K-065. β-Ketoacyl-Acyl Carrier Protein Synthase III (FabH) of Listeria monocytogenes Has Increased Preference for the Precursor of Anteiso Fatty Acids over Iso Branched-Chain Fatty Acid Precursors at Low Temperature

A. K. Singh1, Y. M. Zhang2, K. Zhu2, C. Gatto1, R. K. Jayaswal1, C. O. Rock2, B. J. Wilkinson1;
1Illinois State Univ., Normal, IL, 2St Jude Children's Research Hosp., Memphis, TN.

L. monocytogenes is the causative agent of listeriosis, a severe and life-threatening infection encompassing meningoencephalitis, meningitis, focal infections in the immunocompromised, stillbirths and neonatal sepsis from infection of pregnant women. The ability of L. monocytogenes to grow at refrigeration temperature is a critical aspect of its role as a foodborne pathogen. Modulation of fatty acid composition to ensure sufficient membrane fluidity is an important strategy for growth at low temperature. Anteiso C15:0 represent upto 65% of the total fatty acids at low temperature and plays a major role in ensuring membrane fluidity. The amino acids isoleucine, valine and leucine, are converted to 2-methylbutyryl-CoA, isobutyryl-CoA and isovaleryl-CoA, precursors of anteiso-fatty acids, even-numbered and odd-numbered iso-fatty acids respectively. These acyl-CoA precursors are then utilized by FabH as starting units to initiate branched chain fatty acid (BCFA) synthesis. We are investigating the hypothesis that FabH displays a greater preference for the precursor of anteiso fatty acids at low temperature. We have determined the kinetic constants of L. monocytogenes FabH at 300C and 100C. At 300C, the catalytic efficiency (Kcat/Km) for FabH utilization of 2-metylbutyryl-CoA was 1.8-times and 3.2-times higher than conversion of isovaleryl-CoA and isobutyryl-CoA, respectively. At 100C, the efficiency of FabH conversion of 2-methylbutyryl-CoA exceeded that of isovaleryl-CoA and isobutyryl-CoA by seven and two-fold, respectively. Thus, the kinetic properties of the FabH are consistent with the switching of fatty acid branching from iso to anteiso that occurs as part of the adaptation of L. monocytogenes to low temperature. Measurements of L. monocytogenes amino acid pools at 370C and 100C revealed only minor changes in levels of most amino acids at 100C compared to 370C. However, isoleucine and leucine each increased more than four fold; valine increased 1.4 fold, whereas glutamate increased 7.3 fold. Thus, cold adaptation may also include altered fluxes in fatty acid precursor pools.