Ph.D. Molecular Biology
1993, University of Pennsylvania
School of Medicine
Research Focus Areas
Eukaryotic Microbial Pathogenesis
In prokaryotic and eukaryotic microbes, including a number of significant human pathogens, three enzymes for the activation and/or production of acetate intersect in interesting and sometimes unusual ways. These three enzymes are ADP-forming acetyl-CoA synthetase (ADS; Eq. 1), acetate kinase (ACK; Eq. 2), and AMP-forming acetyl-CoA synthetase (ACS; Eq. 3):
ADS: acetate + ATP + CoASH ⇄ acetyl-CoA + ADP + Pi [Eq. 1]
ACK: acetate + ATP ⇄ acetyl phosphate + ADP [Eq. 2]
ACS: acetate + ATP + CoASH ⇄ acetyl-CoA + AMP + PPi [Eq. 3]
My current research focus is on the biochemistry and physiological roles of ADS and ACK in eukaryotic microbial pathogens.
Enzymology and physiological role of ADP-forming acetyl-CoA synthetase in parasites.
ADP-forming acetyl-CoA synthetase (ADS) is present in the significant human pathogens Plasmodium falciparum and Entamoeba histolytica, the causative agents of malaria and amoebic dysentery and the two leading causes of morbidity and mortality due to parasitic disease in humans. ADS, which catalyzes the conversion of acetyl-CoA to acetate in a three step reaction that results in ATP formation, represents a key pathway for ATP generation in E. histolytica and is thus expected to be essential in this protist. RNA interference studies are underway to explore the role of ADS in E. histolytica physiology and pathogenesis. Studies on the enzymology of ADS are limited and have primarily focused on the archaeal enzymes which are encoded as two separate subunits. The eukaryotic enzymes, which in contrast are encoded as a single subunit, have received little attention. My lab is thus taking a structure/function approach to define active site determinants of substrate binding and catalysis in the eukaryotic ACDs using recombinant enzymes from E. histolytica and P. falciparum.
Acetate kinase in the pathogenic protist Entamoeba histolytica
In bacteria, acetate kinase (ACK) typically partners with phosphotransacetylase for the interconversion of acetate and acetyl-CoA. However, in E. histolytica a partner enzyme for ACK has not been identified. Characterization of the recombinant E. histolytica enzyme has shown that unlike all other known ACKs, the E. histolytica ACK (EhACK) utilizes PPi as the phosphoryl donor instead of ATP or other NTPs. Our structure/function approach to understanding substrate binding and catalysis by EhACK is based on the structure determined by our collaborator Dr. Tina Iverson at Vanderbilt University. This enzyme operates strongly in the direction of acetate/PPi-formation rather than in the acetyl phosphate/Pi-forming direction of the reaction, and thus its role may be to generate PPi, an integral high energy compound in E. histolytica metabolism. We are using RNA interference of ACK gene expression in E. histolytica to determine whether ACK knockdown is detrimental to growth or lethal. Investigation of possible interacting partners and metabolic labeling studies will be used to attempt to determine the origin of the acetyl phosphate substrate for EhACK.
ADS and ACK as a target for antimicrobial agents
ACK is widespread in bacteria including a number of significant human pathogens listed under NIAID categories A, B, and C of priority pathogens for biodefense such as Yersina pestis (plague), Bacillus anthracis (anthrax), and Clostridium botulinum (botulism), among others, and eukaryotic microbial pathogens such as E. histolytica, Cryptococcus neoformans, Aspergillus fumigatus, Histoplasma capsulatum, and Coccidiodes species. The absence of ACK in humans, plants, and animals suggests this enzyme may provide a novel target for chemotherapeutic agents. Although ADS is not widespread, its presence in Plasmodium, Entamoeba, and Naegleria species but its absence in the human host makes this enzyme a potential target as well. We are developing HTS assays for screening of small molecule libraries for inhibitors ACKs from E. histolytica, C. neoformans, the causative agent of fungal meningitis, and Salmonella enterica, an important food-borne pathogen, and the E. histolytica and P. falciparum ADSs.
National Institutes of Health
The Presence of a Bacterial Metabolic Pathway in Eukaryotic Fungi
National Science Foundation
Biochemistry and Physiology of Novel Acetate Kinases from Eukaryotic Microbes
Molecular Biology - Genes to Proteins
Special Topics in Biochemistry
M Fowler, C Ingram-Smith, and KS Smith. 2011. Characterization of a novel pyrophosphate-dependent acetate kinase from Entamoeba histolytica. Journal of Visualized Experiments. In press.
RD Barber, L Zhang, M Harnack, MV Olson, R Kaul, C Ingram-Smith, and KS Smith. 2011. Complete genome sequence of Methanosaeta concilii, a specialist in aceticlastic methanogenesis. Journal of Bacteriology 193: 3668-3669.
Y Meng, C Ingram-Smith, LL Cooper, and KS Smith. 2010. Biochemical characterization of an archaeal medium-chain acyl-CoA synthetase. Journal of Bacteriology 192:5982-5990.
MB Shah, C Ingram-Smith, LL Cooper, J Qu, Y Meng, KS Smith, and AM Gulick. 2009. The 2.1 Å crystal structure of an acyl-CoA synthetase from Methanosarcina acetivorans reveals an alternate acyl-binding pocket for small branched acyl substrates. Proteins 77:685-698.
KS Smith and C Ingram-Smith. 2007. Methanosaeta: the forgotten methanogen? Trends in Microbiology. 15:150-155.
C Ingram-Smith and KS Smith. 2007. AMP-forming acetyl-CoA synthetases in Archaea show unexpected diversity in substrate utilization. Archaea 2: 95-107.
C Ingram-Smith, BI Woods, and KS Smith. 2006. Characterization of the acyl substrate binding pocket of acetyl-CoA synthetase. Biochemistry 45:11482-11490.
C Ingram-Smith, SR Martin, and KS Smith. 2006. Acetate kinase: not just a bacterial enzyme. Trends in Microbiology 14:249-253.
C Ingram-Smith, A Gorrell, P Iyer, KS Smith, and JG Ferry. 2005. The acetate-binding pocket of the Methanosarcina thermophila acetate kinase. Journal of Bacteriology 187:2386-2394.