Featured System - December 2008
Short description: Histidine is an essential amino acid with several interesting chemical properties.
Histidine is an essential amino acid with several interesting chemical properties. The imidazole ring of histidine can adopt several different charge states at typical cellular conditions, and it interacts strongly with metal ions. All of these properties are demonstrated in the enzyme imidazolonepropionase. This enzyme is one of the community-nominated targets studied by the New York SGX Research Center for Structural Genomics (NYSGXRC) as part of a larger study of enolase and amidohydrolase structural genomics. It performs the third step in the major pathway of histidine degradation, when a water molecule is used to open the histidine ring. Ironically, the active site relies on a constellation of histidine amino acids to perform the histidine-breaking reaction.
This enzyme is found in all kingdoms of life. The NYSGXRC has studied the enzyme in two different organisms. The structure shown at the top (PDB entry 2q09) was obtained from an unknown bacterium found in Craig Venter's Global Ocean Sampling project. This metagenomics study randomly isolates DNA from an entire environmental sample, building up a picture of all of the organisms in the environment. The enzyme shown here was reconstructed based on the genomic information from a random sample taken from the fertile waters of the Sargasso Sea. The NYSGXRC has also studied the form of the enzyme made by the bacterium Agrobacterium tumefaciens, shown in the Jmol image below (PDB entry 2puz).
The NYSGXRC has studied this enzyme at both the beginning and the end of its reaction. The enzyme from the Sargasso Sea bacterium was solved with an analogue of the substrate, and the similar Agrobacterium enzyme was solved with the product of the reaction. The structures reveal the atomic details of the active site and have revealed many aspects of the reaction mechanism. The active site is centered around an iron ion which is coordinated by a collection of histidine and acidic amino acids. Two additional histidine amino acids and a glutamine perform the hydrogen-shuttling operations in the hydrolysis reaction. Both of these complexes are shown close up in the Jmol images below.
The JSmol tab below displays an interactive JSmol.
One subunit of aquaglyceroporin is shown here. Glycerol molecules (white and pink spheres) and water (turquoise spheres) are lined up single-file through the channel. Two glycerol molecules are bound at the entrance, waiting to enter, and three are bound within the narrow channel. The narrowest portion of the channel is surrounded by an arginine and two aromatic amino acids (shown here in green), that together form the selectivity filter. In this structure, a glycerol molecule is bound inside th
Tyagi R, Eswaramoorthy S, Burley SK, Raushel FM, Swaminathan S. (2008) A common catalytic mechanism for proteins of the HutI family. Biochemistry 47, 5608-5615.
Tyagi R, Kumaran D, Burley SK, Swaminathan S. (2007) X-ray structure of imidazolonepropionase from Agrobacterium tumefaciens at 1.87 Å resolution. Proteins 15, 652-658.
Venter JC, Remington K, Heidelberg JF, Halpern AL, Rusch D, Eisen JA, Wu D, Paulsen I, Nelson KE, Nelson W, Fouts DE, Levy S, Knap AH, Lomas MW, Nealson K, White O, Peterson J, Hoffman J, Parsons R, Baden-Tillson H, Pfannkoch C, Rogers YH, Smith HO. (2004) Environmental genome shotgun sequences of the Sargasso Sea. Science 304, 66-70.