Amylopectin synthase of Eimeria tenella: identification and kinetic characterization

A soluble enzyme amylopectin synthase (UDP‐glucose‐α 1,4‐glucan α‐4‐glucosyltransferase) which transfers glucose from uridine 5′‐diphosphate glucose (UDP‐glucose) to a primer to form α‐I,4‐glucosyl linkages has been identified in the extracts of unsporulated oocysts of Eimeria tenella. UDP‐glucose and not ADP‐glucose was the most active glucosyl donor. Corn amylopectin, rabbit liver glycogen, oyster glycogen and corn starch served as primers; the latter two were less efficient. The enzyme has an apparent pH optimum of 7.5 and exhibited typical Michaelis‐Menten kinetics with dependence on both the primer and substrate concentrations. The Michaelis constants (Km). with respect to UDP‐glucose, was 0.5 mM; and 0.25 mg/ml and 1.25 mg/ml with respect to amylopectin and rabbit liver glycogen. The product formed by the reaction was predominantly a glucan containing α‐1,4 linkages. The specificity of the enzyme suggests that this enzyme is similar to glycogen synthase in eukaryotes and has been designated as amylopectin synthase (UDP‐glucose‐α‐1,4‐glucosetransferase EC 2.4.1.11).