Glyceraldehyde 3-phosphate exists as the geminal diol and the free aldehyde in the molar ratio 29:1 in aqueous solution. The rate constant of the conversion of diol into aldehyde is 8·7×10−2sec.−1 in the pH range 7·3–8·6 at 20°. The free aldehyde is the substrate for d-glyceraldehyde 3-phosphate dehydrogenase. Over a wide concentration range of enzyme the rate of conversion of diol into aldehyde is the rate-limiting process in the catalytic oxidation of d-glyceraldehyde 3-phosphate by NAD+. Aldolase and triose phosphate isomerase both liberate d-glyceraldehyde 3-phosphate as the aldehyde. This suggests that the relatively slow diol–aldehyde interconversion does not restrict the rate of glycolysis.

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