Tautomerases

De Jong RM, W Brugman, GJ Poelarends, CP Whitman, BW Dijkstra (2004) The X-ray structure of trans-3-chloroacrylic acid dehalogenase reveals a novel hydration mechanism in the tautomerase superfamily. JBiol Chem 279 11546-11552. 

Poelarends GJ, R Saunier, DB Janssen (2001) fra 5-3-chloroacrylic acid dehalogenase from Pseudomonas pavonaceae 170 shares structural and mechanistic similarities with 4-oxalocrotonate tautomerase. J Bacteriol 183 4269-4277. 

Figure 1. The biosynthetic pathway from tyrosine to melanin (according to Hearing and Tsukamoto, 1991 Tsukamoto et al., 1992). Tyrosinase catalyzes three different reactions in this pathway (1, 2, 3). The reaction catalyzed by the product of TRP-2, DOPAchrome tautomerase, is indicated by 4. DOPA = 3,4-dihydroxyphenylalanine DHICA = 5,6-dihydroxyin-dole-2-carboxylic acid DHI = 5,6-dihydroxyindole.

Tsukamoto, K., Jackson, I. J., Urabe, K., Montague, P. M., and Hearing, V. J. (1992). A second tyrosinase-related protein, TRP-2, is a melanogenic enzyme termed DOPAchrome tautomerase. EMBO J. 11 519-526. 

Keywords QM/MM, 4-oxalocrotonate tautomerase, Free energy Perturbation, Enzyme catalysis... 

Cisneros GA, Liu H, Zhang Y, Yang W (2003) Ab initio QM/MM study shows there is no general acid in the reaction catalyzed by 4-oxalocrotonate tautomerase. J Am Chem Soc 125 10384—10393... 

Cisneros GA, Wang M, Silinski P, Fitzgerald MC, Yang WT (2004) The protein backbone makes important contributions to 4-oxalocrotonate tautomerase enzyme catalysis understanding from theory and experiment. Biochemistry 43 6885-6892... 

Metanis N, Brik A, Dawson PE, Keinan E (2004) Electrostatic interactions dominate the catalytic contribution of arg39 in 4-oxalocrotonate tautomerase. J Am Chem Soc 126 12726-12727... 

A major class of enzymes that catalyze changes within one molecule. Examples include racemases, epimerases, mutases, and tautomerases. 

The isomerases that catalyze the simplest reactions are tautomerases that promote the oxo-enol (keto-enol) transformation. The widely distributed oxaloacetate tautomerase (Eq. 13-24) is especially active in animal tissues.97120 Oxaloacetate exists to a substantial extent in the enolic form at 38°, 6% enol, 13% oxo, and 81% covalent hydrate.120121 A mammalian phenylpyruvate tautomerase has also been investigated.122... 

Oxalocrotonate tautomerase. This bacterial enzyme, which functions in the degradation of toluene (Chapter 25), is actually an isomerase. It catalyzes rapid interconversion of an unconjugated unsaturated a-oxoacid such as 4-oxalocrotonate with an intermediate enol (which may leave the enzyme) and the isomeric conjugated oxoacid (Eq. 13-31).168-170 A related 5-carboxymethyl-2-hydroxymuconate isomerase... 

The pKa of 6.2 has been associated with the amino-terminal proline l.172 4-Oxalocrotonate tautomerase is one of a small group of enzymes that have been synthesized nonenzymatically with both l amino acids and as a mirror image constructed with d amino acids.173... 

Figure 25-5 shows the principal catabolic pathways, as well as a few biosynthetic reactions, of phenylalanine and tyrosine in animals. Transamination to phenylpyruvate (reaction a) occurs readily, and the product may be oxidatively decarboxylated to phen-ylacetate. The latter may be excreted after conjugation with glycine (as in Knoop s experiments in which phenylacetate was excreted by dogs after conjugation with glycine, Box 10-A). Although it does exist, this degradative pathway for phenylalanine must be of limited importance in humans, for an excess of phenylalanine is toxic unless it can be oxidized to tyrosine (reaction b, Fig. 25-5). Formation of phenylpyruvate may have some function in animals. The enzyme phenylpyruvate tautomerase, which catalyzes interconversion of enol and oxo isomers of its substrate, is also an important immunoregulatory cytokine known as macrophage migration inhibitory factor.863...

The backbone dynamics of 4-oxalocrotonate tautomerase, a 41-kDa homo-hexamer with 62 residues per subunit, and its complex with a substrate analogue have been analyzed by the model-free formalism.60 Binding of the analogue freezes the motion of some of the backbone NH vectors in the active site, leading to a loss of entropy (Chapter 2). 

Ty initiates melanin synthesis by the hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (Dopa) and the oxidation of dopa to dopaquinone. In the presence of L-cysteine, dopaquinone rapidly combines with the thiol group to form cysteinyldopas, which undergo nonen-zymatic conversion and polymerization to pheomelanin via benzothiazine intermediates. In the absence of thiol groups, dopaquinone very rapidly undergoes conversion to dopachrome, which is transformed to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase. Alternatively, dopachrome is converted nonenzymatically to 5,6-dihydroxyindole (DHI). Oxidation of DHICA and DHI to the corresponding quinones and subsequent polymerization leads to eumelanins. It is still questionable if Ty is involved in this step. 

Relatively little is known about applications of this class of enzymes. In general, they catalyze geometric or structural changes within one molecule. According to the type of isomerization effected, they may be called racemases, epimerases, (cis-trans) isomerases, and tautomerases. Currently, about 126 isomerases are known, of which only about six are available commercially. 

Pennock JL, Behnke JM, Bickle QD, Devaney E, Grends RK, Isaac RE, Joshua GWR Selkirk ME, Zhang Y, Meyer DJ (1998) Rapid purification and characterization of L-dopachrome-methyl ester tautomerase (macrophage-migration-inhibitory factor) from Trichinella spiralis, Trichuris muris and Brugia pahangi. Biochem J 335 495 98... 

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