Proteins lysine side-chains, modification

Primary amine groups on proteins consisting of N-terminal a-amines and lysine side-chain e-amines are typically present in abundant quantities for modification or conjugation reactions. Occasionally, however, a protein or peptide will not contain sufficient amounts of available amines to allow for an efficient degree of coupling to another molecule or protein. For instance, horseradish peroxidase (HRP), a popular enzyme to employ in the preparation of antibody conjugates, only possesses two free amines that... 

Both amine and lysyl oxidases possess a single Type 2 copper center and a redox-active cofactor per subunit. As shown in Figure 18, amine oxidases contain 2,4,5-trihydroxyphenylalanine quinone (TPQ) and lysyl oxidase contains lysyl tyrosine quinone (LTQ), in which a lysine side chain has been cross-linked to TPQ. These cofactors are produced from the oxidation of a specific active-site tyrosine residue via novel self-processing events that require only copper and O2 see Metal-mediated Protein Modification). 

Distinct from the lysosomal pathway are cytosolic mechanisms for degrading proteins. Chief among these mechanisms is a pathway that includes the chemical modification of a lysine side chain by the addition of ubiquitin, a 76-residue polypeptide, followed by degradation of the ubiquitin-tagged protein by a specialized proteolytic machine. Ubiquitination is a three-step process (Figure 3-13a) ... 

Histidine, glutamate, aspartate, and lysine side chains accessible on the protein surface have been used as sites for modification by redox-active metal species. The original modification procedure involved direct reaction... 

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