The haemoglobin of Invertebrate animals differ from that of vertebrate animals is in having :

Question

Accepted Answer

The correct option is 2: less histidine

The significant difference in the functional properties of haemoglobin (Hb) between vertebrates and invertebrates is primarily linked to the concentration of the amino acid Histidine. Vertebrate haemoglobin is a highly regulated tetrameric protein that exhibits cooperativity and the Bohr effect—meaning its affinity for oxygen changes with changes in pH and carbon dioxide concentration. Histidine is crucial for this regulation: its imidazole side chain has a pKa close to physiological pH, making it the most important buffering group in the molecule. These Histidine residues are responsible for binding H+ ions, which triggers a conformational change that promotes oxygen release in the tissues, a mechanism essential for efficient oxygen delivery. In contrast, many types of invertebrate haemoglobin (like the large, extracellular erythrocruorins or chlorocruorins) often function at a higher intrinsic oxygen affinity and may show minimal or no Bohr effect, indicating a much simpler allosteric control mechanism. Consequently, they require far fewer Histidine residues to fulfill their oxygen transport role, resulting in a protein with notably less histidine content than its vertebrate counterpart.

The incorrect option is 1: more lysine

Lysine is a basic, positively charged amino acid whose primary function in protein structure is to stabilize its overall fold and participate in electrostatic interactions. While Lysine residues are important and do interact with allosteric effectors like 2,3-bisphosphoglycerate (2,3-BPG) in mammalian red blood cells, a relative *increase* in Lysine is not the definitive molecular feature differentiating invertebrate and vertebrate haemoglobin. The most impactful functional difference relating to oxygen transport regulation is linked to the capacity for pH buffering, which is the unique role of Histidine.

The incorrect option is 3: less arginine

Arginine is another positively charged, basic amino acid, playing roles similar to Lysine in maintaining the protein's charge and structural integrity. Although any change in the content of basic amino acids will affect the protein's overall charge, the difference in the number of Arginine residues does not account for the fundamental functional divergence in allosteric regulation and pH-sensitivity observed between the two types of haemoglobin. Therefore, a difference in Arginine content is secondary to the functional consequences of reduced Histidine.

The incorrect option is 4: less cystine

Cystine is formed when two Cysteine residues form a disulfide bond, which is a covalent cross-link that dramatically stabilizes a protein's three-dimensional structure, particularly in extracellular environments. Many invertebrate haemoglobins are indeed large, complex, extracellular proteins (dissolved in the haemolymph), which would typically necessitate multiple disulfide bridges for stability. Therefore, a general reduction in Cystine is unlikely to be a universal distinguishing feature. The core functional difference lies in the mechanism of oxygen and proton binding, which directly points to the relative deficiency of Histidine.