ABSTRACT
Changes in glycolytic enzyme binding to particulate matter during anoxia are consistent with the hypothesis that bound enzymes are more active than their free counterparts. Data such as these form the basis of a proposal which states that glycolytic enzymes can bind F-actin to form a multienzyme complex and that reversible enzyme binding is a mechanism for controlling glycolytic rates during periods of depressed metabolism. However, this proposal has met with considerable criticism. First, measurements of enzyme binding have been discredited because they make use of procedures that dilute the cellular milieu during homogenization. Second, in vitro enzyme binding studies have shown that increasing salt concentrations can completely dissociate any enzyme F-actin complexes suggesting that binding does not occur in vivo. Third, although it is impossible to measure enzyme binding at the high protein concentrations found in vivo, mathematical calculations suggest that only a small amount of most glycolytic enzymes would be found to F-actin in vivo whereas a significant proportion of phosphofructokinase (PFK) and pyruvate kinase would be bound. And fourth, kinetic studies of F-actin-bound enzymes showed that only PFK is activated when bound to F-actin; all other enzymes are inhibited. We conclude that the overall glycolytic flux through an F-actin-bound complex would be slower than that of a similar collection of soluble enzymes and that multienzyme complexes do not exist in vivo.
