Structure and Function of Globular Hemeproteins: Myoglobin and Hemoglobin

Globular Hemeproteins

Overview of Globular Proteins

Globular proteins are compact, generally spherical ("globelike") proteins that are somewhat water-soluble. Their structure and solubility are determined by the distribution of hydrophilic and hydrophobic amino acids.

• Hydrophilic amino acids are typically found on the outer surface, interacting with the aqueous environment.

• Hydrophobic (nonpolar) amino acids are buried in the protein's interior, stabilized by hydrophobic interactions.

• These interactions help maintain the protein's globular structure and function.

• Clinically important examples include hemoglobin and myoglobin.

Hemeproteins

Hemeproteins are a class of globular proteins that contain a heme prosthetic group, which is essential for their biological function.

• The heme group is a planar, cyclic structure (porphyrin ring) with a central iron ion (Fe2+).

• The protein's three-dimensional structure dictates the orientation and function of the heme group.

• Heme allows for rapid and reversible electron transfer (as in cytochromes) or oxygen binding (as in myoglobin and hemoglobin).

• In enzymes like catalase, the heme group is involved in the breakdown of hydrogen peroxide.

• Structural changes in the protein can affect the alignment of the iron atom, influencing binding affinity and function.

Structure of the Heme Group

The heme group is a prosthetic group essential for oxygen binding and electron transfer in hemeproteins.

• Composed of a porphyrin ring with a central Fe2+ ion coordinated to four nitrogen atoms.

• The iron atom can form two additional bonds perpendicular to the plane of the ring:

  • One bond is typically to a histidine residue of the protein (proximal histidine).

  • The other bond is available for binding oxygen (O2).

Myoglobin

Structure and Function

Myoglobin is an oxygen-binding protein found primarily in heart and skeletal muscle. It serves as an oxygen reservoir and facilitates oxygen transport within muscle cells.

• Consists of a single polypeptide chain (~153 amino acids).

• Structurally similar to the individual subunits of hemoglobin.

• Contains one heme group and can bind one molecule of O2.

Structural Features

• Compact structure: ~80% of the polypeptide is folded into eight α-helices (labeled A–H).

• Hydrophobic core: Nonpolar amino acids are packed inside, stabilized by hydrophobic interactions.

• Hydrophilic surface: Polar amino acids are mostly on the surface, forming hydrogen bonds with water or other residues.

Heme Group in Myoglobin

• The heme group is located in a crevice lined with nonpolar amino acids.

• Two histidine residues are critical:

  • Proximal histidine (F8): Directly binds the Fe2+ of heme.

  • Distal histidine (E7): Does not bind iron directly but stabilizes the binding of O2 to Fe2+.

• This arrangement creates a microenvironment that facilitates reversible oxygen binding and prevents oxidation of Fe2+ to Fe3+.

Hemoglobin

Structure and Function

Hemoglobin is the major oxygen-transport protein in red blood cells (RBCs), delivering O2 from the lungs to tissues and facilitating CO2 and H+ transport back to the lungs.

• Adult hemoglobin (HbA) is a tetramer: two α and two β polypeptide chains (α2β2).

• Each subunit contains a heme group, allowing one hemoglobin molecule to bind up to four O2 molecules.

• Subunits are held together by noncovalent interactions (hydrophobic, ionic, and hydrogen bonds).

• Hemoglobin can also transport protons (H+) and CO2.

• Oxygen-binding properties are regulated by allosteric effectors (e.g., pH, CO2, 2,3-BPG).

Comparison of Myoglobin and Hemoglobin

Property	Myoglobin	Hemoglobin
Location	Muscle tissue	Red blood cells
Subunit structure	Monomer (1 polypeptide)	Tetramer (2 α, 2 β chains)
Heme groups per molecule	1	4
O2 binding sites per molecule	1	4
Function	O2 storage and diffusion in muscle	O2 transport in blood

Key Terms

• Globular protein: A protein with a compact, spherical shape, generally soluble in water.

• Heme: An iron-containing prosthetic group essential for oxygen binding.

• Proximal histidine (F8): The histidine residue directly bonded to the heme iron.

• Distal histidine (E7): The histidine residue that stabilizes O2 binding to the heme iron.

Example: Oxygen Binding

• Myoglobin binds O2 with high affinity, facilitating O2 storage in muscle.

• Hemoglobin's quaternary structure allows cooperative binding, enabling efficient O2 uptake in the lungs and release in tissues.

Additional info: The cooperative binding of O2 by hemoglobin is a classic example of allosteric regulation, which is not observed in myoglobin due to its monomeric structure.