Effects Of Specific Ligands Research Articles

The susceptibility of the glycoprotein from the purified (Na +, K +)-activated adenosine triphosphatase to tryptic and chymotryptic degradation with and without Na + and K +

Purified (Na +, K +)-activated adenosine triphosphatase ((Na +, K +)-ATPase, ATP phosphohydrolase, EC 3.6.1.3) has been subjected to trypsin and chymotrypsin hydrolysis. The glycoprotein is much more resistant to proteolysis than the large chain. This differential susceptibility to proteolysis is not due to differences in the number of trypsin or chymotrypsin sensitive bonds because the two subunits are equally susceptible to proteolysis after isolation by preparative gel electrophoresis in sodium dodecyl sulfate. It is also not due to steric “shielding’ of the glycoprotein by the large chain or its proteolytic products: (1) The rate of digestion of the glycoprotein is not increased after 90% of the large chain is digested. (2) The majority of the large chain peptides are released into the supernatant upon degradation. It is concluded that the greater resistance of the glycoprotein to proteolysis is due to its native conformation. In the absence of the large chain, the susceptibility of the glycoprotein to tryptic degradation by K + and Na +. The evidence suggest that this decreased susceptibility was due to conformational changes in the glycoprotein. These specific ligand effects on proteolysis of the glycoprotein suggest that the glycoprotein may participate in Na + and K + binding by (Na +, K +)-ATPase.

Polymérisation linéaire et fixation de ligands

Summary The present paper studies a polymerisation system where elementary units reversibly link together to form indefinite linear polymers. A first part is devoted to a description of the system leading to the definitions of polymerisation levels and polymer distribution. The second part defines the binding action of a ligand upon the polymerisation state of the system. This part of the study relies upon the theory of linked effects making it possible to understand reciprocal relations between saturation fraction and polymerisation level. This pattern provides a very satisfactory interpretation of the glutamate dehydrogenase behaviour as a function of concentration as well as the effect of specific ligands upon its polymerisation level.

Conformational Changes in Aspartate Transcarbamylase

Abstract The effects of specific ligands on the intersubunit interactions in aspartate transcarbamylase of Escherichia coli were studied by treating the enzyme with low concentrations (up to 0.5 mm) of sodium dodecyl sulfate (SDS). The detergent was used to amplify the tendency of certain ligands to relax the oligomeric structure, by converting this relaxation to actual dissociation into subunits. The effect was measured by the increase in catalytic activity at subsaturating aspartate concentrations, known to accompany dissociation into subunits. The extent of dissociation was evaluated by column chromatography using Sephadex G-200. The following observations were made. (a) Ligands binding at the active sites such as orthophosphate, carbamyl phosphate, aspartate (in the presence of orthophosphate), and succinate (in the presence of carbamyl phosphate), in this order, enhanced the dissociating effect of the detergent. (b) A linear correlation was found between the extent of dissociation and the product of the net charge and molecular volume of these ligands. (c) All these ligands also protected the dissociated catalytic subunit against SDS inactivation. (d) The allosteric activator, ATP, and inhibitor, CTP, both stabilized the oligomeric structure, and prevented SDS-induced dissociation. (e) The allosteric effectors also protected the enzyme against heat-induced dissociation. These observations are shown to support the functional allosteric model recently proposed for this enzyme (Markus, G., McClintock, D. K., and Bussel, J. B. (1971) J. Biol. Chem. 246, 762).