Structural Studies on Cerebratulus lacteus Mini-Hb K(E10)W and L(G12)A Mutants

A very short hemoglobin (CerHb; 109 amino acids) binds O2 cooperatively in the nerve tissue of the nemertean worm Cerebratulus lacteus to sustain neural activity during anoxia. The structure of oxygenated wild-type CerHb displays a substantial editing of the globin fold which makes CerHb unique among the known globin fold evolutionary variants. Here we present the crystal structures of two CerHb mutants: Lys(E10)Trp (at 2.3 Å resolution) and Leu(G12)Ala (at 1.6 Å resolution) and its complex with xenon atoms (at 2.3 Å resolution). The single mutation Lys(E10)Trp, intended to perturb the protein heme binding, has also a dramatic and unexpected effect on the Hbond network stabilizing the O2 ligand, and it makes the protein more susceptible to heme-iron oxidation. The Leu(G12)Ala mutant and its complex with xenon atoms map a wide protein matrix tunnel connecting the distal site to a surface cleft between the E and H helices, thus suggesting a novel ligand access to heme.