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:

-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....).

© Dr. Noel Sturm 2013