Comparative Enzymology of the Glyceraldehyde 3-Phosphate Dehydrogenases from Pisum sativum

Abstract

Glyceraldehyde 3-phosphate dehydrogenases (EC 1.2.1.12 and 1.2.1.13) have been purified from the seed, root, etiolated, and green shoot of peas (Pisum sativum). These enzymes are tetramers of 140,000 daltons, with subunits of 35,000 daltons. The enzymes differ in isoelectric point. The seed enzyme has a pI of 5.1, and the root enzyme has a pI of 4.5. The cytoplasmic enzyme from etiolated shoots is slightly acidic with a pI of 5.7 to 6.1 and is found in two separable forms. The chloroplast enzyme (from green shoots) is most basic with a pI of 8.0. In immunodiffusion experiments, the seed, root, and cytoplasmic enzymes of the etiolated shoot share antigenic homology, while the chloroplast enzyme does not cross react antigenically with the extra-chloroplast enzymes. The antiserum to the pea chloroplast enzyme did, however, cross react with glyceraldehyde 3-phosphate dehydrogenase purified from the spinach chloroplast. Therefore, the chloroplast enzyme is significantly different from the extra-chloroplast enzymes with respect to primary sequence. The NADP analog phosphoadenosine diphosphoribose showed competitive inhibition to the chloroplast enzyme with either pyridine nucleotide. The NAD analog pyridine 3-aldehyde NAD was competitive with respect to the NAD activity but was hyperbolic competitive in the presence of NADP, indicating a complexity in the binding of pyridine nucleotide to the chloroplast enzyme.