SpermWhaleMyoglobinH64AwithNitrosoamphetamine

Myoglobin H64A with nitrosoamphetamine (PDB ID: 5KD1) from Physeter catodon, is a monomeric, globular protein responsible for the storage and transportation of oxygen to cardiac and skeletal muscles of animals (1). Myoglobin is typically more abundant in aquatic, diving, animals such as sperm whales, due to myoglobin being able to bind to oxygen more readily as opposed to the tetrameric hemoglobin (2). It is important to understand how myoglobin functions and to distinguish it from hemoglobin and other globins.

Myoglobin is a monomer consisting of one subunit. Myoglobin is also a globular (globin) protein. All globins contain a heme group and are actively involved in storage and transport of O2. This heme group is necessary for the functions of globins. Furthermore, different globins are found in different locations in the body. Hemoglobin is found in blood and myoglobin is found in muscle tissue. The heme group is also what gives muscles and blood a red color (3).

Multiple types of myoglobin exist. Deoxymyoglobin, which is myoglobin without a bound oxygen substrate. Oxymyoglobin is myoglobin with a bound oxygen substrate, and carbon monoxide myoglobin has a carbon monoxide bound substrate (4). Carbon monoxide has a 60% stronger affinity towards myoglobin than hemoglobin, which makes carbon monoxide a potent cause of muscle degradation as the myoglobin in muscles are affixed with carbon monoxide instead of oxygen and thus, muscle tissue is deprived of oxygen (5).

Myoglobin, has a molecular weight of 17199.90 Da and an isoelectric point of 8.71 (6). Myoglobin is a 153-residue monomer composed of eight, right-handed, alpha helices, in addition to random coils. Each helix is tagged with letters A through H. Myoglobin is found to be 75% helical (7). More notably, myoglobin contains a single heme prosthetic group that is integral in myoglobin's function. The heme group contains an iron atom bound to four pyrrol rings, which is collectively referred to as protoporphyrin IX. The iron atom within the ring is covalently attached to a proximal His-93 residue while the protoporphyrin ring itself is ionically bonded to a distal His-64. Though His-64 is relatively remote, it is still able to react with any oxygen substrate and therefore favors the binding of oxygen instead of other molecules such as carbon monoxide. The importance of the distal His-64 is that it prevents oxygen from oxidizing the iron (converting from Fe(II) to Fe(III) and forming a linear Fe-O-O bond). Instead, the His-64 induces a bent Fe-O2 bond that is weaker but that allows easier release of oxygen (8). Myoglobin also contains sulfate ions and glycerol for structural stability as well as a 2-nitroso-1-phenylpropane free ligand bound to the heme group through the nitroso atom. Because myoglobin only contains one heme group as opposed to hemoglobin which contains four, myoglobin is able to bind more tightly and readily to oxygen. As a result the main function of myoglobin is the storage of oxygen in muscle cells while hemoglobin is responsible for transport through the body (1).

To better understand the differences and similarities between myoglobin and hemoglobin, two protein search engines were used. PSI-BLAST, which utilizes the primary structure of a given protein to find other similar proteins (an E-score of less than 0.05 indicates significant similarity) (9), and the Dali Server, which compares tertiary structures to find similar proteins. The Dali Server employs a sum-of-pairs method, and a Z-score above 2 indicates the proteins are signifacntly more similar (10). Using these resources, the protein, hemoglobin (PDB ID: 3D4X) from in Felis silvestris catus was shown to be vastly similar to myoglobin (the E-score was 4e-69 and the Z-score was 19.9) (9) (10).Tetrameric hemoglobin is more intricate than myoglobin in that its quaternary structure is composed of two alpha subunits and two beta subunits, but the secondary structures are the same as myoglobin. Hemoglobin also contains a prosthetic heme group. However, hemoglobin has four heme groups, one on each subunit. Because hemoglobin binds to four oxygen molecules, it is better suited for transport through the blood stream as it is more efficient (11). Furthermore, because hemoglobin has four subunits, it illustrates cooperative, allosteric binding which is not seen the monomer of myoglobin. This cooperative binding effect is why myoglobin becomes saturated with oxygen faster than hemoglobin, which is essential for myoglobin's function as a storage unit (12). As seen in Figure 1, myoglobin's saturation curve is hyperbolic while hemoglobin's saturation curve is sigmoidal, indicative of the cooperative binding effect (13).

Both myoglobin and hemoglobin will bind to oxygen in high concentration of oxygen. But because hemoglobin binds to oxygen less tightly and myoglobin binds more tightly, in concentrations of low oxygen, hemoglobin will release oxygen, making it suitable for transport (14). Myoglobin will release oxygen in extreme conditions, which is why it is found in muscles. During anaerobic physical activities such as swimming or holding one's breath, the muscles become deprived of oxygen, which triggers myoglobin stored in the tissues to release oxygen. Thus, aquatic, diving animals have more myoglobin in their muscles to accommodate for frequent oxygen deprivation associated with deep sea diving (14).

Myoglobin not only supplies oxygen to muscle cells, but it also supplies oxygen directly to the electron transport chain in the mitochondria during respiration. Specifically, deoxymyoglobin reduces cytochrome c (4). Both cytochrome c and myoglobin are hemoproteins of a single subunit. When comparing deoxymyoglobin, oxymyoglobin, and carbon monoxide myoglobin, the deoxymyoglobin is the more reactive species, and the latter two proteins have been found to be unreactive with cytochrome c (4). This lack of reactivity is attributed to the fact that oxymyoglobin and carbon monoxide myoglobin are liganded, and it is difficult to remove an electron from liganded hemoglobin. Therefore, myoglobin reduces cytochrome c to aid in the process of the electron transport chain as well as in apoptosis where cytochrome c plays a role in activating specific enzymes and receptors (4).

Another similar globin, human neuroglobin (PDB ID: 1OJ6), also plays a role in supplying oxygen. Neuroglobin was found to be significantly similar to Myoglobin as the Z-score is 18.5 (10). However, neuroglobin specializes in interacting with the central and peripheral nervous system. Neuroglobin also has eight alpha helices but only 151 residues (15). Besides using its prosthetic heme group to store oxygen for the brain, neuroglobin is also found to protect and enhance cells under hypoxia and reactive nitrogen species (ROS) concentrations, which is important especially in neural cells (15). Neuroglobin's protective nature is achieved by the neuroglobins acting as NO-dioxygenase when oxygen concentrations are low and subsequently scavenging and binding to NO2- to form NO. The neuroglobins also can bind to NO to form a Ngb-Fe2+-NO complex that decomposes ROS compounds to create Ngb-Fe3+-NO. Thus, neuroglobin is able to create or destroy NO which could have implications regarding vasoconstriction or relaxation (15).

Overall, myoglobin and the globin family are integral in supplying the body with oxygen via prosthetic heme groups that transport and store oxygen molecules. However, some globins have additional roles such as reducing cytochrome c or protecting neural cells from ROS. It is important to understand and study these hemoproteins because very little is known about their functions beyond their oxygen transporting capabilities, and the more that is known, the better equipped scientists and researchers will be at combating diseases and mutations that arise.