I need help with BIOCHEMISTRY!!!!!!?

[Edur R]

Do enzymes change equilibrium constant of the reaction, velocity of the reaction, or both? How do enzymes accomplish that?

[tgd]

An enzyme, being a catalyst, cannot change the equilibrium constant of a reaction. It WILL, of course, change the velocity. Reactions that are thermodynamically favourable but could take years to go to equilibrium may occur in a matter of mintues in the presence of an appropiate enzyme.


Take alcohol dehydrogenase, for example, which catalyzes the reaction

NADH + H(+) + acetaldehyde = ethanol + NAD(+).

In liver it catalyzes the NAD(+)-linked oxidation of ethanol to acetaldehyde (that is how, at least in part, how injested ethanol is got rid of), but in yeast it catalyzes the NADH-linked reduction of acetaldehyde (that is where the ethanol comes from). [In yeast it acts as an aldehyde reductase].

In the presence of low concentrations of alcohol dehydrogenase, the equilibrium constant will of course be identical (the reaction catalyzed is the same).

Finally, the isolated enzyme from liver can act both as an aldehyde reductase and an alcohol dehydrogenase, and this also applies to the isolated enzyme from yeast. [The kinetic constants (Kms and Vmax etc) however, are different for the two enzymes]

tgd