Some effects of post-translational N-terminal acetylation of the human embryonic zeta globin protein.

Using site-directed mutagenesis we have produced the first mutant form of a human embryonic haemoglobin. We have mutated the N-terminal Ser residue of the zeta-chain of haemoglobin Portland, zeta 2 gamma 2, (which is normally acetylated) to a Val (which possesses a free amine terminus). The protein spontaneously assembles into a fully functional tetramer which shows cooperative oxygen binding. Determination of the reactivity of the mutant protein with 2,3-diphosphoglycerate indicates that the mutation process does not lead to any major disruption of the protein structure. A comparison of the properties of the mutant and wild-type proteins identifies a significant role for the normal N-terminal acetylation of the zeta-chain with regard to the alkaline Bohr effect and the sensitivity of the oxygen affinity of the protein towards chloride ions. The possible physiological significance of this modification is discussed.