Phosphorylase plants

P + G-G-G-G-G. degradative G-P + G-G-G-G-. starch chain y synthetic ot-Glc-l-p so-called primer 

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Previously, it has been shown that most of the residues directly interacting with AMP as well as the phosphorylatable Ser14 and its surroundings in the rabbit muscle enzyme are far less conserved in the potato type-L isozyme sequence.63 Likewise, the amino-terminal region of the potato type-H isozyme is completely different from that of the rabbit muscle enzyme over the first 80 amino acid residues, in which the sites of covalent phosphorylation and of allosteric regulation by AMP are all included. These variances in sequence are compatible with the lack of regulation in the plant phosphorylase isozymes. 

In conclusion, it should be pointed out that in marked contrast to the very extensive studies on rabbit muscle phosphorylase, little attention has been paid to enzymes from other sources. However, primary structures of plant phosphorylases have now been determined and bacterial expression systems for the plant enzymes have also been made available as reviewed in this article. We hope that future studies on the structure and function of plant phosphorylases without allosteric regulation and comparison with those of the highly regulated animal enzyme will provide valuable information on this interesting group of enzymes, phosphorylases. 

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. 

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