Does Km (Michaelis constant) vary with enzyme concentration

ratatat said:

On a Kaplan test, they said that Km varies with temp, pH and enzyme concentration.

On an EK lecture manual, it said Km only varies with temp and pH. I don't understand how it varies with enzyme concentration, as Kaplan says, so I'm just looking for confirmation from others as to whether EK is right.

Thanks

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My kinetics suck (disclaimer).
I can't see off-hand why it would vary with enzyme concentration. The only thing I can think of, is that if the enzymes are inter-dependent, like more enzymes leads to an increase in the enzyme activity of each individual enzyme (or a decrease) because the enzymes slightly inhibit or stimulate each other, then they could affect Km. If they follow norma Michaelis-Menton Kinetics, then they would not affect Km I think? Any physical chemists around? lol.

why not? if you have more enzymes, then you need a larger concentration of substrate to reach the maximum speed. you're not reaching the systems kinetic potential if you double the # of enzymes but have the same concentration of substrate.

thebillsfan said:

why not? if you have more enzymes, then you need a larger concentration of substrate to reach the maximum speed. you're not reaching the systems kinetic potential if you double the # of enzymes but have the same concentration of substrate.

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Since km is used as a measure of an enzyme's affinity for a given substrate, it must either be independent of enzyme concentration, or have a set (standard) concentration value. It is the former, and this is why km and not Vmax is used.

Only Vmax is dependent on enzyme concentration.

If you use the Michaelis-Menten equation, you'll see that Km is not affected by Vmax and thus, not affected by enzyme concentration. Km is equal to the substrate concentration at 1/2 Vmax.

Vo = (Vmax*)/(Km + )
1/2 Vmax = (Vmax*)/(Km + )
Km + = 2
Km =

is the substrate concentration. So you can see that Km is NOT dependent on enzyme concentration; otherwise, a change in Vmax (which IS affected by enzyme concentration) would affect Km as well. Hope this helps.