Homogenization and crystallization of histidine ammonia-lyase by exchange of a surface cysteine residue.
Histidase (histidine ammonia-lyase, EC 126.96.36.199) from Pseudomonas putida was expressed in Escherichia coli and purified. In the absence of thiols the tetrameric enzyme gave rise to undefined aggregates and suitable crystals could not be obtained. The solvent accessibility along the chain was predicted from the amino acid sequence. Among the seven cysteines, only one was labeled as 'solvent-exposed'. The exchange of this cysteine to alanine abolished all undefined aggregations and yielded readily crystals diffracting to 1.8 A resolution. (+info)
Histidine-imbalanced diets stimulate hepatic histidase gene expression in rats.
A high protein concentration in the diet induces the gene expression of several amino acid degrading enzymes such as histidase (Hal) in rats. It is important to understand whether the amino acid pattern of the dietary protein affects the gene expression of these enzymes. The purpose of the present work was to study the effect of a histidine-imbalanced diet on the activity and mRNA concentration of rat hepatic histidase. Seven groups of six rats were fed one of the following diets: 1) 6% casein (basal), 2) 20% casein, 3) 35% casein, 4) an imbalance diet containing 6% casein plus a mixture of indispensable amino acids (IAA) equivalent to a 20% casein diet without histidine (I-20), 5) 6% casein plus a mixture of IAA equivalent to a 35% casein diet without histidine (I-35), 6) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 20% casein diet, 7) a corrected diet containing 6% casein plus IAA including histidine equivalent to a 35% casein diet. Serum histidine concentration was inversely proportional to the protein content of the diet, and it was significantly higher in rats fed the corrected diets compared to their respective imbalanced diet groups. Hal activity increased as the protein content of the diet increased. Greater histidine imbalance resulted in lower food intake and higher Hal activity. Rats fed histidine-corrected diets had lower activity than their respective imbalanced groups. Differences in Hal activity were associated with differences in the concentration of Hal mRNA. These results indicate that rats fed a histidine-imbalanced diet exhibit reduced food intake and weight gain and increased Hal gene expression as a consequence of an increased amino acid catabolism. (+info)
Histidine ammonia-lyase. The use of 4-fluorohistidine in identification of the rate-determining step.
The alpha,beta eliminations of NH3 from L-histidine and 4-fluoro-L-histidine by histidine ammonia-lyase appear to occur by similar mechanisms, although a large difference in Vmax for the two reactions was observed. Both reactions were shown to be reversible with an equilibrium constant of 4 to 5. The presteady state kinetics of the deamination of 4-fluoro-L-histidine indicates that the rate-determining step precedes the dissociation of ammonia from the enzyme. The isotope effect of 1.4 to 2.0 observed with 4-fluoro-DL-[beta-2-H2]histidine or DL-[beta-2-H2]histidine or DL-[beta-2-H2]histidine indicates that the C-H bond breakage is at least partially rate-determining for the deamination of both substrates. (+info)
Characterization of the active site of histidine ammonia-lyase from Pseudomonas putida.
Elucidation of the 3D structure of histidine ammonia-lyase (HAL, EC 188.8.131.52) from Pseudomonas putida by X-ray crystallography revealed that the electrophilic prosthetic group at the active site is 3,5-dihydro-5-methylidene-4H-imidazol-4-one (MIO) [Schwede, T.F., Retey, J., Schulz, G.E. (1999) Biochemistry, 38, 5355-5361]. To evaluate the importance of several amino-acid residues at the active site for substrate binding and catalysis, we mutated the following amino-acid codons in the HAL gene: R283, Y53, Y280, E414, Q277, F329, N195 and H83. Kinetic measurements with the overexpressed mutants showed that all mutations resulted in a decrease of catalytic activity. The mutants R283I, R283K and N195A were approximately 1640, 20 and 1000 times less active, respectively, compared to the single mutant C273A, into which all mutations were introduced. Mutants Y280F, F329A and Q277A exhibited approximately 55, 100 and 125 times lower activity, respectively. The greatest loss of activity shown was in the HAL mutants Y53F, E414Q, H83L and E414A, the last being more than 20 900-fold less active than the single mutant C273A, while H83L was 18 000-fold less active than mutant C273A. We propose that the carboxylate group of E414 plays an important role as a base in catalysis. To investigate a possible participation of active site amino acids in the formation of MIO, we used the chromophore formation upon treatment of HAL with l-cysteine and dioxygen at pH 10.5 as an indicator. All mutants, except F329A showed the formation of a 338-nm chromophore arising from a modified MIO group. The UV difference spectra of HAL mutant F329A with the MIO-free mutant S143A provide evidence for the presence of a MIO group in HAL mutant F329A also. For modelling of the substrate arrangement within the active site and protonation state of MIO, theoretical calculations were performed. (+info)
Autocatalytic peptide cyclization during chain folding of histidine ammonia-lyase.
