Transferases and Phosphorylation

Of primary importance are enzymes which catalyse phosphate transfer or phosphorylation. These have been known variously as kinases, phosphotransferases phosphorylases, phosphokinases, trans-phosphorylases and so on. They show a wide range of molecular weights. Adenylate kinase, for example, has a molecular weight of 21,000, whereas yeast fructokinase has a molecular weight of 580,000. 

Some of the reactions involve a hydrolysis as well as a transfer and there is some inconsistency of nomenclature in use. The often-used expression phosphate group transfer may, strictly speaking, involve either what biochemists call the phosphate group (11.34a) or the phosphoryl group (11.34b), or their esterified derivatives. These whole groups are sometimes represented in biochemistry simply as P (the term phosphoryl group usually represents P=0 (Chapter 6.1). 

If the hydrolysis of glucose-l-phosphate is involved, for example, the group transferred may be either (11.34a) or (11.34b) depending upon whether rupture occurs at P-0 or 0-C. This will be determined by chemical conditions and by the enzymes present. 

Virtually all enzymes that transfer phosphate gronps require metal cation activators. Most use Mg, but Mn + and other divalent cations serve in some cases. 

There are two phosphorylation processes of fundamental importance in biochemistry. These are photophosphorylation, the process by which green plants convert light energy to chemical energy, and oxidative phosphorylation, the process by which a large part of the energy in foods is conserved and made available to the cell. 

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