Leucyl aminopeptidases (, leucine aminopeptidase, LAPs, leucyl peptidase, peptidase S, cytosol aminopeptidase, cathepsin III, L-leucine aminopeptidase, leucinaminopeptidase, leucinamide aminopeptidase, FTBL proteins, proteinates FTBL, aminopeptidase II, aminopeptidase III, aminopeptidase I) are enzymes that preferentially catalyze the hydrolysis of leucine residues at the N-terminus of peptides and proteins. Other N-terminal residues can also be cleaved, however. LAPs have been found across superkingdoms. Identified LAPs include human LAP, bovine lens LAP, porcine LAP, Escherichia coli (E. coli) LAP (also known as PepA or XerB), and the solanaceous-specific acidic LAP (LAP-A) in tomato (Solanum lycopersicum). Enzyme description, structure, and active site The active sites in PepA and in bovine lens LAP have been found to be similar. Shown in the picture below is the proposed model for the active site of LAP-A in tomato based on the work of Strater et al. It is also known that the biochemistry of the LAPs from these three kingdoms is very similar. PepA, bovine lens LAP, and LAP-A preferentially cleave N-terminal leucine, arginine, and methionine residues. These enzymes are all metallopeptidases requiring divalent metal cations for their enzymatic activity Enzymes are active in the presence of Mn+2, Mg+2 and Zn+2. These enzymes are also known to have high pH (pH 8) and temperature optima. At pH 8, the highest enzymatic activity is seen at 60 °C. PepA, bovine lens LAP and LAP-A are also known to form hexamers in vivo. The Gu et al. from 1999 demonstrated that six 55kDA enzymatically inactive LAP-A protomers come together to form the 353kDa bioactive LAP-A hexamer. Structures of the bovine lens LAP protomer and the biologically active hexamer have been constructed can be found through Protein Data Bank (2J9A). Mechanism(s) Historically, the mechanisms of carboxypeptidases and endoprotease have been much more well-studied and understood by researchers (Ref #6 Lipscomb 1990). Work within the past two decades has provided vital knowledge regarding the mechanisms of aminopeptidases. The mechanism of bovine lens LAP and PepA have been elucidated (Ref 1 and 2), however, the exact mechanism of tomato LAP-A is unknown at this time. A search of current literature does not indicate that new research is underway to determine the exact mechanism of LAP-A. Based on the biochemical similarities of the LAPs between kingdoms, the mechanism of LAP-A may be similar to bovine lens LAP and PepA. Biological function Once thought of as a housekeeping gene necessary only for protein turnover, studies