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I am wondering what the exact definition of biochemical coupling between two enzymatic reactions is. I know that these two enzymatic reactions have to

  1. share the same intermediate(s)

  2. one is thermodynamically favorable and the other is not.

So the thermodynamically unfavorable reaction can happen when it coupled with the thermodynamically favorable reaction. I just want to confirm that : Do these two enzymes have to be physically associated with each other? Since I do not have any evidence to support the physically interactions between these two enzymes, can I still use the term "biochemical coupling"?

Many thanks.

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The enzymes don't need physical interactions to couple reactions. Some enzymes do share subunits which are physically associated, but this is a special case. The following review gives a sort of general example during glycolysis,

For example, in the biochemical pathway that breaks down glucose for energy, two enzymes work one after the other to create a high-energy ATP molecule:

We can also look at the peptidyltransferase reaction during the elongation step in translation. Earlier, an aminoacyl tRNA synthetase catalyzed the covalent addition of an amino acid to it's associated tRNA. The energy from breaking this bond in the ribosome provides energy for the peptidyltransferase to link the two amino acids together. This isn't assuming the ATP/GTP needed by elongation factors, etc.

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In any coupled system the activity of one component is dependent on the other. For example translation and transcription are coupled; here translation is dependent on transcription but not vice versa. Coupling need not be bidirectional.

There is no question of unfavourable thermodynamics.

For coupled biochemical reactions:

  • One of the reactions should be dependent on the other. In multicomponent systems there should be at least a pairwise unidirectional dependency (A→B, B→C, C→D). In other words there should be a pathway that connects all the components.
  • There need not be any physical interaction
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