Proteins Using the Same Residues for Different Functions
These are multidomain proteins (listed in Table 2.3) which utilize the same functional site for carrying out their primary and moonlighting function(s).
GAPDH E. coli
The E. coli glyceraldehyde-3-phosphate dehydrogenase (GAPDH; EC 18.104.22.168) (Function 1) is a multifunctional housekeeping protein. It also catalyzes its own NAD+ -dependent ADP-ribosylation which has been implicated in host-pathogen interactions (Function 2) . GAPDH consists of two chains, each comprising a Rossmann fold and a a3p5 sandwich domain . The three catalytic residues of GAPDH are Cys149, His179, and Ser238, which are located in the sandwich domain (Fig. 2.9). However, mutational analyses have shown that the catalytic Cys149 (shown in red) is also the target residue of the ADP-ribosylation.
Leukotriene A4 hydrolase, Homo sapiens
Leukotriene A4 hydrolase (EC 22.214.171.124) is a bifunctional zinc metalloenzyme that converts the fatty acid epoxide leukotriene A4 (LTA4) into a potent chemoattractant, leukotriene B4 (LTB4) (Function 1) and also exhibits an anion-dependent aminopeptidase activity (EC 126.96.36.199) (Function 2) . Both the enzymatic activities require the presence of the catalytic zinc which is coordinated by the
Figure 2.9 GAPDH. The catalytic site residue Cys149 (shown in red) is the residue known to be involved for both the canonical and moonlighting functions of E. coli GAPDH. The other catalytic residue His179 is shown in blue (PDB:1DC5). (See color plate section for the color representation of this figure.)
three zinc-binding residues His295, His299, and Glu318. The crystal structure of the LTA4 hydrolase consists of three domains: a p-sandwich, an a-orthogonal bundle, and a a-a superhelix (Fig. 2.10) . It also contains the 269 GX M EN272 motif in the Ml family of zinc peptidases. Mutation of the catalytic residues Glu296 or Tyr383 resulted in loss of the aminopeptidase activity, and mutation of the catalytic residue Glu271 abolished both the epoxide hydrolase activity and the aminopeptidase activity. Glu271 is a unique example of a catalytic residue that has distinct roles in two separate catalytic reactions for two chemically different substrates. Based on the LTA4 hydrolase structure and structure activity studies, two mechanistic models for the role of Glu271 in the epoxide hydrolase activity and in the aminopeptidase reaction were proposed .