TABLE OF CONTENTS
Structure and Function of Hemoglobin in Animals
The structure and function of hemoglobin in animals are very important, as it is the main structural component that carries oxygen and transports other gases as well.
Hemoglobin is an iron-containing, oxygen-transporting metalloprotein present in the red blood cells of all vertebrates. It carries oxygen from the lungs to the tissues and transports carbon dioxide from the tissues back to the lungs.
The oxygen-carrying capacity of hemoglobin is 60% higher than that of plasma. It also functions as a buffer in the regulation of acid-base balance.
A hemoglobin molecule is a complex substance consisting of a conjugated protein, composed of the pigment heme and a protein, globin.
The protein portion of each of these chains is called globin. The α (alpha) and β (beta) globin chains are very similar in structure. In this case, α and β refer to the two types of globin.
Globin is a conjugated protein, and heme contains iron in the ferrous state.
Hemoglobin consists of four polypeptide chains, namely two alpha and two beta chains. Each of these four chains binds to a heme group, forming a complete hemoglobin molecule.
Concentration of Hemoglobin in Various Species
| Species | Hb (g/dl) |
| Dog | 12-18 (15) |
| Cat | 8-15 (12) |
| Cow | 8-15 (11) |
| Sheep | 8-16 (12) |
| Goat | 8-14 (11) |
| Horse | 11-19 (15) |
| Pig | 10-16 (13) |
Types of Hemoglobin
Based on physiological functions, hemoglobin is classified into adult hemoglobin and fetal hemoglobin. Electrophoretically, hemoglobins are categorized as HbA, HbB, HbC, and HbF.
Humans exhibit three types of hemoglobin: HbA (98%) and HbA₂ (2%) in adults, and HbF in fetuses and newborns.
- HbA consists of two α-chains and two β-chains.
- HbA₂ is composed of two α-chains and two δ-chains.
- HbF consists of two α-chains and two γ-chains.
In adult sheep, HbA (two α, two β chains) is electrophoretically fast and has a higher oxygen affinity than HbB, which contains four γ-chains. Under anemia or hypoxic conditions, sheep with HbA or HbB develop another type of hemoglobin, HbC, which partially or completely replaces HbA. A similar change is observed in goats. HbC is the naturally occurring hemoglobin in sheep during the growth period.
HbF has a higher oxygen affinity. In many animal species, fetal hemoglobin (HbF) is replaced by adult hemoglobin within 4 to 8 weeks after birth. In adult cats, HbA and HbB are found in the same erythrocyte.
Some hemoglobin variants, such as HbS, HbC, and HbE, are associated with specific hematologic disorders:
- HbS is responsible for sickle cell anemia, particularly in individuals of African descent.
- HbC and HbE cause a failure in the synthesis of α or β chains, leading to α- or β-thalassemia.