Structure and mechanism of bovine lens leucine aminopeptidase

Enzymatic and molecular characterisation of leucine aminopeptidase of Burkholderia pseudomallei

Background

Leucine aminopeptidase (LAP) has been known to be a housekeeping protease, DNA-binding protein and repressor or activator in the operon regulation of virulence-associated genes in several bacterial species. LAP activity was consistently detected in overnight cultures of Burkholderia pseudomallei, the causative agent of melioidosis and this enzyme was partially purified and characterised in this study. The intra- and inter-species nucleotide and deduced amino acid sequence variation of LAP encoding gene (pepA) was determined. A pepA/PCR-RFLP assay was designed to facilitate the identification of major LAP sequence types amongst clinical and environmental isolates of B. pseudomallei.

Results

LAP activity was detected in B. pseudomallei culture supernantants by zymographic analysis. Optimum activity was at pH 9 and stable at 50°C. Enhanced enzymatic activity was observed in the presence of metallic ions Mg²⁺, Ca²⁺, Na⁺ and K⁺. LAP activity was inhibited by EDTA, 1,10-phenanthroline, amastatin, Mn²⁺ and Zn²⁺. Sequence analysis of the complete nucleotide and deduced amino acid sequences of LAP-encoding (pepA) gene showed close genetic relatedness to B. mallei (similarity 99.7%/99.6%), but not with B. thailandensis (96.4%/96.4%). Eight pepA sequence types were identified by comparison with a 596 bp DNA fragment encompassing central regions of the pepA gene. A pepA/PCR-RFLP was designed to differentiate pepA sequence types. Based on restriction analysis with StuI and HincII enzymes of the amplified pepA gene, clinical and environmental isolates showed different predominant RFLP types. Type I was the most predominant type amongst 73.6% (67/91) of the clinical isolates, while Type II was predominant in 55.6% (5/9) of the environmental isolates.

Conclusions

This study showed that LAP is a secretory product of B. pseudomallei with features similar to LAP of other organisms. Identification of major LAP sequence types of B. pseudomallei was made possible based on RFLP analysis of the pepA gene. The high LAP activity detected in both B. pseudomallei and B. thailandensis, suggests that LAP is probably a housekeeping enzyme rather than a virulence determinant.

Profiling of lens protease involved in generation of αA-66-80 crystallin peptide using an internally quenched protease substrate

Proteins of lens fiber cells are prone to accumulate extensive post-translational modifications because of very little protein turnover. Lens proteins are degraded via the lens proteolytic systems into peptides, which are subsequently hydrolyzed by downstream aminopeptidases. Inefficient degradation can lead to accumulation of protein fragments and subsequent aggregation. Previously we showed that αA-66-80 peptide and its truncated products accumulate in aging and cataract human lenses. These peptides interact with crystallins, causing crystallin aggregation and precipitation. N- and C-terminal-blocked peptides that have the cleavage sites to generate the αA-66-80 fragment were used to test lens extracts for sequence-specific proteases in lens extracts. An internally quenched fluorogenic peptide substrate containing the sequence-specific site for a lens protease to generate αA-66-80 peptide was designed, synthesized and used to characterize protease(s) that are capable of generating this peptide in bovine and human lenses. We show that proteases with the potential to generate αA-66-80 peptide are present in bovine and human lenses. We also show that the αA-66-80 peptides are resistant to hydrolysis by aminopeptidases present in the lenses and they can suppress the degradation of other peptides. Failure of complete hydrolysis of these peptides in vivo can lead to their accumulation in the lens and subsequent lens protein aggregation, which may ultimately lead to the formation of cataract.

Biochemical properties and potential applications of recombinant leucine aminopeptidase from Bacillus kaustophilus CCRC 11223

Experiments were carried out to investigate the effects of various factors on the activity and conformation of recombinant leucine aminopeptidase of Bacillus kaustophilus CCRC 11223 (BkLAP) and potential utilization of BkLAP in the hydrolysis of anchovy protein. Optimal temperature and pH of BkLAP were 70 °C and 8.0 in potassium-phosphate buffer, respectively, and the activity was strongly stimulated by Ni²⁺, followed by Mn²⁺ and Co²⁺. Conformational studies via circular dichroism spectroscopy indicated that various factors could influence the secondary structure of BkLAP to different extents and further induce the changes in enzymatic activity. The secondary structure of BkLAP was slightly modified by Ni²⁺ at the concentration of 1×10⁻⁴ M, however, significant changes on the secondary structures of the enzyme were observed when Hg²⁺ was added to the concentration of 1×10⁻⁴ M. The potential application of BkLAP was evaluated through combination with the commercial or endogenous enzyme to hydrolysis the anchovy protein. Results showed that combining the BkLAP with other enzymes could significantly increase the degree of hydrolysis and amino acid component of hydrolysate. In this regard, BkLAP is a potential enzyme that can be used in the protein hydrolysate industry.

The induction of tomato leucine aminopeptidase genes (LapA) after Pseudomonas syringae pv. tomato infection is primarily a wound response triggered by coronatine

Tomato plants constitutively express a neutral leucine aminopeptidase (LAP-N) and an acidic LAP (LAP-A) during floral development and in leaves in response to insect infestation, wounding, and Pseudomonas syringae pv. tomato infection. To assess the physiological roles of LAP-A, a LapA-antisense construct (35S:asLapA1) was introduced into tomato. The 35S:asLapA1 plants had greatly reduced or showed undetectable levels of LAP-A and LAP-N proteins in healthy and wounded leaves and during floral development. Despite the loss of these aminopeptidases, no global changes in protein profiles were noted. The 35S:asLapA1 plants also exhibited no significant alteration in floral development and did not impact the growth and development of Manduca sexta and P. syringae pv. tomato growth rates during compatible or incompatible infections. To investigate the mechanism underlying the strong induction of LapA upon P. syringae pv. tomato infection, LapA expression was monitored after infection with coronatine-producing and -deficient P. syringae pv. tomato strains. LapA RNA and activity were detected only with the coronatine-producing P. syringae pv. tomato strain. Coronatine treatment of excised shoots caused increases in RNAs for jasmonic acid (JA)-regulated wound-response genes (LapA and pin2) but did not influence expression of a JA-regulated pathogenesis-related protein gene (PR-1). These results indicated that coronatine mimicked the wound response but was insufficient to activate JA-regulated PR genes.

A bicarbonate ion as a general base in the mechanism of peptide hydrolysis by dizinc leucine aminopeptidase

The active sites of aminopeptidase A (PepA) from Escherichia coli and leucine aminopeptidase from bovine lens are isostructural, as shown by x-ray structures at 2.5 A and 1.6 A resolution, respectively. In both structures, a bicarbonate anion is bound to an arginine side chain (Arg-356 in PepA and Arg-336 in leucine aminopeptidase) very near two catalytic zinc ions. It is shown that PepA is activated about 10-fold by bicarbonate when L-leucine p-nitroanilide is used as a substrate. No activation by bicarbonate ions is found for mutants R356A, R356K, R356M, and R356E of PepA. In the suggested mechanism, the bicarbonate anion is proposed to facilitate proton transfer from a zinc-bridging water nucleophile to the peptide leaving group. Thus, the function of the bicarbonate ion as a general base is similar to the catalytic role of carboxylate side chains in the presumed mechanisms of other dizinc or monozinc peptidases. A mutational analysis shows that Arg-356 influences activity by binding the bicarbonate ion but is not essential for activity. Mutation of the catalytic Lys-282 reduces k(cat)/K(m) about 10,000-fold.