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What exactly does the reaction direction mean

asked 2020-06-23 01:37:02 -0500

BorisGorelik gravatar image

The reactions.dat file contains (at least) possible values for direction: PHYSIOL-LEFT-TO-RIGHT, LEFT-TO-RIGHT, nan, RIGHT-TO-LEFT, REVERSIBLE, IRREVERSIBLE-LEFT-TO-RIGHT, and PHYSIOL-RIGHT-TO-LEFT

I get what LEFT-TO-RIGHT means. I also think that I understand REVERSIBLE (equilibrium reactions, I suppose).

But what the purpose of RIGHT-TO-LEFT? Does that mean that one can take a left-to-right reaction such as 2H2+O2 -> 2H2O, reverse it and record a right-to-left reaction 2H2O <- 2H2 + O2 ?

If this is the case, what's the purpose of this complication?

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answered 2020-06-23 02:04:29 -0500

Suzanne Paley gravatar image

See https://biocyc.org/PGDBConceptsGuide....:_tex2pagesec_4.2.

Short answer: left and right are assigned according to EC conventions (e.g., all hydrolases are written in the hydrolysis direction), which may not accurately reflect thermodynamic or physiological constraints. The value of the direction slot is intended to indicate the actual direction in biological contexts. So if your example reaction were listed as right-to-left, that would indicate that evidence suggests the reaction proceeds in the direction of electrolysis (which of course would be highly unlikely in this particular case). The purpose of doing things this way is to be consistent with EC conventions.

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Asked: 2020-06-23 01:37:02 -0500

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Last updated: Jun 23