Constant-ratio alternative substrate approach for differentiation of enzyme mechanisms

A new kinetic diagnostic for enzymatic mechanisms using alternative substrates

When the concentration of substrate A is varied against fixed and saturating concentrations of alternative substrates B, B', B", etc., and B is similarly varied against alternatives of A, unique pairs of kinetic patterns are generated for each of the major classes of kinetic mechanism for enzymatic catalysis involving two substrates. Thus, a determination of mechanism may be obtained from just two kinetic patterns. The diagnostic has the following advantages over previous usages of alternative substrates and the traditional approach using initial velocity patterns: efficiency (fewer assays are needed), accuracy (large changes in slopes and intercepts are possible), and precision of assays (one substrate is always saturating).

Kinetic mechanism of the exchanges catalysed by the adenine-nucleotide carrier

Initial rates of the exchange ADPin/ADPout catalysed by the adenine-nucleotide carrier of rat-heart mitochondria have been studied under conditions where internal and external ADP may be varied. The initial rate was measured within 1 s by the carboxyatractyloside-stop method, using a rapid-mixing technique. The double-reciprocal plots v0(-1) versus [ADP]out-1 at different internal-ADP concentrations and v0(-1) versus [ADP]in-1 at different external-ADP concentrations exhibit straight-line relationships having a common point of intersection on the axis of ordinates. These results demonstrate the essential role of a ternary complex and thus exclude the ping-pong mechanism generally accepted. The kinetic equation implies a strong positive cooperativity in the binding of the two substrates. Two models are proposed: (a) the ternary complex performs the exchange and the transport of the substrates in a single step; (b) the carrier is mobile and transports the substrates one by one, the formation of a ternary complex being needed to release the first product.