The Dissociation of the Extracellular Hemoglobin of Lumbricus Terrestris: A Model of Its Subunit Structure

Abstract

The invertebrate extracellular hemoglobins can be divided into four distinct groups based on their subunit structure (1). One of these groups is comprised of the annelid extracellular hemoglobins and chlorocruorins. These 60S molecules possess a characteristic appearance of a two-tiered hexagonal array of twelve pentagonallyshaped subunits (2) and, unlike the preponderant majority of all other hemoglobins and myoglobins, have a low iron content of 0.24 ± 0.03% (3–5). The hemoglobin of Lumbricus terrestris, the common earthworm, is the best studied of the annelid hemoglobins: it has a molecular mass of 3.9 × 10⁶, a diameter of 30nm, a height of 20nm and an iron content of 0.22% (4–6). SDS PAGE of its reduced form shows that it consists of at least six polypeptide chains: I through IV (M_r 16–19 kDa) and V (M_r 31kDa) and VI (M_r 37kDa) (7). SDS PAGE of the unreduced hemoglobin shows that it consists of four subunits: M (chain I), D1 (chain V), D2 (chain VI) and subunit T, a disulfide-bonded trimer of chains II, III and IV (4–7). Figure 1 shows the relationship between the two sets of subunits.