Control of blood proteins by functional disulfide bonds

Abstract

Most proteins in nature are chemically modified after they are made to control how, when, and where they function. The 3 core features of proteins are posttranslationally modified: amino acid side chains can be modified, peptide bonds can be cleaved or isomerized, and disulfide bonds can be cleaved. Cleavage of peptide bonds is a major mechanism of protein control in the circulation, as exemplified by activation of the blood coagulation and complement zymogens. Cleavage of disulfide bonds is emerging as another important mechanism of protein control in the circulation. Recent advances in our understanding of control of soluble blood proteins and blood cell receptors by functional disulfide bonds is discussed as is how these bonds are being identified and studied.

Figure 1

The 3 fundamental types of posttranslational modifications. The ribbon structure of a model protein is shown. The amino acid side chains and the peptide and disulfide bonds that bind the polypeptide backbone can be posttranslationally modified. Type 1 modifications are covalent additions of a molecule to an amino acid side chain. The side chains of 15 of the 20 common amino acids of proteins can be covalently modified in reactions that usually involve an enzyme and a cosubstrate. The lysine and tyrosine side chains are shown. The most frequent Type 1 modification in humans is phosphorylation of tyrosine. Type 2 modifications are hydrolytic cleavage or isomerization of certain peptide bonds. Hydrolytic cleavage is catalyzed by proteases, which are tightly regulated in space and time because the cleavage is irreversible. Isomerization of the peptide bond on the C-terminal side of proline residues is catalyzed by peptidyl prolyl cis-trans isomerases. Type 3 modifications are reductive cleavage of certain disulfide bonds, known as allosteric disulfides. Allosteric disulfide bonds control the function of the mature protein in which they reside by mediating a change when they are cleaved by oxidoreductases or by thiol-disulfide exchange.

Figure 2

Mechanisms of cleavage of allosteric disulfide bonds. Disulfide bond reduction occurs via a second-order nucleophilic substitution (S_N2)-type reaction mechanism in which the 3 sulfur atoms involved must form an ∼180° angle. (A) For oxidoreductase cleavage, the active site sulfur ion nucleophile of the oxidoreductase (green) attacks 1 of the sulfur atoms of the allosteric disulfide bond (gray). The mixed disulfide that forms then spontaneously decomposes, releasing oxidized oxidoreductase and the substrate protein containing a reduced allosteric disulfide. (B) For thiol-disulfide exchange, the protein contains a sulfur ion nucleophile that is unreactive until a conformational change brings the sulfur ion in line with the allosteric bond, where it attacks 1 of the sulfur atoms of the disulfide, cleaving the bond. The conformational change can be mediated by ligand binding or by mechanical shear of the protein. Intramolecular cleavage is shown, but this can also occur intermolecularly.

Figure 3

Configurations of allosteric disulfide bonds. (A) Classification of disulfide bonds based on their geometry.^, The geometry is defined by the 5 bond angles (χ angles) linking the 2 α-carbons of the cystine residue. C_α, main chain carbon atom; C_β, side chain carbon atom of each cysteine residue. The χ angles are recorded as being either positive or negative. The 3 basic types of bond configurations (spirals, hooks, and staples) are based on the signs of the central 3 angles, and they can be either RH or LH depending on whether the sign of the χ³ angle is positive or negative, respectively. These 6 bond types expand to 20 when the χ¹ and χ¹′ angles are taken into account. (B) Examples of the structures of the emerging allosteric configurations: –RHStaple, –LHHook, and −/+RHHook.