Abstract
Succinic thiokinase (EC 6.2.1.4 and EC 6.2.1.5) is one of the most neglected enzymes of intermediary metabolism. Although it was isolated from pig kidney and liver in 1956 by Sanadi et al. (1956) and from spinach by Kaufman & 41ivisatos (1955), following earlier work by Kaufman et al. (1953) subsequent studies have been sparse and from a metabolic point of view, unhelpful. Burnham (1963) purified a succinic thiokinase from Rhodopseudomonas spheroides. Cha and co-workers purified the enzyme from pig heart (see Cha, 1969). Subsequent work has been almost entirely concerned with the mechanism of the E. co/i enzyme, in particular with the interaction of the subunits and the location of the catalytically active sulphydryl groups (Nishimura et al.. 1978; Matula & Nishimura, 1978). The enzyme from both mammalian and bacterial sources consists of two types of subunit, a larger (/J) unit, mol wt approx 40,000, and a smaller (r) unit of mol wt approx 30.000. Succinic thiokinase from pig heart contains one mole of each type of subunit (Brownie & Bridger, 1972) while the E. coli enzyme consists of two of each type (Bridger. 1971). The mechanism of the enzyme was for a long time in dispute, as has been the case with other 3 substrate. 3 product enzymes. Of the many mechanisms proposed, two have gained most popularity:
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