Protomer structure of oligomeric enzymes: symmetry and allosteric interactions in yeast hexokinase

Structural aspects of oligomerization taking place between the transmembrane alpha-helices of bitopic membrane proteins

Recent advances in biophysical methods have been able to shed more light on the structures of helical bundles formed by the transmembrane segments of bitopic membrane proteins. In this manuscript, I attempt to review the biological importance and diversity of these interactions, the energetics of bundle formation, motifs capable of inducing oligomerization and methods capable of detecting, solving and predicting the structures of these oligomeric bundles. Finally, the structures of the best characterized instances of transmembrane alpha-helical bundles formed by bitopic membrane proteins are described in detail.

Dissociation of enzyme oligomers: a mechanism for allosteric regulation

Most enzymes exist as oligomers or polymers, and a significant subset of these (perhaps 15% of all enzymes) can reversibly dissociate and reassociate in response to an effector ligand. Such a change in subunit assembly usually is accompanied by a change in enzyme activity, providing a mechanism for regulation. Two models are described for a physical mechanism, leading to a change in activity: (1) catalytic activity depends on subunit conformation, which is modulated by subunit dissociation; and (2) catalytic or regulatory sites are located at subunit interfaces and are disrupted by subunit dissociation. Examples of such enzymes show that both catalytic sites and regulatory sites occur at the junction of 2 subunits. In addition, for 9 enzymes, kinetic studies supported the existence of a separate regulatory site with significantly different affinity for the binding of either a substrate or a product of that enzyme. Over 40 dissociating enzymes are described from 3 major metabolic areas: carbohydrate metabolism, nucleotide metabolism, and amino acid metabolism. Important variables that influence enzyme dissociation include: enzyme concentration, ligand concentration, other cellular proteins, pH, and temperature. All these variables can be readily manipulated in vitro, but normally only the first two are physiological variables. Seven of these enzymes are most active as the dissociated monomer, the others as oligomers, emphasizing the importance of a regulated equilibrium between 2 or more conformational states. Experiments to test whether enzyme dissociation occurs in vivo showed this to be the case in 6 out of 7 studies, with 4 different enzymes.

Magnetic resonance studies of the spatial arrangement of glucose-6-phosphate and chromium (III)-adenosine diphosphate at the catalytic site of hexokinase

The interaction of CrADP, an exchange-inert paramagnetic analogue of Mg-ADP, with yeast hexokinase has been studied by measuring the effects of CrADP on the longitudinal nuclear relaxation rate (1/T1) of the protons of water and the protons and phosphorus atom of enzyme-bound glucose-6-P. The paramagnetic effect of CrADP on 1/T1 of water protons is enhanced upon complexation with the enzyme. Titrations measuring this paramagnetic effect at several enzyme concentrations in the presence of glucose-6-P yielded a characteristic enhancement factor for 1/T1 of water protons and the dissociation constant of CrADP from the ternary enzyme . ADPCr . glucose-6-P complex. The latter value (2 mM) is similar to that obtained from kinetic inhibition studies (Danenberg and Cleland [1975]. Biochemistry. 14:28). The presence of glucose-6-P increased the enhancement of the water relaxation rate by enzyme-bound CrADP, suggesting the formation of an enzyme . CrADP . glucose-6-P complex. The existence of such a complex was confirmed by the observation of a paramagnetic effect of enzyme-bound CrADP on the l/T1 of the 31P-nucleus and protons of enzyme-bound glucose-6-P. From the paramagnetic effects of enzyme-bound CrADP on the relaxation rates of the 31P-nucleus and the carbon-bound protons of glucose-6-P in the enzyme . ADPCr . glucose-6-P complex, using the correlation time of approximately 0.7 ns, determined from the magnetic field-dependence of 1/T1 of water protons over the range 24.3-360 MHz, a Cr3+ to phosphorus distance of 6.6 +/- 0.7 A and Cr3+ to alpha- and beta-anomeric proton distances of 8.9 and 9.7 A were calculated. These results imply the absence of a direct coordination of the phosphoryl group of glucose-6-P by the nucleotide-bound metal on hexokinase but indicate van der Waals contact between a phosphoryl oxygen of glucose-6-P and the hydration sphere of the nucleotide-bound metal. The distances are consistent with a model that assumes molecular contact between the phosphorus of glucose-6-P and a beta-phosphoryl oxygen of ADP suggesting an associative phosphoryl transfer. Because after phosphorylation of ADP, the metal ion is coordinated to the transferred phosphoryl group, the overall migration of the phosphoryl group during the phosphoryl transfer is approximately 3.6 A toward the nucleotide-bound metal. Little or no catalysis of phosphoryl transfer from glucose-6-P to alpha, beta-bidentate or beta-monodentate CrADP ( less than or equal to 0.05% of the rate found with MgADP) occurred in the presence of hexokinase, as monitored by glucose formation in a coupled assay system using glucose oxidase and peroxidase. The ability of beta, gamma-bidentate CrATP to act as a substrate (Danenberg and Cleland [1975].