Unifying concept for the coupling between ion pumping and ATP hydrolysis or synthesis.

Abstract

A mechanism is proposed for the coupling between ion transport and enzyme catalysis. The basic concept is that enzymes associated with transport exist in two possible conformations. Each conformation has the potential of catalyzing the enzymatic reaction, and pumping is associated with the conversion of one conformational form to the other. The conformational transition is triggered by the kinetic blockage of specific mechanistic steps for each conformation. Such blockages can cause a cycling between the two conformations concomitant with catalysis. This mechanistic concept is consistent with a variety of results obtained with the Na+,K+-ATPase, the Ca2+,Mg2+-ATPase, and ATP synthesizing enzymes (coupling factors).The association ofATP hydrolysis and synthesis with ion pumping processes is an accepted tenet of modern biochemistry. However, the detailed enzyme mechanism that couples ATP hydrolysis/synthesis and ion pumping is not well understood.The molecular structure and kinetics of several ion-pumping enzymes have been extensively studied. The purpose of this communication is to show that a single mechanistic concept can be used to understand the coupling between ion transport and catalysis for a diverse group of enzymes. Specifically, this concept is that all of these enzymes exist in two possible conformations and that pumping is associated with the conversion of one conformational form to the other. This conformational transition occurs because specific mechanistic steps are blocked for each conformation. Such a mechanism accounts for a variety of complex kinetic observations. A briefreview ofsome ofthe specific enzymes is first presented. The general mechanism then is proposed and discussed. Na',K+-ATPaseFor a recent review ofthis enzyme see ref.1. The enzyme transports K+ into the cell and Na+ out of the cell, both against concentration gradients. Apparently 3 Na+ and 2 K+ are pumped for each ATP hydrolyzed inside the cell (2). The enzyme contains two types ofpolypeptides in equal amounts with molecular weights ofapproximately 105,000 and 40,000 (3). A phosphoryl enzyme is formed by ATP, and an aspartyl residue on the larger polypeptide is phosphorylated (4). Phosphorylation of the enzyme by MgATP requires Na+, whereas dephosphorylation requires K+. Equal binding stoichiometries have been found for tightly bound ATP (5-7), phosphate (5), ouabain (5, 7), and vanadate (7). Probably one site per large polypeptide is present (8). Both ouabain and vanadate are potent inhibitors ofcatalytic activity. Approximately three Na+ binding sites are present per phosphorylation site (9) and two K+ sites are present per ouabain binding site (10). The Na+ binds tightly to the inside and K+ binds tightly to the outside ofthe cell. Both Na' and K+ may bind at the same location, but the access to the binding sites may be different for different ions and different enzyme conformations. Kinetic studies ofATP hydrolysis indicate apparent negative cooperativity with respect to MgATP (cf. refs. 1 and 11). Both negative c...

Properties of the Stochastic Energization-Relaxation Channel Model for Vectorial Ion Transport

“…Läuger proposed a model of ion pumps in which an essential part of the pump molecule is an ion channel (Läuger, 1979(Läuger, , 1984(Läuger, , 1991. In ATP utilizing ion pumps, Hammes stressed the importance of two distinct conformational states (Hammes, 1982). In the so-called cubic model for redox proton pumps, two distinct conformational states were also clearly proposed (Wikström et al, 1981;Malmström, 1985).…”

Fabrication of biomolecule–copolymer hybrid nanovesicles as energy conversion systems

“…For example, basic descriptions have explained the enzymatic reaction process as a single-ion transport step [44]. In several cases, ATP-utilizing pumps as well as redox pumps have all been characterized as having two conformational states [45,46]. A stochastic energizationrelaxation model has also been applied specifically to BR and was based upon the derivation of an external energy supply from photoreactions.…”

Translocation pathway in the catalysis of active transport.

“…An equally important aspect of the mechanism of active transport is the need to avoid reaction steps that would lead to catalysis of uncoupled partial reactions, as has been stressed by Jencks (4) and Hammes (22). In relation to translocation, the need to avoid a transmembrane channel open to both sides is a restriction of this kind.…”