Myoglobin and hemoglobin are hemeproteins whose physiological importance is principally related to their ability to bind molecular oxygen.

Myoglobin and Hemoglobin

Myoglobin (Mb)

The oxygen storage proteinIt also enhances oxygen diffusion within the cellFound in many organisms, including unicellular organisms

Binds O2 reversibly:

O2 + Mb MbO2

O2 + myoglobin oxymyoglobin

Myoglobin

Single polypeptide chain (153 amino acids)

No disulfide bonds

8 right handed alpha helicesform a hydrophobic pocket which contains heme molecule protective sheath for a hemegroup

Myoglobin is a monomeric heme protein found mainly in muscle tissue where it serves as an intracellular storage site for oxygen During periods of oxygen deprivation oxymyoglobin releases its bound oxygen which is then used for metabolic purposes

The tertiary structure of myoglobin is that of a typical water soluble globular proteinIts secondary structure is unusual in that it contains a very high proportion (75%) of α-helical secondary structure

A myoglobin polypeptide is comprised of 8 separate right handed a-helices, designated A through H, that are connected by shortnon helical regions

Amino acid R-groups packed into the interior of the molecule arepredominantly hydrophobic in character while those exposed on the surface of the molecule are generally hydrophilic, thus making the molecule relatively water soluble

Each myoglobin molecule contains one heme prosthetic group inserted into a hydrophobic cleft in the protein

Each heme residue contains one central coordinately bound iron atom that is normally in the Fe 2+ , or ferrous, oxidation state

The oxygen carried by hemeproteins is bound directly to the ferrous iron atom of the heme prosthetic group

ProtoporphyrinIX + Fe 2+ (heme)

Molecular weight 616(C34H32N4O4Fe1)

The heme group is located in a creviceExcept for one edge, non polar side chains surround the hemeFe 2+ is octahedrally coordinatedFe 2+ covalently bonded to the imidazole group of histidine 93 (F8)O2 held on the other side by histidine 64 (E7)

Hydrophobic interactions between the tetrapyrrole ring and hydrophobic amino acid R groups on the interior of the cleft in the protein strongly stabilize the heme protein conjugate.

In addition a nitrogen atom from a histidine R group located above the plane of the heme ring is coordinated with the iron atom further stabilizing the interaction between the heme and the protein. In oxymyoglobin the remaining bonding site on the iron atom (the 6th coordinate position) is occupied by the oxygen, whose binding is stabilized by a second histidine residue

Carbon monoxide also binds coordinately to heme iron atoms in a manner similar to that of oxygen, but the binding of carbon monoxide to heme is much stronger than that of oxygen. The preferential binding of carbon monoxide to heme iron is largely responsible for the asphyxiation that results from carbon monoxide poisoning.

Hemoglobin (Hb)

Oxygen transporter

Four polypeptide chains

Tetramer

Each chain has a heme group

Hence four O2 can bind to each Hb

Two alpha (141 amino acids)and two beta (146 amino acids) chains

Hemoglobin

Hemoglobin is an [α(2):β(2)] tetrameric hemeprotein found in erythrocytes where it is responsible for binding oxygen in the lung and transporting the bound oxygen throughout the body where it is used in aerobic metabolic pathways

Each subunit of a hemoglobin tetramer has a heme prosthetic group identical to that described for myoglobin.

Although the secondary and tertiary structure of various hemoglobin subunits are similar, reflecting extensive homology in amino acid composition, the variations in amino acid compositionthat do exist impart marked differences in hemoglobin's oxygen carrying properties

In addition, the quaternary structure of hemoglobin leads to physiologically important allosteric interactions between the subunits, a property lacking in monomeric myoglobin which is otherwise very similar to the α-subunit of hemoglobin

Sequence alignment between whale myoglobinand human α, β hemoglobin – Page 163

Allosteric properties of hemoglobin

Results from its quaternary structure and differentiate hemoglobin's oxygen binding properties from that of myoglobin

The curve of oxygen binding to hemoglobin is sigmoidal typical of allosteric proteins in which the substrate, in this case oxygen, is a positive homotropic effector

When oxygen binds to the first subunit of deoxyhemoglobin it increases the affinity of the remaining subunits for oxygen. As additional oxygen is bound to the second and third subunits oxygen binding is further, incrementally, strengthened, so that at the oxygen tension in lung alveoli, hemoglobin is fully saturated with oxygen. As oxyhemoglobin circulates to deoxygenated tissue, oxygen is incrementally unloaded and the affinity ofhemoglobin for oxygen is reduced

Thus at the lowest oxygen tensions found in very active tissues the binding affinity of hemoglobin for oxygen is very low allowingmaximal delivery of oxygen to the tissue. In contrast the oxygenbinding curve for myoglobin is hyperbolic in character indicating the absence of allosteric interactions in this process

The allosteric oxygen binding properties of hemoglobin arise directly from the interaction of oxygen with the iron atom of the heme prosthetic groups and the resultant effects of these interactions on the quaternary structure of the protein

When oxygen binds to an iron atom of deoxyhemoglobin it pulls the iron atom into the plane of the heme. Since the iron is also bound to histidine F8, this residue is also pulled toward the plane of the heme ring. The conformational change at histidine F8 is transmitted throughout the peptide backbone resulting in a significant change in tertiary structure of the entire subunit

Conformational changes at the subunit surface lead to a new set of binding interactions between adjacent subunits. The latterchanges include disruption of salt bridges and formation of new hydrogen bonds and new hydrophobic interactions, all of which contribute to the new quaternary structure.

The latter changes in subunit interaction are transmitted, from the surface, to the heme binding pocket of a second deoxysubunit and result in easier access of oxygen to the iron atom of the second heme and thus a greater affinity of the hemoglobin molecule for a second oxygen molecule.

The tertiary configuration of low affinity, deoxygenated hemoglobin (Hb) is known as the taut (T) state. Conversely, the quaternary structure of the fully oxygenated high affinity form of hemoglobin (HbO2) is known as the relaxed (R) state.

Sickle-cell anemia

Sickle-cell anemia

Sickle-cell anemia

Each chain of Hb similar (same fold) to Mb

In sickle-cell hemoglobin Glu 6 in the beta chain is mutated to Val

Creating a hydrophobic patch on the surface of the molecule

This patch fits and can bind into a hydrophobic pocket in the deoxygenated form of another hemoglobin molecule

Effect of one mutationin one protein

Gene therapy