The Mini-hemoglobins in Neural and Body Wall Tissue of the Nemertean Worm, Cerebratulus lacteus *

Hemoglobin (Hb) occurs in circulating red blood cells, neural tissue, and body wall muscle tissue of the nemertean worm, Cerebratulus lacteus. The neural and body wall tissue each express single major Hb components for which the amino acid sequences have been deduced from cDNA and genomic DNA. These 109-residue globins form the smallest stable Hbs known. The globin genes have three exons and two introns with splice sites in the highly conserved positions of most globin genes. Alignment of the sequences with those of other globins indicates that the A, B, and H helices are about one-half the typical length. Phylogenetic analysis indicates that shortening results in a small tendency of globins to group together regardless of their actual relationships. The neural and body wall Hbsin situ are half-saturated with O₂ at 2.9 and 4.1 torr, respectively. The Hill coefficient for the neural Hb in situ, ∼2.9, suggests that the neural Hb self-associates in the deoxy state at least to tetramers at the 2–3 mm (heme) concentration estimated in the cells. The Hb must dissociate upon oxygenation and dilution because the weight-average molecular mass of the HbO₂ in vitro is only about 18 kDa at 2–3 μm heme concentration. Calculations suggest that the Hb can function as an O₂ store capable of extending neuronal activity in an anoxic environment for 5–30 min.