REDUCTION OF AN 8-STATE MECHANISM OF COTRANSPORT TO A 6-STATE MODEL USING A NEW COMPUTER-PROGRAM

A computer program was developed to allow easy derivation of steady-st ate velocity and binding equations for multireactant mechanisms includ ing or without rapid equilibrium segments. Its usefulness is illustrat ed by deriving the rate equation of the most general sequential iso or dered ter ter mechanism of cotransport in which two Na+ ions bind firs t to the carrier and mirror symmetry is assumed. It is demonstrated th at this mechanism cannot be easily reduced to a previously proposed si x-state model of Na+-D-glucose cotransport, which also includes a numb er of implicit assumptions. In fact, the latter model may only be vali d over a restricted range of Na+ concentrations or when assuming very strong positive cooperativity for Na+ binding to the glucose symporter within a rapid equilibrium segment. We thus propose an equivalent eig ht-state model in which the concept of positive cooperativity is best explained within the framework of a polymeric structure of the transpo rt protein involving a minimum number of two transport-competent and i dentical subunits. This model also includes an obligatory slow isomeri zation step between the Na+ and glucose-binding sequences, the nature of which might reflect the presence of functionally asymmetrical subun its.