Abstract
The penetration of D-[14C] glucose into human red blood cells (RBCs) features kinetic parameters which are readily distinguishable from passive permeation. It would be expected to require activation energy above 80 kJ/mol for permeation of glucose with five hydroxyls capable of forming hydrogen bonds, but the measured activation energy is approximately 16 kJ/mol. As a consequence, glucose permeates RBC membrane about five orders of magnitude faster than would be expected for passive permeation. Glucose transporter protein 1, or GLUT1 and SGLT1, present in all human tissues, but especially in RBCs. It is also anchored in the protective sheet of flat cells that line up the blood vessels of the brain. GLUT1 has a strong affinity for glucose and it ensures that both RBCs and the brain receive appropriate levels of glucose that they need to be able to function. The brain consumes ~120g of glucose per day; the blood glucose level in a typical person 80mg/100ml. The binding site of glucose faces intracellular and extracellular of the membrane alternately when it is loaded by a glucose. The transport is accomplished by conformational changes within GLUT1 , and not by rotation of the whole single long polypeptide chain (55kD, ~500 residues) with the presence of 12 trans membrane α-helices segments. The super family of related GLUT sugar transporters comprises 14 identified isoforms in the human genome, all adopting a 12-membrane–spanning domain structure that delineate 6 extracellular loops .The erythrocyte glucose transporter GLUT1 has an ~10-fold-lower affinity for D-glucose, Km ≈ 10–15 mM, at the inside face for net export than on the outside (Km = 1–2 mM) for net import of glucose (zero-trans net flux) at 24°C , pertaining a liganded consequential asymmetric transporter.
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