Biochemistry question

Erich Kriegler · 5 May 2013 at 15:14

Anyone have any idea on the answer to this? I know it isn't D, and I think the answer is C, but I'm not certain.

Erich Kriegler · 5 May 2013 at 16:19

Judgeneo · 5 May 2013 at 16:31

Warning: I am not a Chemist or Biologist!

The difference between experiments 1 and 4 is the k_cat values, which I'm assuming measure the effectiveness of the enzyme as a catalyst in this experiment? As the value decreased by a factor of 20, we can assume that substance X decreases the catalytic effect.
a factor of 20 seems a bit big for a 'regulator', and a second substrate shouldn't effect the speed of the reaction massively. So that leaves options C and D.

No idea what the difference is between competitive and non-competitive inhibitors are, but you seem sure its not D, which leaves the answer as C.

anjs · 5 May 2013 at 18:07

Well the affinity for the substrate by the enzyme stays the same (Km Michaelis constant). The but the reaction rate is reduced (Kcat) meaning that less substrate is turned over in the same time frame.
So applying these principles we can say that:-

1. X is an inhibitor as Kcat is reduced between 1 and 4
2. X does not compete with the substrate as Km stay the same between 1 and 4.

This means that C is the correct answer.

I am a Biochemist by the way

Erich Kriegler · 5 May 2013 at 18:31

Thanks for the help, so an allosteric regulator would change Km?

anjs · 5 May 2013 at 18:36

Because if it was then the kcat would be increased signifying an increased rate of substrate turn over.

Erich Kriegler · 5 May 2013 at 18:41

Could an allosteric inhibitor not be inhibitory and decrease the rate of substrate turn over?

anjs · 5 May 2013 at 19:10

Yes but an allosteric inhibitor binds to the enzyme changes the confirmation of the active site and would prevent the binding of the substrate leading to increased Km.