共查询到20条相似文献,搜索用时 31 毫秒
1.
Ovechkina L. G. Zinoviev V. V. Gorbunov Yu. A. Malygin E. G. 《Russian Journal of Bioorganic Chemistry》2000,26(12):844-847
The structural and catalytic properties of the phage T4 DNA-(adenine-N
6)-methyltransferase (EC 2.1.1.72) were studied at different enzyme–substrate concentration ratios by chemical crosslinking of the protein subunits and by measuring the presteady state kinetics of the reactions. Various structural states of the methyltransferase were correlated with its catalytic activity, and it was shown that the oligomeric forms of the enzyme are catalytically active but are characterized by the reaction parameters different from those of the monomer. 相似文献
2.
The individual and interactive effects of physicochemical parameters on ellagitannin acyl hydrolase activity and ellagic acid
production by Aspergillus
oryzae using ellagitannins from acorn fringe of oak as substrate were studied. Ellagitannins concentration, incubation time were
identified as important physicochemical parameters influencing the enzyme synthesis and the production accumulation, and the
substrate concentration with initial pH was determined to has an interactive effect on the enzyme synthesis, while ellagitannins
concentration and initial pH with incubation time were found to have interactions on the production accumulation. Furthermore,
the parameters were optimized by quadratic programming. Under optimum condition, the fermentation run lasted 84 h with 4 g L−1 ellagitannins concentration, yielding 17.7% ellagic acid. However, the maximum enzyme activity was obtained in 96 h with 5 g L−1 substrate concentration. The research demonstrated a possible way to develop an efficient approach for recovery of higher
added-value product (ellagic acid) from forestry byproduct (acorn fringe of oak). 相似文献
3.
Douglas B. Craig 《The protein journal》2010,29(1):55-61
Single enzyme molecule assays were performed on β-galactosidase from the thermophilic bacteria Geobacillus stearothermophilus using a capillary electrophoresis-based continuous flow assay and the substrate DDAO-β-d-galactoside. The enzyme was found to be heterogeneous with respect to catalytic rate, electrophoretic mobility and activation
energy of catalysis. Catalytic rate was also found to vary over time for individual molecules at elevated temperature. Comparison
with β-galactosidase from the mesophilic bacteria Escherichia coli showed that the variation in activity over time was less pronounced and the average activation energy of catalysis was lower
for the Geobacillus stearothermophilus enzyme. Attempts to measure the properties of individual β-galactosidase molecules from the thermophilic bacteria Thermus thermophilus and the cold-adapted bacteria Pseudoalteromas haloplanktis using this assay were unsuccessful. 相似文献
4.
The effects of thyroidectomy (Tx) and subsequent treatment with 3,5,3′-triiodothyronine (T3) or combined replacement therapy (TR) with T3 and thyroxine (T4) on the substrate and temperature kinetics properties of Na+,K+-ATPase and lipid/phospholipid makeup of rat kidney microsomes were examined. Enzyme activity was somewhat high in the hypothyroid
(Tx) animals and increased significantly following T3 treatment, while TR treatment caused a decrease. In the Tx and T3 groups enzyme activity resolved in two kinetic components, while in the TR group the enzyme showed allosteric behavior up to 0.5 mm ATP concentration. The K
m and V
max values of both the components decreased in Tx animals without affecting the catalytic efficiency. T3 treatment caused a significant increase in the V
max of both the components, with a significant increase in the catalytic efficiency, while the K
m values were not upregulated. The TR regimen lowered the K
m and V
max of component II but improved the catalytic efficiency. Thyroid status-dependent changes were also noted in the temperature
kinetics of the enzyme. Regression analysis revealed that changes in the substrate and temperature kinetics parameters correlated
with specific phospholipid components. 相似文献
5.
Enzyme access, kinetic behavior, and protein–protein interactions are critical for explaining reaction of the metabolites
contained within the myriad compartments of biological systems. To explore these relationships, the reaction kinetics of oil
bodies versus oil emulsions as substrates for lipolytic reactions were measured. The initial rate of hydrolysis for the oil
body system was comparatively very low due to a brief latency period. However, the complete activation of the lipase at the
interface resulted in an enzyme–membrane complex that was catalytically enhanced 3–15-fold over the emulsion system for substrate
concentrations in the measured range of approximately 1–5.5 mM. This disparity is explained by the availability of substrate
to the enzyme active site (defined as the availability parameter “A”) which varies between the two substrates by 40-fold. A simple hyperbolic kinetic mechanism is proposed with K
m replaced by the parameter, A, to account for this phenomenon, leading to a maximum rate of approximately 1450 IU/mg protein. The interaction is verified
through separation of the enzyme–membrane complex which shows nearly double the activity towards an emulsified soybean oil
substrate (activity ratio of 5:3) when compared to the native enzyme. 相似文献
6.
