Ligand-binding: Myoglobin and Hemoglobin

Globular Proteins

• Proteins which are typically spherical and mainly consist of secondary structures
• Many globular proteins are enzymes. Meaning they bind to a molecule and facilitate a reaction

Myoglobin

• Found in the muscles of aquatic vertebrates
• It's structural arrangement is seen in globin proteins and contains a binding pocket for a heme group
• The heme group binds oxygen and is stored by the protein until used
• Myoglobin is 153 amino acids long with 121 of those making up 8 α-helix units which are folded via β-turns

Hemoglobin

• Is a tetramer consisting of 2 α subunits and 2 β subunits (globins) found in red blood cells
• Each subunit is bound to a heme molecule (iron center), which binds oxygen

• Heme shows cooperative binding, binding of oxygen depends on the partial pressure of oxygen in its immediate environment
• In lungs the heme are ~98% saturated with oxygen, but that drops to ~32% saturation in tissues
• A much greater potential for O2 release compared to myoglobin alone

• The binding of oxygen to the first subunit results in a conformational change due to the His residue bound to the Iron center
• This leads to a change in the α/β interface converting the neighbouring subunit to change from the T-state (deoxyhemoglobin) into the R-state (oxyhemoglobin).

Allosteric Regulation

• 2,3-Bisphosphoglycerate (2,3-BPG) is an allosteric regulator of hemoglobin.
• Binds to the T-state and stabilizes it
• In order to bind O2 (R-state) the 2,3-BPG must be removed from binding site

• Lower pH values will induce the release of O2 from heme
• Lower pH results in protonation of His195, allowing it to form a salt bridge that stabilizes the T-state

• Muscles release CO2 and H+ (products of aerobic metabolism)
• The binding of CO2 to terminal amino groups forms carbamate, which forms salt bridges that stabilize the T-state
• Lowering O2 affinity and releasing oxygen
• Hemoglobin provides a way for CO2 to be taken up in tissues and released in the lungs and then exhaled

Disorders in Hemoglobin

• Mutation in both hemoglobin genes (α and β)
• Creates long fibers that deform red blood cells and leads to easy lysis

• The loss of either the α hemoglobin chain (α-thalassemia) or the β hemoglobin chain (β-thalassemia)
• This prevents cooperative O2 binding

Ligand Binding

• Specificity= how specific is the ligand-protein interaction. Can the ligand bind to more than one protein or vice versa?
• Affinity= how strong is the ligand-binding interaction. Is it easy for the ligand to dissociate from the protein?
• Kd is a measure of ligand-protein affinity.
• For a particular ligand binding interaction: PL <----> P + L

• When Kd is SMALL, the protein-ligand interaction is STRONG (high affinity) and vice versa

• Allostery= when a ligand (allosteric effector) binds to a protein at one ligand binding site and effects the binding affinity at ANOTHER ligand-binding site on the same protein by inducing a conformational change in the protein's tertiary or quaternary structure.
• Hemoglobin contains FOUR subunits which each contain one heme molecule. When the heme molecule in one subunit binds to O2, it increases the affinity for the other three heme molecules (i.e. they bind O2 more easily)