Glycogen/starch/alpha-glucan phosphorylase (Glycg_phsphrylas)
Type: Family

The biosynthesis of disaccharides, oligosaccharides and polysaccharides involves the action of hundreds of different glycosyltransferases. These enzymes catalyse the transfer of sugar moieties from activated donor molecules to specific acceptor molecules, forming glycosidic bonds. A classification of glycosyltransferases using nucleotide diphospho-sugar, nucleotide monophospho-sugar and sugar phosphates ([ec:2.4.1.-]) and related proteins into distinct sequence based families has been described . This classification is available on the CAZy (CArbohydrate-Active EnZymes) web site. The same three-dimensional fold is expected to occur within each of the families. Because 3-D structures are better conserved than sequences, several of the families defined on the basis of sequence similarities may have similar 3-D structures and therefore form 'clans'.

Proteins in this entry are glycosyltransferases with phosphorylase activities. Members use phosphate to break alpha 1,4 linkages between pairs of glucose residues at the end of long glucose polymers, releasing alpha-D-glucose 1-phosphate. The nomenclature convention is to preface the name according to the natural substrate, as in glycogen phosphorylase, starch phosphorylase, maltodextrin phosphorylase, etc.

The main role of glycogen phosphorylase (GPase) is to provide phosphorylated glucose molecules (G-1-P) . GPase is a highly regulated allosteric enzyme. The net effect of the regulatory site allows the enzyme to operate at a variety of rates; the enzyme is not simply regulated as "on" or "off", but rather it can be thought of being set to operate at an ideal rate based on changing conditions at in the cell. The most important allosteric effector is the phosphate molecule covalently attached to Ser14. This switches GPase from the b (inactive) state to the a (active) state. Upon phosphorylation, GPase attains about 80% of its Vmax. When the enzyme is not phosphorylated, GPase activity is practically non-existent at low AMP levels.

There is some apparent controversy as to the structure of GPase. All sources agree that the enzyme is multimeric, but there is apparent controversy as to the enzyme being a tetramer or a dimer. Apparently, GPase (in the a form) forms tetramers in the crystal form. The consensus seems to be that `regardless of the a or b form, GPase functions as a dimer
in vivo
. The GPase monomer is best described as consisting of two domains, an N-terminal domain and a C-terminal domain . The C-terminal domain is often referred to as the catalytic domain. It consists of a β-sheet core surrounded by layers of helical segments . The vitamin cofactor pyridoxal phosphate (PLP) is covalently attached to the amino acid backbone. The N-terminal domain also consists of a central β-sheet core and is surrounded by layers of helical segments. The N-terminal domain contains different allosteric effector sites to regulate the enzyme.

Bacterial phosphorylases follow the same catalytic mechanisms as their plant and animal counterparts, but differ considerably in terms of their substrate specificity and regulation. The catalytic domains are highly conserved while the regulatory sites are only poorly conserved. For maltodextrin phosphorylase from Escherichia coli the physiological role of the enzyme in the utilisation of maltidextrins is known in detail; that of all the other bacterial phosphorylases is still unclear. Roles in regulatuon of endogenous glycogen metabolism in periods of starvation, and sporulation, stress response or quick adaptation to changing environments are possible .