Although most enzymes are quite specific, they can catalyze side reactions with compounds that are structurally similar to their physiological substrates, but usually at much slower rates. For example, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), which normally catalyzes the oxidative phosphorylation of glyceraldehyde-3-phosphate, can slowly convert erythrose-4-phosphate, an intermediate in the pentose phosphate pathway, to 1,4-bisphosphoerythronate:

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H H-C-OH H-C-OH + NAD+ + P H₂C-OPO₂²- Erythrose-4- phosphate GAPDH OPO₂²- H-C-OH H-C-OH + NADH + H+ H₂C-OPO₂²- 1,4-Bisphosphoerythronate