Enzymology and regulation of amino acid metabolsim
The research interests of this laboratory center on control of the synthesis and activity of key enzymes involved in the metabolism of the amino acids histidine and ergothioneine. These studies include regulation of enzyme activity and gene expression, mechanisms of coenzyme function, and structure-function analysis of the enzymes required for the breakdown of these amino acids.
Our principal activity in the enzymology of histidine degradation deals with the first two enzymes in the pathway, namely histidase and urocanase, plus the repressor protein which controls expression of all histidine utilization genes and a specialized histidine transporter. Both histidase and urocanase are known to be reduced or absent in the human genetic disorders histidinemia and urocanic acidemia, respectively, and thus an understanding of these enzymes and their genetics could prove useful in overcoming the consequences of their deficiency. Using Pseudomonas putida as our model system, we seek to understand more thoroughly the nature of the histidase electrophilic coenzyme, methylidene-imidazolone (MIO), and how this coenzyme is formed. This investigation utilizes the cloned histidase gene overexpressed in Escherichia coli to allow structural analysis of the coenzyme and its mode of production in the protein, as well as studies on the kinetic properties of the enzyme's deamination process. The second enzyme in the histidine dissimilatory pathway, urocanase, catalyzes the addition of water to urocanic acid, the product of histidase action. We established that urocanase uses NAD in a novel mechanism that does not involve NADH formation. Present efforts aim to characterize the NAD binding site in order to better understand the extremely tight binding and how NAD functions in this unusual reaction process. The availability of complete amino acid sequences for both histidase and urocanase facilitate these studies.
In addition to histidase and urocanase, we also are characterizing the repressor protein for the hut regulon in Pseudomonas putida and determining the locations and sequence of the various operator regions recognized by the repressor in order to understand if the repressor’s binding to two dissimilar inducers (urocanate and N-formylglutamate) leads to selectivity in gene expression. Additionally, a yet-uncharacterized gene co-transcribed with the hut repressor gene is being studied for its possible role in directing the repressor’s action on other systems. Recent observations by others have revealed that the histidine utilization repressor coordinates expression of the virB gene in Brucella abortus; this gene is associated with the type IV secretion system used in the pathogenesis associated with this organism.
Lastly, we are investigating the microbial breakdown of ergothioneine, the betaine of thiolhistidine. This little studied amino acid is readily detected in the tissues of many higher species but it is thought to be biosynthesized only by fungi (e.g. mushrooms) and closely related organisms such as the mycobacteria. Present interest in ergothioneine stems from its ability to serve as an effective anti-oxidant, and thus possibly promoting increased longevity by means of its free radical trapping properties. Its degradation seems to be carried out by reactions similar, but not identical, to those involved in histidine degradation. We are currently cloning the genes for this function from Agrobacterium radiobacter and plan to use these to investigate the complete ergothioneine catabolic pathway as well as to provide sufficient enzymes for their purification and characterization. Ergothionase, the initial enzyme for ergothioneine breakdown, is extremely specific in its catalytic action and thus can be a useful tool for evaluating the presence and concentration of ergothioneine in a variety of sources.
- Hernandez, D., and A. T. Phillips. 1993. Purification and characterization of Pseudomonas putida histidine ammonia-lyase expressed in Escherichia coli. Protein Expression and Purification 4:473-478.
- Hernandez, D., J. G. Stroh, and A. T. Phillips. 1993. Identification of Ser143 as the site of modification in the active site of histidine ammonia-lyase. Arch. Biochem. Biophys. 307:126-132.
- Guo, L., A. T. Phillips, and R. N. Arteca. 1993. Amino acid N-malonyltransferases from mung beans: action on 1-aminocyclopropane-1-carboxylic acid and D-phenylalanine. J. Biol. Chem. 268:25389-25394.
- Hernandez, D., A. T. Phillips, and J. Zon. 1994. 1-Amino-2-imidazol-4'-ylethylphosphonic acid is a potent reversible inhibitor of Pseudomonas putida histidine ammonia-lyase. Biochemistry and Molecular Biology International 32:189-194.
- King, R. S., L. L. Sechrist, and A. T. Phillips. 1994. A revised map location for the histidine utilization genes in Pseudomonas putida. J. Basic Microbiol. 34:253-257.
- Hernandez, D. and A. T. Phillips. 1994. Ser-143 is an essential active site residue in histidine ammonia-lyase of Pseudomonas putida. Biochem. Biophys. Res. Commun. 201:1433-1438.
- Walters, F. S., C. A. Kulesza, A. T. Phillips, and L. H. English. 1994. A stable oligomer of Bacillus thuringiensis delta-endotoxin, CryIIIA. Insect Biochem. Molec. Biol. 24:963-968.
Wilfinger, W. W., C. S. Baker, E. L. Kunze, A. T. Phillips, and R. H. Hammerstedt. 1996. Versatile fluid-mixing device for cell and tissue microgravity research applications. J. Spacecraft and Rockets 33:126-130.
- Teo, B., Kidd, R.D., Mack, J., Tiwari, A., Hernandez, D., Phillips, A.T., and Farber, G.K. (1998). Crystallization and preliminary X-ray studies of Pseudomonas putida histidine ammonium-lyase. Acta Cryst. D54:681-683.
- Hausinger, R. P. and A. T. Phillips. 2007. Enzymatic activity. In: Methods for General and Molecular Microbiology, 3rd Ed., C. A. Reddy, ed., American Society for Microbiology, Washington, DC, pp. 504-526.
- Briczinski, E.P., Phillips, A.T., and Roberts, R.F. (2008). Transport of glucose by Bifidobacterium animalis subsp. lactis occurs via facilitated diffusion. Appl. Environ. Microbiol. 74:6941-6948.