The histidine-rich glycoprotein of serum has a domain rich in histidine, proline, and glycine that binds heme and metals

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Abstract
Histidine-rich glycoprotein (HRG) from rabbit serum was digested with plasmin, reduced and carboxymethylated, and the fragments produced were resolved by reverse-phase high-performance liquid chromatography. Several peptide fractions were obtained that contain unusually high contents of histidine, proline and glycine. One His-Pro-Gly-rich peptide (apparent MW 30,000) was obtained in sufficient yield and purity for further study. This peptide is 29 mol % histidine, 37% proline and 16% glycine, indicating that most of these 3 amino acids are located in 1 region of HRG. The peptide contains 9% by weight carbohydrate and is devoid of tyrosine or tryptophan. The far-ultraviolet circular dichroism spectrum of the peptide has an minimum at 203 nm, indicating that the peptide contains polyproline II helical sections. The peptide represents a binding domain of HRG since it retains much of the ability of intact HRG to bind heme and metals including Zn2+, Ni2+ and Cu2+. As with the parent HRG molecule, interaction of the peptide with heme and metals is dependent on pH and intact histidine residues.