Myoglobin/Hemoglobin
Myoglobin (Mb):
single chain, high a-helical content, globular
Both Myoglobin and hemoglobin (Hb)contain the Heme Prosthetic Group
Inorganic Co-factor: iron, Fe2+ (ferrous), the binding of O2 keeps Fe2+ from oxidizing to Fe3+
Myoglobin and hemoglobin contain both organic and inorganic co-factors
The six coordination sites in the Heme Prosthetic Group:
four sites are occupied by nitrogen (relatively the same plane)
fifth site by nitrogen from the proximal histidine residue
sixth site by oxygen or carbon monoxide (nitrogen from distal histidine)
Carbon monoxide is a toxic by-product of metabolism and heme actually has a higher affinity for CO then O2.
Mb and Hb have developed a mechanism (as yet unknown) to discriminate against binding by CO, still, CO binds better than O2 and a measurable amount of Mb and Hb in the body exist bound to CO.
Myoglobin
Normal O2 Pressures:
| Location | Approximate pO2 (mmHg) | Comments |
| Air | 150 | Normal atmospheric pressure at sea level |
| Lung Capillaries | 100 | Where direct gas exchange occurs |
| Venous Blood | 40 | Where gas exchange to tissues occurs |
| Muscle | 20 | Under normal activity |
| O2-Deprived Muscle | >5 | Under conditions of strenuous exercise |
Myoglobin binds oxygen when the pO2is high and releases oxygen at very low pO2
Oxygen Affinity Curve for Myoglobin:
When all Mb molecules have O2 bound-100%
When no Mb molecules have O2 bound-0%
Looking at curve:
1. high affinity for O2, at most physiological conditions O2 remains bound to Mb.
2. only when O2 falls very low, exercise etc., Mb releases O2 to other tissues
3. "the role of Mb is to store O2 for release during crisis involving O2 deprivation"
i.e. seals and whales contain abundant Mb (deep dives)
Hemoglobin (Hb): has a structure very similar to Mb
with one very notable exception, Hb is a tetramer with two types of subunits, alpha and beta.
two a and two b (a2b2, referred to as HbA-normal hemoglobin)
This means that hemoglobin has quaternary structure and must have all four subunits, each with it's own individual primary, secondary, tertiary structure, to be a fully functioning protein.
Cooperativity/Cooperative Binding:
When an O2 molecule binds to the first site a conformational change occurs in the tetrameric structure which affects binding to the other subunits.
Oxygen binding curve is a characteristic sigmoidal ("S") shape.
Comparison of Oxygen Affinity Curve for Hemoglobin vs. Myoglobin:
Hb takes up O2 in the lungs and releases it to interior tissues
At pO2of 100 mmHg, Hb is saturated
At pO2 of 20 mmHg, Hb loses most of its O2
Even at 40 mmHg Hb is loosing some of it's O2
Note:
© Dr. Noel Sturm 2014-The amount of Hb in the blood is so high that it contributes to the buffering capacity of blood.
-However, the bicarbonate system is still the major blood buffer.
-Hb does have the ability to accept or donate protons through its His residues (36 His residues per tetramer, so this cannot be discounted....).