Introduction to Molecular Biology: From Cells to Atoms - Anthony Rees, Michael Sternberg 2002

Proteins
Proteins as Transporters: Globins

Class="center">Introduction/introduction.files/image038.jpg" width="641"/>

Fig. 15.1.

Hemoglobin is a protein that transports oxygen from the Lungs to the Tissues and carries carbon dioxide back from the tissues to the lungs. Hemoglobin is localized in red Blood Cells, or erythrocytes. A hemoglobin molecule consists of four polypeptide chains: two identical α-chains (designated α₁ and α₂) and two identical β-chains (designated β₁ and β₂). Each chain is bound to a specific prosthetic group known as a heme.

Myoglobin is the protein responsible for Oxygen transport within Muscle cells. It consists of a single polypeptide chain and contains one heme group.

The Amino Acid Sequence of myoglobin differs from the sequences of both the α- and β-chains of hemoglobin; however, a clear structural similarity exists among all three sequences (Chap. 16). Each chain contains approximately 140–150 residues.

Using X-ray crystallography, Kendrew, Perutz, and their coworkers determined the three-dimensional structures of Myoglobin and hemoglobin.

The tertiary structures of the α- and β-chains of hemoglobin and the myoglobin chain were found to be remarkably similar. In all three Polypeptides, about three-quarters of the residues are incorporated into α-helices. Each of the three chains consists of six long α-helical segments, designated A, B, E, F, G, and H, each containing between 10 and 25 residues (Chap. 10). The chain structures differ in the region between the B and E helices. Myoglobin contains two additional α-helices, C and D, which are either absent or distorted in hemoglobin chains. The overall folding of the α-helices in all three chains is so similar that it is customary to speak of a "globin fold." Adjacent α-helices along the chain are connected by loops consisting of several residues, which adopt non-regular Conformations. In globins, residues are sometimes designated according to their sequential position and the specific α-Helix they belong to. For example, HisF8 of the β-chain is the eighth residue in the F α-helix, ValFG5 is the fifth residue of the loop connecting the F and G α-helices, and TyrHC2 is the second residue in the C-terminal non-regular segment located following the H α-helix.

Heme consists of carbon, nitrogen, and hydrogen atoms forming a flat ring called a porphyrin. At the center of this ring lies an iron atom, which is coordinated to the nitrogen atoms of the ring through four of its six possible coordination bonds.

Two His residues adjoin the heme group. The proximal Histidine (HisF8) is bound to the iron atom via the fifth coordination bond, while oxygen is attached at the sixth position.

Fig. 5.2.

The distal histidine (HisE7) is located on the opposite side of the porphyrin ring relative to HisF8 and does not form a bond with the heme iron—leaving sufficient space to accommodate an oxygen molecule.

Deoxyhemoglobin is hemoglobin that is not bound to oxygen. In deoxyhemoglobin, the iron atom sits out of the plane of the porphyrin ring, and furthermore, there are several salt bridges and Hydrogen Bonds that are absent in the oxygenated form of the protein. Some of these bonds form between atoms within the same chain (such as the Hydrogen bond between the OH group of TyrHC2 and the carbonyl oxygen of ValFG5 in the β-chain), while others form between atoms belonging to different chains (such as the salt bridge between LysC5 of the α-chain and the terminal COO^- group of the β-chain).

Oxyhemoglobin is hemoglobin with bound oxygen. Oxygen is attached to the sixth coordination position of the heme iron and is positioned adjacent to the distal histidine residue. Upon oxygen binding, the iron atom moves into the plane of the porphyrin ring. Concurrently, several other conformational changes occur, including the Displacement of the E and F helices (Fig. 15.1). The primary structural reorganization takes place in The quaternary Structure of hemoglobin, where the α₁ and β₁ chains rotate as a rigid unit relative to the β₂ and β₂ chains. As a result of these changes, oxyhemoglobin contains fewer salt bridges than deoxyhemoglobin.

The binding of oxygen by myoglobin and hemoglobin can be described using binding curves, as shown in Fig. 15.3. The degree of heme oxygenation depends on the concentration of oxygen in the medium (expressed on the graph as its partial pressure). For myoglobin, the curve is hyperbolic, as expected for a simple, single-step binding process. In contrast, the hemoglobin curve is sigmoidal, indicating positive cooperativity—meaning that the oxygenation of one or more subunits increases the oxygen affinity of the remaining subunits.

Fig. 5.3.

The sigmoidal shape of the oxygen-hemoglobin binding curve explains how oxygen is released from the protein in the tissues. The drop in oxygen partial pressure from the arterial to the Venous Circulation leads to a decrease in oxygen saturation from nearly 100% to about 75%. Oxygen that dissociates from the hemoglobin complex in the Veins is then picked up by myoglobin, which has a higher affinity for it.

The cooperative effects of oxygen binding by hemoglobin can be explained within the framework of the concerted (Symmetry) model proposed by Monod, Wyman, and Changeux (Chap. 13). Indeed, as predicted by the model, hemoglobin can exist in two distinct states that differ in the spatial arrangement of their subunits and the number of intersubunit contacts. The Cell/13.html">Protein Structure in one state corresponds to deoxyhemoglobin, and in the other, to oxyhemoglobin. The first state exhibits lower oxygen affinity and possesses more salt bridges and hydrogen bonds than the second. According to the Monod-Wyman-Changeux model, the first state of hemoglobin is the T (tense) state, and the second is the R (relaxed) state.

The equilibrium position between the T and R states depends on oxygen concentration: the higher the concentration, the more the equilibrium shifts toward the R form. Consider a hemoglobin molecule that is unliganded and resides in the T state. The binding of an oxygen molecule to one of the chains induces conformational changes within that heme group and subsequently throughout the entire subunit. These changes increase the probability that the entire hemoglobin molecule will transition from the T to the R state. The binding of a second and third oxygen molecule further increases this probability. Once in the R conformation, hemoglobin binds the final, fourth oxygen molecule with even greater ease. Consequently, a hemoglobin molecule is far more likely to bind either zero or four oxygen molecules than any intermediate number.

The Bohr effect, which involves the release of protons upon oxygen binding to hemoglobin, plays a crucial role in the Transport of Carbon dioxide from the tissues to the lungs.

2,3-Bisphosphoglycerate (BPG), found in erythrocytes, participates in regulating oxygen binding to hemoglobin. BPG readily forms a complex with deoxyhemoglobin by binding in the cavity between the two β-chains, but it has a low affinity for oxyhemoglobin. Therefore, high concentrations of BPG lower hemoglobin's oxygen affinity, thereby promoting oxygen release in the tissues.

Human fetal hemoglobin differs from adult hemoglobin in that its two β-chains are replaced by two γ-chains. Fetal hemoglobin binds BPG less strongly than adult deoxyhemoglobin and consequently exhibits a higher oxygen affinity. As a result, when fetal blood is oxygenated via maternal blood in the Placenta, oxygen is transferred from the mother to the fetus with high efficiency.