Research Interests

Protein Structure and Function, Enzyme Catalysis and Inhibition, and Drug Discovery

Methionine aminopeptidase: A promising target for the development of antibacterial and anticancer drugs

Our research currently focuses on studying structure and function of bacterial methionine aminopeptidases (MetAPs). By discovering novel enzyme inhibitors and elucidating their mechanisms of inhibition, we are working toward the long-term goal of developing these inhibitors as novel antibiotics with broad antibacterial spectrum and low toxicity to human. In addition to using standard approaches of biochemistry, medicinal chemistry and structural biology, We are especially interested in applying chemical biology and high throughput screening technologies to biomedical research.

Methionine aminopeptidase (MetAP) removes the N-terminal methionine residue from nascent proteins in all types of cells. It is an attractive target for development of novel antibiotics, because it is a protein coded by a single gene in all prokaryotes, and it is essential for bacterial survival. Deletion of the MetAP gene was shown to be lethal in Escherichia coli or Salmonella typhimurium. In contrast, there are two genes in eukaryotic cells, coding for type I and type II MetAPs, respectively, and deletion of both MetAP genes in Saccharomyces cerevisiae was shown to be lethal. Inhibition of human type II MetAP has been related to antiangiogenic activity of fumagillin and its analogs. Bengamides showed potent antiproliferative activity at nanomolar concentrations in cellular assays and inhibit both types of MetAP non-discriminatively. Therefore, human MetAPs may also serve as targets for development of new anticancer therapeutics.

We discovered novel small molecule inhibitors with selectivity towards different metalloforms of methionine aminopeptidases by high throughput screening

Divalent metal ions directly participate in the removal of N-terminal methionine from nascent polypeptides by MetAP. MetAP can be activated in vitro by Co(II), Mn(II), Ni(II), Zn(II) and Fe(II), but it is not clear which of the metals is the most important inside cells. Most of the current MetAP inhibitors were discovered by using the Co(II)-form of MetAP, but it has been suggested that Fe(II) is the intrinsic metal of E. coli MetAP, and Mn(II) is the metal for human type II MetAP under physiological conditions. We have shown that inhibitors have significantly different binding affinities to enzymes with different metals at the active site. Therefore, to be therapeutically useful, the inhibitors of MetAP must inhibit the physiologically relevant metalloform of MetAP.

Without lead structures for such unique inhibitors available, we discovered several classes of MetAP inhibitors by high throughput screening that show great selectivity towards the Mn(II)-form or Co(II)-form of MetAP. Selective inhibitors for other metalloforms were also discovered. We further showed their binding modes at the enzyme active site by solving crystal structures of enzyme-inhibitors complexes, which shed some lights on their potency and selectivity.

Based on our experimental evidence, methionine aminopeptidase likely functions as a monometalated enzyme in cells

Not only the identity of metal ion at the active site is in question, the exact number of metal ion also remains to be clarified. Almost all of the available X-ray structures of MetAP have at least two metal ions, either Co(II) or Mn(II), bound at the active site. However, due to the weak affinity at the second site, the dimetalated form is less likely to exist in cells. Recently, we showed that only one metal equivalent [Co(II) or Mn(II)] is required for full activation of E. coli MetAP and described the X-ray structure of E. coli MetAP with a transition-state inhibitor norleucine phosphonate bound as the first structure of a monometalated MetAP. Further, we discovered an inhibitor by high throughput screening at a low metal concentration that specifically inhibits the monometalated MetAP.