The Structure of Artemia Hemoglobin and Hemoglobin Domains

Abstract

Unlike the hemoglobins of the vertebrates, which are almost invariably intracellular and tetrameric, the intra- and extracellular hemoglobins of the invertebgates show a wide variety in their molecular size (Mr 16,000 to ~ 1.7.10⁶) and architecture [1–4]. Intracellular hemoglobins usually have low Mr’s whereas extracellular hemoglobins have high Mr’s which are advantageous in minimizing excretion and avoiding excessive osmotic pressure. A high Mr can be achieved either by aggregation of many low Mr chains into a functional hemoglobin, as in annelids, or by concatenation of the low Mr chains into polymerie globins, as in molluscs and arthropods [4]. Despite this heterogeneity, Svedberg & Hedenius [5] suggested that all these pigments are built up from myoglobin-like polypeptide chains of Mr 16,000 containing one heme group and able to bind oxygen reversibly. Polypeptide chains, or fragments of much longer chains having these characteristics (Mr 16,000; one heme), were defined by Vinogradov [4] as “hemebinding domains”. Based on the number of domains and subunits in the native molecule, the invertebrate extracellular hemoglobins can be classified into four groups

• a) Single-domain, single-subunit hemoglobins, consisting of a single polypeptide chain, containing one heme group and having a Mr ~ 16,000 (Chironomus)

• b) Single-domain, multi-subunit hemoglobins consisting of aggregates of monomeric subunits, some of which are connected by disulfide bonds (Annelida)

• c) Two-domain, multi-subunit hemoglobins consisting of aggregates of dimeric Polypeptide chains (Mr 30,000–40,000), each containing two heme-binding domains (Arthropoda)

• d) Multi-domain, multi-subunit hemoglobins, consisting of two or more polypeptide chains each comprising eight to twenty heme binding domains (Arthropoda).