Swinging arms in multifunctional enzymes and the specificity of post-translational modification

Covalently attached prosthetic groups serve as swinging arms in several different and wellcharacterized multifunctional enzymes. Notable among these are the lipoyl-lysine residues in 2-0x0 acid dehydrogenase multienzyme complexes [l-31 and the glycine cleavage system [4-61, the biotinyl-lysine residues in various ATP-dependent carboxylases [7,8] and the phosphopantetheinyl-serine residues in fatty acid [9] and polyketide [lo] synthases. In all these instances, the swinging arm is used to ferry substrate between the active sites that function successively in a multistep catalytic reaction. Several explanations have been proposed for the advantages supposedly conferred on a multifunctional protein by virtue of these swinging arms [ 11,121: an enhancement of catalytic efficiency, the potential for substrate channelling, and an opportunity to protect an otherwise unstable catalytic intermediate (the 'hot potato hypothesis'). Another important feature is the mechanism by which the target side chain in the parent protein is selected for post-translational modification. In each case it has become apparent that there is an enzyme, a ligase or transferase, that catalyses the formation of an amide bond with the amino group of a lysine residue (lipoylation or biotinylation) or phosphodiester bond with the hydroxyl group of a serine residue (pantetheinylation) as a means of attaching the prosthetic group ([ 13-16] and references therein). However, only recently has it become clear, at least in the case of lipoylation [17] and biotinylation [18], how these ligases recognize their target protein and identify the particular side chain for modification. We review here our current knowledge of the lipoyl-lysine and biotinyl-lysine swinging arms in multienzyme complexes. Parallels between lipoic acid and biotin have long been drawn and it has now become apparent that there is an underlying structural similarity in the proteins that become lipoylated or biotinylated.