a theoretical question about enzymes(most likely beyond the scope of the mcat)

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10+ Year Member
Jul 24, 2013
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I majored in biochem and seem to remember that enzymes can have differential Km values for the substrate and product, thus differentially effecting the rates of the forward and back reactions(if not, is the rate change zero because of a corresponding vmax change?). Does anyone know how this fits with the ever so publicized notion that the addition of enzymes do not change the equilibrium concentration of a reaction? Sorry I know this is almost certainly beyond the scope of the mcat, but its just something I've been wondering.

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Feb 21, 2014
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Wrote a big thing, but honestly (http://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme/Michaelis_and_Menten_Equation) explained it better than I could.
Basically Km is not different for the forward and reverse but instead k1 and k2 are different than k-1 and k-2.

k1 = rate that enzyme and substrate form ES complex.
k2 = rate that ES complex forms enzyme and product.
k-1 = reverse from ES complex back to Enzyme and substrate.
k-2 = reverse from separate enzyme and product to ES complex.

So an enzyme could have a higher affinity for a reactant than a product, but if than affinity actually produces a product is still going to be determined by your normal free energy diagram and equilibrium concentration.

Explanation also found here: http://www.worthington-biochem.com/introbiochem/equilibrium.html