3.5 Role of Respiratory Pigments | Respiratory System Of Man | Biology 12

Respiratory pigments are coloured, protein-based molecules that act as oxygen carriers by binding reversibly to oxygen. They all contain a coloured, non-protein component, such as the haem group in haemoglobin and myoglobin. These pigments are essential for meeting the high oxygen demands of aerobic respiration in large, active multicellular organisms.

Haemoglobin (Hb)

Structure: Composed of four globin protein chains. Each chain is associated with a haem group, which is an iron-containing ring structure.

Function: The iron atom in each haem group combines loosely and reversibly with one oxygen molecule. Therefore, one haemoglobin molecule can carry up to four oxygen molecules.

Oxygen Binding: It binds to oxygen in environments with high oxygen concentration (like the lungs) and releases oxygen in environments with low oxygen concentration (like respiring tissues). This process is influenced by cooperativity, where the binding of one oxygen molecule makes it easier for subsequent molecules to bind.

Location: Found in Red Blood Cells (RBCs). At normal arterial $P O_{2}$ (100 mmHg), 100ml of blood carries approximately 20ml of oxygen.

Myoglobin (Mb)

Structure: Consists of a single polypeptide chain associated with one iron-containing haem group. It can bind to only one molecule of oxygen.

Function: Serves as an oxygen storage molecule and an intermediate for transferring oxygen from haemoglobin to the aerobic metabolic processes within muscle cells.

Oxygen Binding: It has a higher affinity for oxygen than haemoglobin. It only releases its stored oxygen when the partial pressure of oxygen ( $P O_{2}$ ) drops below 20 mmHg, such as during strenuous exercise when oxyhaemoglobin supplies are depleted.

Location: Found in skeletal muscles, giving meat its red colour.

Comparison: Haemoglobin vs. Myoglobin

Feature	Haemoglobin	Myoglobin
Structure	Four polypeptide chains	One polypeptide chain
Haem Groups	Four per molecule	One per molecule
$O_{2}$ Capacity	Binds up to four $O_{2}$ molecules	Binds one $O_{2}$ molecule
Location	Red Blood Cells (RBCs)	Muscle tissue
Primary Function	Oxygen transport	Oxygen storage
Oxygen Affinity	Lower affinity	Higher affinity
$O_{2}$ Release	Loses $O_{2}$ at $P O_{2}$ ≈ 60 mmHg	Loses $O_{2}$ at $P O_{2}$ < 20 mmHg

Factors Affecting Oxygen Binding (Bohr Effect)

The affinity of haemoglobin for oxygen is affected by environmental factors. The Bohr Effect describes how an increase in $C O_{2}$ concentration or a decrease in pH (acidic conditions) decreases haemoglobin's affinity for oxygen, causing the dissociation curve to shift to the right. This facilitates the release of oxygen to active tissues.

Carbon Monoxide (CO) Poisoning

Cause: Inhaling carbon monoxide, often produced by incomplete combustion from sources like faulty gas heaters in enclosed spaces.

Mechanism:

CO binds to the iron atom in haemoglobin with an affinity ~250 times greater than that of oxygen.
This binding forms a stable compound called carboxyhaemoglobin, which prevents haemoglobin from picking up oxygen in the lungs.
CO also increases haemoglobin's affinity for any oxygen it is still carrying, preventing it from releasing that oxygen to the tissues.

Symptoms: Nausea, vomiting, headache, changes in mental status, and a characteristic cherry-red lip colour.

Feature

Haemoglobin

Myoglobin

Structure

Four polypeptide chains

One polypeptide chain

Haem Groups

Four per molecule

One per molecule

O_{2}

Capacity

Binds up to four

O_{2}

molecules

Binds one

O_{2}

molecule

Location

Red Blood Cells (RBCs)

Muscle tissue

Primary Function

Oxygen transport

Oxygen storage

Oxygen Affinity

Lower affinity

Higher affinity

O_{2}

Release

Loses

O_{2}

P O_{2}

≈ 60 mmHg

Loses

O_{2}

P O_{2}

< 20 mmHg