Nerve globins in invertebrates

The expression of nerve hemoglobins in invertebrates is a well-established fact, but this occurrence is uncommon. In the species where nerve globins occur, they probably function as an oxygen store for sustaining activity of the nerves during anoxic conditions. Although invertebrate nerve globins are functionally similar with respect to O2 affinity, they are by no means uniform in structure and can differ in size, cellular localization and heme-coordination. The best-studied nerve globin is the mini-globin of Cerebratulus lacteus, which belongs to a class of globins containing the polar TyrB10/GlnE7 pair in the distal pocket. The amide and phenol side chains normally cause low rates of O2 dissociation and ultra-high O2 affinity by forming strong hydrogen bonds with bound ligands. Cerebratulus hemoglobin, however, has a moderate O2 affinity, due to the presence of a third polar amino-acid in its active site, ThrE11, which inhibits hydrogen bonding to bound oxygen by the B10 tyrosine side chain.