UDPglucose pyrophosphorylase

A specific example is UDPglucose pyrophosphorylase, which catalyzes reaction (9). 

The intermediate form with the binding sites in both subunits occupied does not accumulate, but it is the only form that can release a bound nucleotide at a significant rate. UDPglucose pyrophosphorylase exists in several multimeric forms, including dimers, tetramers, and octamers (26). In view of the fact that the calf liver enzyme tightly binds up to one uridine nucleotide per subunit (28), it may be that the former binding mechanism is operative. Either mechanism is consistent with the compulsory ordered binding of substrates, as revealed by the kinetics. 

Reactions (35a) and (35b) are catalyzed by galactose-1-P uridylyltransferase and UDPglucose pyrophosphorylase, respectively reactions (36a) and (36b) are catalyzed by nucleoside diphosphate kinase and adenylate kinase, respectively and reactions (37a) and (37b) are catalyzed by nucleoside phosphotransferase and adenosine kinase, respectively. 

The sugar nucleotides UDPglucose and ADPglucose can be synthesized in plants either by a pyrophosphorylase-type reaction (reactions 3a and 3b Espada, 1962) or via a reversal of the sucrose synthase reaction (reaction 4 Cardini et ai, 1955 de Fekete and Cardini, 1964)... 

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