Hemoglobina

Abstract

Hemoglobin (Hb) is a globular protein located inside the erythrocytes which function is carry oxygen from the lungs to the tissue capillaries. The normal levels are 12-15 g/dl in women and 13-16 g/dl in men. Structurally, the Hb is a hetero-protein formed by two types of peptide chains, each of them attached to a prosthetic group called hemegroup, consisting of a complex of protoporphyrin IX and ferrous iron. The protein subunits are held together by non-covalent bonds and they occupy different relative positions in oxyhemoglobin and deoxyhemoglobin. The deoxy form called tense form (T), has low affinity by oxygen, the oxygen binding to the Hb causes a disruption of ionic and hydrogen bonding favoring the relaxed form (R).Oxygen binding to hemoglobin presents a sigmoidal curve of saturation which reflects the cooperative binding of oxygen due to the quaternary structure of hemoglobin. The ability of hemoglobin to bind oxygen is reversibly modulated by the pressure of CO₂ (pCO₂), pH, and availability of 2,3-bisphosphoglycerate. Therefore, the release of oxygen from hemoglobin is intensified when the pH decreases or increases pCO₂ (Bohr Effect). On the other hand, regarding the porphyrins metabolism; the heme biosynthesis is performed in the liver and in the erythrocytes-producing cells in bone marrow; begins with the condensation of glycine and succinil-CoA. The determining step in heme synthesis is the formation of δ-aminolevulinic acid (ALA) to form porphobilinogen. This reaction is catalyzed by ALA synthase. The porphyrin ring is formed by condensation of four molecules of porphobilinogen with the subsequent incorporation of the iron to form heme group. The hemoproteins degradation occurs in the liver and spleen. The first stage in the degradation is the production of biliverdin, which is reduced to bilirubin. Bilirubin is transported to the liver where solubility increases with the glucuronic acid conjugation. The diglucuronide bilirubin is transported to the bile canaliculi which is hydrolyzed by intestinal bacteria and reduced to stercobilin.