Thioester protein crosslinks

Thioester bonds between cysteine and glutamine sidechains can form covalent cross-links between polypeptide chains. An accurate estimate of the number of thioester bonds in the PDB may not be available (see Protein crosslinks#Thioester Crosslinks). A search for "thioester" in Proteopedia yields >500 hits. Thioester bonds are one way that ubiquitin may be attached to proteins (ubiquitinylation; see also Ubiquitin and Ubiquitination). Thioester bonds in complement proteins C3 and C4 are involved in tagging pathogens for destruction by the immune system, via the alternate complement activation pathway. Example: 2b39. Thioester bonds occur in thioester domains (TED) of Gram-positive bacteria, where they are thought to mediate covalent adhesion of bacteria to host cells, or during bacterial conjugation involved in antibiotic resistance. Many surface proteins of Gram-positive bacteria are predicted to contain TED as well as isopeptide and ester cross-links. This family of proteins is termed TIE proteins for thioester, isopeptide, ester proteins. Examples: 2xi9, 6fwy. In a dramatic example, the pilus tip adhesin of Streptococcus pyogenes forms dimers in the presence of spermidine, a di-amine that forms thiopeptide bonds with the Gln211 residues in two adhesin protein chains, illustrated in 4c0z.

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Other Protein CrosslinksOther Protein Crosslinks

In addition to the thioester bonds discussed above, other covalent cross-links between polypeptide chains include:

• Disulfide bonds
• Isopeptide bonds
• Thioether protein crosslinks
• Ester protein crosslinks
• Histidine-tyrosine protein crosslinks
• Lysine-cysteine NOS bonds