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I have always been taught that enzymes can catalyze both the forward and reverse reaction, and will increase the reaction rate in both directions. I understand that the thermodynamics of the reaction are not altered by the enzyme, but I have yet to find a good answer / example for an enzyme that actually does this. (E.g. can a nuclease join DNA together if the products are in excess? Can a protease form a peptide bond?) I am having difficulty finding a resource online that actually justifies this claim without just saying "Yes enzymes work both ways."

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    $\begingroup$ you are having trouble becasue it is only technically true. In practice becasue of energy profiles, inhibitors, and a few other factors many reactions will never be reversed or cannot be reversed in conditions that the enzyme will survive. This may help bip.cnrs-mrs.fr/bip10/stellen.htm $\endgroup$ – John May 5 '17 at 5:11
  • $\begingroup$ Remember that these chemical reactions have some equilibrium. This can be pushed to to the right side of the reaction by having an excess of educts - then the reaction goes completely in this direction. If it is the other way, the reaction can be reversed... $\endgroup$ – Chris May 5 '17 at 6:19
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Enzymes alter the rate of a reaction by lowering activation energy; they have no effect on the reaction equilibrium ($\ce{K_{eq}}$). Since $\ce{K_{eq}=\frac{k_f}{k_r}}$ and $\ce{K_{eq}}$ is constant, an increase in forward rate ($\ce{k_f}$) requires a corresponding increase in the reverse rate ($\ce{k_r}$). Intuitively it may help to think that the same effect an enzyme has on lowering the energy of the transition state in the forward direction should also be present in the reverse direction. Whether or not the reaction actually proceeds in reverse depends on the free energy difference between the reactants and products and their concentrations (among other things). See this answer for a great explanation.

Because their reactions are in equilibrium ($\Delta G\approx0,\ce{K_{eq}\approx1}$), many enzymes in the glycolytic pathway also catalyze the reverse reaction in gluconeogenesis.

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[image source]

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What about the ATP synthase? https://en.wikipedia.org/wiki/ATP_synthase it uses proton flow to generate ATP but it can also burn ATP to generate proton flow.

Like other enzymes, the activity of F1FO ATP synthase is reversible. Large-enough quantities of ATP cause it to create a transmembrane proton gradient, this is used by fermenting bacteria that do not have an electron transport chain, but rather hydrolyze ATP to make a proton gradient, which they use to drive flagella and the transport of nutrients into the cell.

In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy. The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in the mitochondria, where ATP synthase is located in the inner mitochondrial membrane and the F1-part projects into mitochondrial matrix. The consumption of ATP by ATP-synthase pumps proton cations into the matrix.

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Examples of enzymes working in reverse?

Except three enzymes of Glycolysis (Hexokinase, PFK-I and Pyruvate kinase) all catalyse reversible reactions.

As these enzymes catalyse the backward reactions too they are part of Gluconeogenesis pathway.

(A comparison between the two pathways)

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  • $\begingroup$ Beat me to the example! $\endgroup$ – canadianer May 5 '17 at 6:57
  • $\begingroup$ At first I felt it was a coincidence. :) $\endgroup$ – Tyto alba May 5 '17 at 6:59
  • $\begingroup$ I forgot to upvote. This seems like the most salient example. $\endgroup$ – canadianer May 5 '17 at 7:15
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You mention nucleases and proteases, but if you turn these processes around and think about the actual nucleic acid or protein synthesis reactions an interesting point emerges:

These synthetic processes involve the production of pyrophosphate — not orthophosphate — from ATP (etc.).

(In the case of nucleic acid synthesis this should be obvious; in the case of protein synthesis I am refering to the aminoacyl-tRNA synthetase ‘amino acid activation’ reaction which drives peptide bond formation.)

Why pyrophosphate? The answer commonly given is that this is to prevent the reversal of the reaction. The generation of pyrophosphate achieves this because it is rapidly degraded to orthophosophate by pyrophosphatases in the cell. (See discussion of DNA polymerization in Berg et al.)

If this argument is accepted, it paradoxically illustrates the potential reversibility of the synthetic reactions.

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