Histidine ammonia-lyase requires a 4-methylidene-imidazole-5-one group (MIO) that is produced autocatalytically by a cyclization and dehydration step in a 3-residue loop of the polypeptide. The crystal structures of three mutants have been established. Two mutants were inactive and failed to form MIO, but remained unchanged elsewhere. The third mutant showed very low activity and formed MIO, although it differed from an MIO-less mutant only by an additional 329-C(beta) atom. This atom forms one constraint during MIO formation, the other being the strongly connected Asp145. An exploration of the conformational space of the MIO-forming loop showed that the cyclization is probably enforced by a mechanic compression in a late stage of chain folding and is catalyzed by a well-connected internal water molecule. The cyclization of the respective 3-residue loop of green fluorescent protein is likely to occur in a similar reaction. (+info)
Structures of two histidine ammonia-lyase modifications and implications for the catalytic mechanism.
Histidine ammonia-lyase (EC 184.108.40.206) catalyzes the nonoxidative elimination of the alpha-amino group of histidine using a 4-methylidene-imidazole-5-one (MIO), which is formed autocatalytically from the internal peptide segment 142Ala-Ser-Gly. The structure of the enzyme inhibited by a reaction with l-cysteine was established at the very high resolution of 1.0 A. Five active center mutants were produced and their catalytic activities were measured. Among them, mutant Tyr280-->Phe could be crystallized and its structure could be determined at 1.7 A resolution. It contains a planar sp2-hybridized 144-N atom of MIO, in contrast to the pyramidal sp3-hybridized 144-N of the wild-type. With the planar 144-N atom, MIO assumes the conformation of a putative intermediate aromatic state of the reaction, demonstrating that the conformational barrier between aromatic and wild-type states is very low. The data led to a new proposal for the geometry for the catalyzed reaction, which also applies to the closely related phenylalanine ammonia-lyase (EC 220.127.116.11). Moreover, it suggested an intermediate binding site for the released ammonia. (+info)
An active site homology model of phenylalanine ammonia-lyase from Petroselinum crispum.
The plant enzyme phenylalanine ammonia-lyase (PAL, EC 18.104.22.168) shows homology to histidine ammonia-lyase (HAL) whose structure has been solved by X-ray crystallography. Based on amino-acid sequence alignment of the two enzymes, mutagenesis was performed on amino-acid residues that were identical or similar to the active site residues in HAL to gain insight into the importance of this residues in PAL for substrate binding or catalysis. We mutated the following amino-acid residues: S203, R354, Y110, Y351, N260, Q348, F400, Q488 and L138. Determination of the kinetic constants of the overexpressed and purified enzymes revealed that mutagenesis led in each case to diminished activity. Mutants S203A, R354A and Y351F showed a decrease in kcat by factors of 435, 130 and 235, respectively. Mutants F400A, Q488A and L138H showed a 345-, 615- and 14-fold lower kcat, respectively. The greatest loss of activity occurred in the PAL mutants N260A, Q348A and Y110F, which were 2700, 2370 and 75 000 times less active than wild-type PAL. To elucidate the possible function of the mutated amino-acid residues in PAL we built a homology model of PAL based on structural data of HAL and mutagenesis experiments with PAL. The homology model of PAL showed that the active site of PAL resembles the active site of HAL. This allowed us to propose possible roles for the corresponding residues in PAL catalysis. (+info)
Soy protein, casein, and zein regulate histidase gene expression by modulating serum glucagon.
Glucagon has been postulated as an important physiological regulator of histidase (Hal) gene expression; however, it has not been demonstrated whether serum glucagon concentration is associated with the type and amount of protein ingested. The purpose of the present work was to study the association between hepatic Hal activity and mRNA concentration in rats fed 18 or 50% casein, isolated soy protein, or zein diets in a restricted schedule of 6 h for 10 days, and plasma glucagon and insulin concentrations. On day 10, five rats of each group were killed at 0900 (fasting), and then five rats were killed after being given the experimental diet for 1 h (1000). Rats fed 50% casein or soy diets showed higher Hal activity than the other groups studied. Rats fed 50% zein diets had higher Hal activity than rats fed 18% casein, soy, or zein diets, but lower activity than rats fed 50% casein or soy diets. Hal mRNA concentration followed a similar pattern. Hal activity showed a significant association with serum concentrations of glucagon. Serum glucagon concentration was significantly correlated with protein intake. Thus the type and amount of protein consumed affect Hal activity and expression through changes in serum glucagon concentrations. (+info)