Vitamin C transporters

Vitamin C is a wide spectrum antioxidant essential for humans, which are unableto synthesize the vitamin and must obtain it from dietary sources. There are two biologicallyimportant forms of vitamin C, the reduced form, ascorbic acid, and the oxidizedform, dehydroascorbic acid. Vitamin C exerts most of its biological functionsintracellularly and is acquired by cells with the participation of specific membranetransporters. This is a central issue because even in those species capable of synthesizingvitamin C, synthesis is restricted to the liver (and pancreas) from which is distributedto the organism. Most cells express two different transporter systems for vitaminC; a transporter system with absolute specificity for ascorbic acid and a secondsystem that shows absolute specificity for dehydroascorbic acid. The dehydroascorbicacid transporters are members of the GLUT family of facilitative glucose transporters,of which at least three isoforms, GLUT1, GLUT3 and GLUT4, are dehydroascorbicacid transporters. Ascorbic acid is transported by the SVCT family ofsodium-coupled transporters, with two isoforms molecularly cloned, the transportersSVCT1 y SVCT2, that show different functional properties and differentialcell and tissue expression. In humans, the maintenance of a low daily requirement ofvitamin C is attained through an efficient system for the recycling of the vitamininvolving the two families of vitamin C transporters (AU)

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Vitamin C transporters.

Vitamin C is a wide spectrum antioxidant essential for humans, which are unable to synthesize the vitamin and must obtain it from dietary sources. There are two biologically important forms of vitamin C, the reduced form, ascorbic acid, and the oxidized form, dehydroascorbic acid. Vitamin C exerts most of its biological functions intracellularly and is acquired by cells with the participation of specific membrane transporters. This is a central issue because even in those species capable of synthesizing vitamin C, synthesis is restricted to the liver (and pancreas) from which is distributed to the organism. Most cells express two different transporter systems for vitamin C; a transporter system with absolute specificity for ascorbic acid and a second system that shows absolute specificity for dehydroascorbic acid. The dehydroascorbic acid transporters are members of the GLUT family of facilitative glucose transporters, of which at least three isoforms, GLUT1, GLUT3 and GLUT4, are dehydroascorbic acid transporters. Ascorbic acid is transported by the SVCT family of sodium-coupled transporters, with two isoforms molecularly cloned, the transporters SVCT1 y SVCT2, that show different functional properties and differential cell and tissue expression. In humans, the maintenance of a low daily requirement of vitamin C is attained through an efficient system for the recycling of the vitamin involving the two families of vitamin C transporters.

Conformational changes induced by cloxacillin in class a beta-lactamase from Bacillus cereus.

Class A beta-lactamases are enzymes that hydrolyse beta-lactam antibiotics such as penicillins and cephalosporins. They also hydrolyse substrate analogues such as oxacillin and cloxacillin, with a biphasic kinetic as it has been reported for Bacillus cereus beta-lactamase. A molecular model of Bacillus cereus beta-lactamase was built and the conformational changes that the substrates benzylpenicillin and cloxacilline produced in the conformation of selected regions of the protein were analyzed. This study was performed using the docking of the substrates, their tetrahedral intermediates and the corresponding acids on the active site, followed by molecular dynamic and subsequent optimisation procedures. The most important changes were produced on Tyr105 and Tyr273, when the tetrahedral intermediate of cloxacillin was docked at the active site, these amino acids are partially responsible for the stabilisation of the substrates at the active site. These changes may explain the kinetic differences observed during the hydrolysis of substrates type S and type A by beta-lactamases class A.