Channelling and Channel Efficiency: Theory and Analytical Implications

Abstract

Channelling of metabolites in a reaction sequence is accepted as the phenomenon where the reaction product of one enzyme is transferred directly to another enzyme as its substrate via transient enzyme-enzyme interactions (Srivastava & Bernhard, 1986, 1987). According to the Srivastava-Bernhard hypothesis such a “one-encounter-type metabolite transfer” can occur if the heterologous enzyme complex associates and dissociates during every catalytic turnover of enzymes. Alternatively, an intermediate can be also channelled if its liberation from the active site of one enzyme is followed by direct transfer to the active site of the next enzyme in the sequence without diffusing into the bulk solution (Srere, 1987; Welch, 1985, Keleti & Ovádi, 1988, and references therein; see also Chapter 21 by Keleti in this book). Both mechanisms of intermediate transfer may result in physiological advantages to an organism, such as (i) segregation of competing pathways by microcompartmentation of intermediates, (ii) reduction of the time required to reach the steady state, and (iii) enhancement of metabolite flux by providing high local metabolite concentrations. A new description of the channelling effect has been elaborated (Tompa et al., 1987) based on inherent parameters such as channel efficiency and intermediate lifetime. These inherent parameters, together with the analytical implications, will be discussed in this chapter. In addition, some examples will be presented to illustrate how the mechanism of intermediate transfer in interacting enzyme systems can be determined and how dynamic channelling complexes of enzymes can be specifically modulated.