Convergent evolution of human and bovine haptoglobin: partial duplication of the genes.

Haptoglobin (Hp) is a hemoglobin-binding plasma protein consisting of two types of chains, called alpha and beta, which originate from a common polypeptide. In humans, but not in other mammals, Hp has been shown to occur in two allelic forms, Hp1 and Hp2, which differ in the length of the alpha-chain. The longer alpha-chain (in Hp2) seems to have arisen by an internal duplication of a gene segment coding for almost the entire alpha-chain of Hp1. In this article we show that Hp of cow (Bos taurus) contains an alpha-chain, the structure of which is similar to that of the human Hp2 alpha-chain. Furthermore, comparison of the structure of bovine Hp and human Hp2 suggests that the bovine gene arose by a duplication of the gene segment homologous to that duplicated in human Hp2. However, a phylogenetic analysis indicates that the two genes were formed independently. The evolutionary pressure that has led to the fixation of the Hps with a longer alpha-chain is not known.