Hemoglobin's Chaperone
The function of hemoglobin in oxygen transport by red blood cells requires precise assembly of a tetramer of two beta-subunits and two alpha-units. If too many beta-subunits are present, then all tetramers of beta-subunits are formed that bind oxygen but do not release it when needed. When alpha-subunits are in excess they form precipitates with oxidized heme that damage red blood cells. Red blood cells require a precise coregulation of alpha and beta subunit expression to maintain the proper ratio of the subunits and normal red blood cell function. Generally red blood cells express slightly more alpha than beta-subunits to avoid forming unproductive beta-tetramers (HbH). The slight excess of alpha units are blocked from precipitation by a molecular chaperone, AHSP, alpha-hemoglobin stabilizing protein. Expression of the AHSP gene is coordinately activated with the globin genes and heme biosynthesis genes by the GATA-1 gene involved in erythrocyte differentiation. AHSP blocks precipitation of alpha-units, and promotes the formation of normal productive alpha-beta tetramers (HbA). People with mutation or deletion of the beta-subunit gene have Beta-thalassemias resulting from an over-abundance of alpha subunits and their precipitation. The severity of the phenotype may depend on the AHSP genotype of these individuals and elevating AHSP levels through gene therapy may provide a treatment strategy for beta-thalassemia.
Contributor: Kosi Gramatikoff, PhD
REFERENCES: Gell D, Kong Y, Eaton SA, Weiss MJ, Mackay JP. Biophysical characterization of the alpha-globin binding protein alpha-hemoglobin stabilizing protein. J Biol Chem. 2002 Oct 25;277(43):40602-9. Kihm AJ, Kong Y, Hong W, Russell JE, Rouda S, Adachi K, Simon MC, Blobel GA, Weiss MJ. An abundant erythroid protein that stabilizes free alpha-haemoglobin. Nature 2002 Jun 13;417(6890):758-63 Luzzatto L, Notaro R. Haemoglobins chaperone. Nature. 2002 Jun 13;417(6890):703-5.
