Hemoglobin wonders: a fascinating gas transporter dive into molluscs

Abstract

Hemoglobin (Hb) has been identified in at least 14 molluscan taxa so far. Research spanning over 130 years on molluscan Hbs focuses on their genes, protein structures, functions, and evolution. Molluscan Hbs are categorized into single-, two-, and multiple-domain chains, including red blood cell, gill, and extracellular Hbs, based on the number of globin domains and their respective locations. These Hbs exhibit variation in assembly, ranging from monomeric and dimeric to higher-order multimeric forms. Typically, molluscan Hbs display moderately high oxygen affinity, weak cooperativity, and varying pH sensitivity. Hb’s potential role in antimicrobial pathways could augment the immune defense of bivalves, which may be a complement to their lack of adaptive immunity. The role of Hb as a respiratory protein in bivalves likely originated from the substitution of hemocyanin. Molluscan Hbs demonstrate adaptive evolution in response to environmental changes via various strategies (e.g. increasing Hb types, multimerization, and amino acid residue substitutions at key sites), enhancing or altering functional properties for habitat adaptation. Concurrently, an increase in Hb assembly diversity, coupled with a downward trend in oxygen affinity, is observed during molluscan differentiation and evolution. Hb in Protobranchia, Heteroconchia, and Pteriomorphia bivalves originated from separate ancestors, with Protobranchia inheriting a relative ancient molluscan Hb gene. In bivalves, extracellular Hbs share a common origin, while gill Hbs likely emerged from convergent evolution. In summary, research on molluscan Hbs offers valuable insights into the origins, biological variations, and adaptive evolution of animal Hbs.

Keywords:

• mollusc
• hemoglobin
• structure
• function
• cooperative mechanism
• sulfide binding
• antibacterial and immunization
• adaptive evolution

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This work was supported by National Natural Science Foundation of China under Grant 32273123 and 32130107; the Department of Science and Technology of Zhejiang Province under Grant LZ20C190001; South China Sea Institute of Oceanology of Chinese Academy of Sciences under Grant 2021011001; Ningbo Science and Technology Bureau under Grant 2023J042 and 2021S014; and Taishan Scholar Project Fund of Shandong Province of China.