Characterization of the honeybee haemolymph alpha-glycosidase specificity and apparent cooperativity as related to substrate-directed aggregation of enzyme asymmetric protomers.

Based on experimentally determined glycosidase molecular forms, their specificity and apparent Hill coefficients against trehalose (1.4) and sucrose (0.6), respectively, in honeybee haemolymph, a theoretical model is proposed involving differential types of non-random aggregation of a single enzyme protomer. This basic unit contains one trehalose-specific site and two asymmetrical subsites: one holds a catalytic zone and both share a proper affinity to any substrate non-specific binding zone. Then, the predicted aggregation possibilities of the promoter to dimers, trimers and tetramers very closely account for all the experimentally determined properties of the enzymes. Moreover, the hypothesis that the enzyme aggregation may be directed by the particular substrate present in major concentrations in the medium is supported by the observed differences in enzyme polymorphism following pre-incubation at high concentrations of either trehalose or sucrose in the medium.