Higher Plant Phosphorylases

Katsuyuki Tanizawa, Hiroyuki Mori, Mitsuo Tagaya and Toshio Fukui 

The Institute of Scientific and Industrial Research, Osaka University, 

Earlier studies on the properties of phosphorylases isolated from various sources have indicated that their subunits are similar in size with about 100,000 daltons.15-17 The reaction proceeds in a rapid equilibrium random Bi-Bi mechanism as has been shown by kinetic studies with rabbit skeletal muscle phosphorylases a18-20 and b,21,22 rabbit liver enzyme,23 potato tuber enzyme,24 and the enzyme from E. coli.25) In contrast, the substrate specificities for various glucans differ considerably depending on the enzyme sources. The rabbit muscle enzyme has high affinity for branched glucans such as glycogen and amylopectin but low affinity for amylose and maltodextrin.26,27 The potato tuber enzyme can act on amylose, amylopectin, and maltodextrin but only poorly on glycogen,28,29 while the E. coli enzyme shows high affinity for maltodextrin.10  

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Each monomer contains one pyridoxal phosphate and a highly conserved primary structure in the vicinity of the cofactor binding site. The enzymes follow the same catalytic mechanism, a rapid equilibrium random Bi Bi mechanism. However, unlike animal phosphorylases, the microbial and plant enzymes are not subjected to covalent or allosteric control. Within the group of higher plant phosphorylases two types of enzyme have been distinguished which differ in monomer size, peptide pattern, glucan specificity, and intracellular location (1-3). Based on immunochemical studies on leaf tissues (4-5), one enzyme form has been localized in the cytosol whereas the other one resides in the chloroplast. Thus, the two plant phosphorylase types represent non-interconvertible proteins which presumably have entirely different metabolic functions. 

A SOLUBLE POLYSACCHARIDE FRACTION FROM HIGHER PLANTS A POSSIBLE PHYSIOLOGICAL SUBSTRATE OF THE CYTOSOLIC PHOSPHORYLASE ISOZYME... 

A Soluble Polysaccharide Fraction from Higher Plants A Possible Physiological Substrate of the Cytosolic Phosphorylase Isozyme 99... 

This enzyme is very specific it does not react with sorbose, fructose phosphates, glucose, or any other compound tested in place of fructose, and glucose-l-phosphate cannot replace UDPG. Unlike the case of sucrose phosphorylase, the equilibrium of this reversible reaction favors sucrose synthesis, and the presence of this enzyme in the cells of higher plants implicates this reaction in sucrose synthesis. 

As might be expected, phosphorylases have been observed in numerous biological materials from animals and higher plants and in various bacteria, protozoa, and yeasts US), An especially good source is the extract of potatoes 114)- The phosphorylase can be concentrated from potato extract by means of fractional precipitation with ammonium sulfate. 

The most important enzyme, which is eifective in the synthesis as well as in the degradation of the starchlike polysaccharides, is phosphorylase. This enzyme is known to be widespread in nature. It is present in animal tissues, such as muscle, liver, and brain in higher plants potato, waxy maize, barley, and beans in yeast and in bacteria, namely. 

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