Molecular Biology: Protein Structure and Functions - Stepanov V.M. 2005

Globins
Myoglobin

Myoglobin and Hemoglobin subunits share a very similar tertiary Structure, with their peptide chains folded in space in a comparable manner. Their primary structures also exhibit a distinct, albeit markedly lesser, degree of similarity. At the same time, myoglobin lacks a quaternary structure, and its function is strictly limited to oxygen storage. Thus, myoglobin is a simpler and more ancient protein than hemoglobin.

The peptide chain of mammalian myoglobin consists of 153 amino acid residues. A comparison of the primary structures of myoglobin from 24 animal species shows that 82 positions are occupied by the exact same Amino Acids across all these Proteins. Consequently, roughly half of the residues in this set of myoglobins are invariant, whereas other residues can be substituted without practically affecting the folding of the polypeptide chain or its tertiary structure. Moreover, many of these substitutions do not significantly alter the Chemical Nature of the given residue—hydrophilic Amino acids are replaced by hydrophilic ones, and hydrophobic by hydrophobic, meaning that so-called conservative substitutions predominantly occur.

Myoglobin, like other globins, is a distinctly a-helical protein (Fig. 8.1). Its Spatial Structure is formed by eight a-helices, sequentially designated by letters from A to H (from the amino to the carboxyl end of the chain). The a-helices contain 75% of all amino acid residues, while the remainder are located in turns and disordered terminal residues of the peptide chain. Non-helical regions are denoted by two letters corresponding to the helices between which the turn is situated, or to a helix and either the amino (N) or carboxyl (C) terminus of the peptide chain. For example, HC corresponds to the region between the final H helix and the C-terminal amino acid. For myoglobin amino acid residues, it is standard practice to indicate the letter designation of the a-helix containing the residue and its sequential number within that helix.

About 30 amino acid residues, or more precisely their side chains, form the internal Hydrophobic core of myoglobin, which stabilizes its spatial structure. The core, in which three segments or "clusters" can be distinguished, contains residues of valine, leucine, isoleucine, phenylalanine, and Alanine. The packing of the side chains of these amino acids within the clusters is sufficiently dense while still allowing a certain degree of freedom in their relative movement. One of the clusters features a cavity large enough to accommodate, for instance, a xenon atom. The clusters are integrated into the hydrophobic core by the planar heme molecule, which is, so to speak, inserted into the interior of The myoglobin structure. In this arrangement, only the edge of the heme molecule comes into contact with Water; the molecule itself is immersed in the protein like a basket formed by two a-helices (E and F) and lined with hydrophobic amino acid side chains. Heme has no covalent bonds with the protein and is held in place by non-covalent interactions. In total, there are 80 interatomic contacts between the protein and the heme, ensuring that the latter is held quite firmly.

Class="center">

Fig. 8.1. Spatial structure of myoglobin. Only C_a atoms are shown. The heme is located between helices E and F (indicated by an arrow). Hemoglobin subunits share a very similar structure.

On both sides of the heme plane, "above" and "below" the iron atom, lie the imidazole rings of two Histidine residues (Fig. 8.2). One of them, occupying the eighth position in the F a-helix (His F8) and positioned closest to the iron atom, is referred to as the proximal histidine. The nitrogen atom of its imidazole ring is in contact with the iron atom, suggesting The formation of a covalent bond between them. The other histidine residue, occupying the seventh position in the E a-helix (His E7), is called the distal histidine because it is relatively distant from the iron atom. There is free space between the latter and the imidazole group of this residue, which accommodates the oxygen molecule bound by myoglobin: one of its oxygen atoms approaches the iron directly, while the other approaches the imidazole of His E7. Replacing the distal histidine with Glycine reduces oxygen binding by a factor of 1,000.

In the myoglobin of certain Mollusks, the E7 position is occupied by valine rather than histidine, which also leads to a significant decrease in oxygen affinity. Clearly, the Hydrogen bond formed between the imidazole group of the distal histidine and the oxygen facilitates the binding of the latter. Thus, around the ferrous iron of the heme, five coordination sites are occupied by nitrogen atoms (four belonging to the porphyrin, and the fifth to His F8), and one by oxygen.

However, the space where the oxygen molecule binds lacks a permanent opening to the water surrounding the myoglobin. It is hypothesized that the side groups of the His E7 residue and the neighboring Arg CD3 residue can occupy several different positions, acting as a kind of "gate." The opening of this gate allows the oxygen molecule to approach the heme.

It remains unclear whether the oxygen "waits" for the accidental opening of the gate resulting from thermal fluctuations or induces it in some way. Evidently, the dynamics of the myoglobin molecule play a crucial role in its function.

Fig. 8.2. Oxygen-binding site in myoglobin and hemoglobin subunits.

The iron atom (Fe²⁺) located at the center of the porphyrin is bonded to the nitrogen atom of the imidazole ring of the proximal histidine F8. Upon binding, the oxygen molecule is positioned between the N-H group of the imidazole ring of the distal histidine E7 and the iron atom at an angle to the heme plane.

Recall that under normal conditions, the attachment of oxygen to myoglobin and other globins is not accompanied by The oxidation of iron. Judging by model experiments, this is somehow related to the immersion of the heme-oxygen complex into the hydrophobic environment provided by the surroundings of the heme in globins. If oxidation does occur and the iron transitions to the ferric state, the resulting product is so-called metmyoglobin, which is no longer functional. Its oxygen-binding site is occupied by water. Note that this same cavity between the iron atom and the distal histidine can also accommodate other molecules. For instance, CO binds to heme approximately 200 times more tightly than oxygen, depriving the globin of its ability to function.

Oxygen is readily bound by myoglobin—complete saturation of the protein is achieved at a partial pressure of about 10 mmHg, and 50% saturation at 1 mmHg ¹ (Fig. 8.3). Thus, myoglobin is well-suited for storing oxygen in Muscles, becoming saturated even at very low partial pressures characteristic of Tissues. However, it could not provide a transport function because, having become saturated with oxygen in the Lungs, it would release it only with great difficulty in the capillaries. This task is accomplished by hemoglobin, a protein related to myoglobin but more complex in structure.

Fig. 8.3. Oxygen saturation curves for myoglobin (1) and hemoglobin (2) as a function of oxygen partial pressure.