The glycine cleavage system of arthrobacter globiformis.

The glycine cleavage system of Arthrobacter globiformis has been shown to catalyse the reaction: glycine + H4PteGlu [right arrow] 5,10-methylene-H4PteGlu + CO2 + NH3 + 2H. The reaction is assayed by the conversion of methylene- tetrahydropteroylglutamate (methylene-H4PteGlu) to 5,10-methylidyne-H4PteGlu, catalysed by methylene-H4PteGlu dehydrogenase. Methylene-H4PteGlu + NADP+ [right arrow] methylidyne-H4PteGlu + NADPH + H+. The formation of methylene-H4PteGlu, methylidyne- H4PteGlu and NADPH has been demonstrated. Equimolar amounts of methylidyne-H4PteGlu and NADPH are formed. It appears that the two hydrogen atoms formed in the reaction are transferred directly to oxygen, possibly via a flavo- protein. It is unlikely that the glycine cleavage system uses the electron transport chain of NADH oxidase. The enzyme concentration plot is non-linear in the absence of boiled extract, but linear in the presence of boiled extract. Extracts of A. globiformis catalyse an exchange reaction between [1 - 14C_] glycine and an added pool of bicarbon-ate or between [-14C] bicarbonate and an added pool of glycine. Reaction kinetics, purification and its adaptive formation suggest that the exchange reaction is catalysed by the glycine cleavage system. The factor in boiled extract, A2, has been studied in detail. It is possible to remove A2 from extracts by a mild heat-treatment followed by dialysis. A2 has been shown to be a low molecular weight protein. Its replacement by lipoate and the effect of thiol reagents on reduced A2 suggest that it is a disulphide. A2 is present at a much higher level in glycine-grown than in glucose-grown cells. The level of the glycine cleavage system rises sharply during the lag period of about 3hr. on transfer from growth on glucose to a glycine medium. On transfer from growth on glycine to a glucose medium the specific activity falls to very low levels over a long period of time but over short periods somewhat variable results are obtained.