Half-of-the-sites reactivity and all-of-the-sites substrate binding in transaldolase

Transaldolase from Candida utilis is a dimeric protein composed of two identical subunits. The cleavage of fructose 6-phosphate by this enzyme was followed in a rapidmixing spectrophotometer. A very rapid reaction was observed during which 1 mol of glyceraldehyde 3-phosphate/mol of enzyme was released, followed by a much slower reaction in which additional glyceraldehyde 3-phosphate was formed. Binding studies carried out with the same substrate showed that two equivalents of dihydroxyacetone were bound. These results indicate that both sites are active, but that only one functions in the rapid catalytic reaction. The half-of-the-sites reactivity of transaldolase may be attributed to a high degree of negative cooperativity between the two subunits.