. The zinc acts as a lewis acid and binds homocysteine, lowering the pK a . [HA] [HA] 1 [HA] pH = pK a - log 7.4 = 6 - log = [A - ] [A - ] 25 [A - ] Enzyme-bound homocysteine is __________________ and reactive at physiological pH! O O 25 + H 3 N CH C O - 1 + H 3 N CH C O - A - A - A - A - CH 2 CH 2 H At CH 2 CH 2 pH = 7.4 S - SH pHs above A A - A - A - the pKa H H pK a = 6 pH=pK a A A A - A - = pHs below the pKa A H A H A H A H A H A H A A - H CHEMICAL EQUILIBRIUM Methionine synthase exists in multiple conformations. These conformations are in equilibrium with each other. The enzyme needs to position three things above the B 12 and there is no room for any of them. Conformational changes need to occur.
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36.5 "Methyl-cap" region must move Methionine synthase is a modular protein folate- binding domain homocysteine- binding domain Methionine synthase must exist in multiple conformations. CH 3 -cap B 12 B 12 -binding domain Activation domain Folate-On Homocysteine-On Conformation Conformation Activation Resting conformation CH 3 -cap conformation Enzymes are dynamic. Chemistry is dynamic. CHEMISTRY IN SOLUTION!!!!!
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