Enzymes increase the rate of a reaction. They cannot affect the rate constant, thus the rate of both the backwards and forwards reaction is increased. This means the same enzyme catalyses the forwards and backwards reaction.

Then why do cells sometimes have different enzymes for the backwards and forwards reaction (eg alcohol hydrogenase and dehydrogenase)?

  • $\begingroup$ Which specific enzymes are you referring to, in what species and cell are these two enzymes present, and what are the entire reactions that each one catalyzes? "Alcohol dehydrogenase" is a name for many different enzymes. I'm not sure what "alcohol hydrogenase" is. $\endgroup$
    – Bryan Krause
    Apr 24 at 18:31
  • $\begingroup$ Welcome to SE Biology. Please do us the courtesy of reading to the end of the Tour to find out how this site works and what is required of questions. One thing is research on the topic, which would perhaps have given you the answer that I have provided instead. $\endgroup$
    – David
    Apr 24 at 18:50
  • 1
    $\begingroup$ Enzymes do change the rate constant. Maybe you meant equilibrium constant? $\endgroup$
    – canadianer
    Apr 24 at 20:37

1 Answer 1


There are two things to consider in relation to an enzymic reaction. The rate is one consideration, but that has no influence on the direction in which the reaction proceeds.

The direction in which the reaction proceeds is determined by the Gibbs Free Energy change — dependent on the intrinsic chemistry, in the form of the standard free energy change (ΔG0), and the concentration of reactants and products. This determines the position of equilibrium. Speeding up the rate of the overall reaction (the combination of forwards and backwards reactions, the rate of both of which increases) has NO effect on the position of equilibrium, and hence the overall directionality.

This is basic enzyme biochemistry, covered in all textbooks on the subject.

So, how can a different enzyme reverse a reaction, if its ΔG0 and equilibrium constant are fixed? The solution to this apparent paradox lies in discarding the lax terminology:

what is happening in the example cited is not the reversal of a reaction, but the reciprocal of an inter-conversion of two metabolites by a different chemical reaction, involving other metabolites. These two reactions have different values of ΔG0, and, being different, are catalysed by distinct enzymes with different catalytic mechanisms.

I am not familiar with the enzymes the OP refers to as alcohol hydrogenases, but this point is illustrated more easily with the familiar glycolytic enzyme, hexokinase, and its gluconeogenic counterpart, glucose 6-phosphatase:

Interconversions of Glucose and G 6-P

It can be seen that the two reactions interconvert glucose and glucose 6-P, but are different in that one involves ATP (the hydrolysis of which produces an overall negative ΔG0, whereas the other involves a hydrolysis to produce inorganic phosphate.

  • $\begingroup$ Yes now I understand . Different catalysts are involved because different reactions are occurring not the backward and forward part of same reaction. Beauty of biochemistry . $\endgroup$
    – User51
    Apr 25 at 10:14
  • $\begingroup$ @Location — glad to hear that. I did an internet search for enzymes entitled "alcohol hydrogenases" but drew a blank. Are you sure this is correct? Have you a reference of an EC number? $\endgroup$
    – David
    Apr 25 at 10:18
  • $\begingroup$ images.app.goo.gl/13ky2sGSiK4UBZNMA this image caught my attention and I $\endgroup$
    – User51
    Apr 25 at 10:40
  • $\begingroup$ It actually says it's a class of enzyme not a particular enzyme $\endgroup$
    – User51
    Apr 25 at 10:44
  • $\begingroup$ @Location — The link you posted is to alcohol DEhydrogenases, which are well known, not hydrogenases. $\endgroup$
    – David
    Apr 25 at 18:03

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