On the molecular conformation of human haemopexin. I. Reactivity of the tyrosine and tryptophan side chains.

The reactivity of the aromatic side chains of Tyr and Trp in human haemopexin were studied by chemical modifications and analysis of spectrophotometric titration curves. It has turned out that: 1. Under non-denaturing conditions the aromatic rings of Tyr resisted both acetylation and nitration. 2. Three indole groups of Trp reacted with the Koshland agent, without the native conformation of the protein being markedly affected (CD spectra). 3. Oxidation by N-bromosuccinimide split the peptide chain and the molecular conformation collapsed. 4. The Tyr residues could be placed into three classes, according to their pK values: 2 (or 1 in the haem-haemopexin complex) were normally accessible to titration, 5 were masked and the remaining 7 (or 8) were buried. 5. The spectrophotometric titration curve could not be analysed in terms of the Linderstrøm-Lang equation. The findings 1 to 3 refer to both haemopexin and its complex with haem; the spectrophotometric titration curves of the two molecules are very similar too. Consequently, the binding of haem is not associated with a profound alteration of the molecular architecture. The generally low reactivity of the side chains studied indicates that the hydrophobic peptide core of this glycoprotein is a compact one, very restricted in its contacts with the environment.