Hemoglobin and Myoglobin

Susanna Herold

Hemoglobin, the constituent of red blood cells responsible for transport of dioxygen, and myoglobin, present in skeletal muscles to store dioxygen, are probably the most thoroughly studied proteins. Hemoglobins were first found in blood because of their very high concentration (15 g per 100 mL in normal human blood). Until recently it seemed that the oxygen carrier function was so specialized that it appeared to be the only role of these

Proximal histidine

Scheme B.10.1. The active site of hemoglobin and myoglobin.

globins. However, recent discoveries of new hemoglobins in archea, bacteria, protozoa, plants, and invertebrate animals have revealed the broad diversity of these proteins and suggest that they are present in most organisms, if not all [1]. The hemoglobin that can be readily isolated from the blood of any vertebrate is a heterotetramer of two a and two b polypeptides, with a heme tightly bound to each monomer via the so-called proximal his-tidine residue (Scheme B.10.1) [2]. Despite the differences between their amino acid sequences, the a and b subunits have the same fold, which is made up of eight a-helices and called the globin fold. Movements and interactions between the a- and b-globin subunits lead to cooperative binding of oxygen, which enables hemoglobin to pick up oxygen readily in the lungs and to unload it efficiently in the peripheral respiring tissues. The coordinated dioxygen is stabilized by hydrogen bonding to an additional highly conserved histidine residue, the so-called distal histidine (Scheme B.10.1).

Myoglobin is a protein of conserved structure and function found in skeletal and heart muscles of vertebrates. It is believed to be used as a short-term reservoir of oxygen in working muscles and also to facilitate oxygen diffusion to mitochondria for respiration [3]. The myoglobin structure, the first protein structure to be revealed at the atomic level [4], has the typical globin fold formed by eight a-helices and is very similar to that of the two hemoglobin chains. The heme pocket is also analogous, with the proximal histidine coordinated to the iron center and the distal histidine stabilizing the coordinated dioxygen molecule (Scheme B.10.1).

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