Statistical Thermodynamic Background

Abstract

By an “equilibrium aggregate” is meant one whose formation involves physical attractive forces only (e.g., van der Waals and electrostatic interactions between protein molecules). This is the simplest case and is the subject of Part I of this book. Part II treats “steady-state aggregates.” This term refers to cases in which the monomers are enzyme molecules and a chemical reaction, catalyzed by the aggregated enzyme, accompanies or follows monomer attachment to the aggregate. Actin and microtubules provide examples of this, in which the chemical reactions are the hydrolysis of ATP or of GTP, which are bound to the respective monomers. Furthermore, in the case of microtubules, at least, it is possible for a microtubule end to spend most of its time in either of two distinct states or phases: the subunits at and near the end have GTP bound (not yet hydrolyzed) or these subunits have GDP bound (i.e., the GTP has been hydrolyzed). In-the first phase, the microtubule end is stable; in the second it is unstable. This two-phase behavior is novel and requires separate treatment (Chapter 8).