Khawar Sohail Siddiqui Don M. Parkin Paul M. G. Curmi Davide De Francisci Anne Poljak Kevin Barrow Malcolm H. Noble Jill Trewhella Ricardo Cavicchioli 《Biotechnology and bioengineering》2009,103(4):676-686
The alkaline protease, savinase was chemically modified to enhance the productivity of the enzyme at low temperatures on a complex polymeric protein (azocasein) substrate. At 5 and 15°C, savinase modified with ficol or dextran hydrolyzed fivefold more azocasein than the unmodified savinase. Kinetic studies showed that the catalytic improvements are associated with changes in uncompetitive substrate inhibition with Ki values of modified savinases sixfold higher than the unmodified savinase. Modeling of small‐angle scattering data indicates that two substrate molecules bind on opposing sides of the enzyme. The combined kinetic and structural data indicate that the polysaccharide modifier sterically blocks the allosteric site and reduces substrate inhibition. In contrast to the properties of cold‐active enzymes that generally manifest as low activation enthalpy and high flexibility, this study shows that increased activity and productivity at low temperature can be achieved by reducing uncompetitive substrate inhibition, and that this can be achieved using chemical modification with an enzyme in a commercial enzyme‐formulation. Biotechnol. Bioeng. 2009;103: 676–686. © 2009 Wiley Periodicals, Inc. 相似文献
7.
The filamentous fungus Stachybotrys sp has been shown to possess a rich β-glucosidase system composed of five β-glucosidases. One of them was already purified
to homogeneity and characterized. In this work, a second β-glucosidase was purified and characterized. The filamentous fungal
A19 strain was fed-batch cultivated on cellulose, and its extracellular cellulases (mainly β-glucosidases) were analyzed.
The purified enzyme is a monomeric protein of 78 kDa molecular weight and exhibits optimal activity at pH 6.0 and at 50°C.
The kinetic parameters, K
m and V
max, on para-nitro-phenyl-β-d-glucopyranosid (p-NPG) as a substrate were, respectively, 1.846 ± 0.11 mM and 211 ± 0.08 μmol min−1 ml−1. One interesting feature of this enzyme is its high stability in a wide range of pH from 4 to 10. Besides its aryl β-glucosidase
activity towards salicin, methylumbellypheryl-β-d-glucoside (MU-Glc), and p-NPG, it showed a true β-glucosidase activity because it splits cellobiose into two glucose monomers. This enzyme has the
capacity to synthesize short oligosaccharides from cellobiose as the substrate concentration reaches 30% with a recovery of
40%. We give evidences for the involvement of a transglucosylation to synthesize cellotetraose by a sequential addition of
glucose to cellotriose. 相似文献
8.
Teuber M Azemi ME Namjoyan F Meier AC Wodak A Brandt W Dräger B 《Plant molecular biology》2007,63(6):787-801
Putrescine N-methyltransferase (PMT) is a key enzyme of plant secondary metabolism at the start of the specific biosynthesis of nicotine,
of tropane alkaloids, and of calystegines that are glycosidase inhibitors with nortropane structure. PMT is assumed to have
developed from spermidine synthases (SPDS) participating in ubiquitous polyamine metabolism. In this study decisive differences
between both enzyme families are elucidated. PMT sequences were known from four Solanaceae genera only, therefore additional
eight PMT cDNA sequences were cloned from five Solanaceae and a Convolvulaceae. The encoded polypeptides displayed between
76% and 97% identity and typical amino acids different from plant spermidine synthase protein sequences. Heterologous expression
of all enzymes proved catalytic activity exclusively as PMT and K
cat values between 0.16 s−1 and 0.39 s−1. The active site of PMT was initially inferred from a protein structure of spermidine synthase obtained by protein crystallisation.
Those amino acids of the active site that were continuously different between PMTs and SPDS were mutated in one of the PMT
sequences with the idea of changing PMT activity into spermidine synthase. Mutagenesis of active site residues unexpectedly
resulted in a complete loss of catalytic activity. A protein model of PMT was based on the crystal structure of SPDS and suggests
that overall protein folds are comparable. The respective cosubstrates S-adenosylmethionine and decarboxylated S-adenosylmethionine, however, appear to bind differentially to the active sites of both enzymes, and the substrate putrescine
adopts a different position. 相似文献
9.
The present study investigates the efficiency of Aspergillus niger to produce invertase, an industrially important enzyme by using powdered stem of Cympopogan caecius (Lemon grass) as sole substrate and sole carbon source for the microorganism. The molecular weight of invertase was estimated
to be 66–70 kDa by sodium do decyl sulphate poly acrylamide gel electrophoresis (SDS PAGE). The production of the enzyme was
studied at different pH scales ranging from pH 4.0 to 7.0 at a constant temperature of 30°C and 2% substrate concentration.
The maximum production of invertase (specific activity −0.0516 μk/mg protein) was obtained at pH 5.5 at 30°C temperature,
and incubation for 48 h. The activity was found to be stable at pH 5.5 for 30 min. The enzyme was found to be stable in the
temperature range of 20–55°C. The effect of divalent metal ions Cu2+, Fe2+, Co2+ on the activity of the enzyme invertase showed that these ions affected the activity by a certain factor. The study can be
further industrially exploited in a country-like India where lemon grass is found in plenty and can be used as substrate for
enzyme production. Moreover, the preparation of the substrate is also a simple process. 相似文献
10.
Kinetic constants for peptide phosphorylation by the catalytic subunit of the dimorphic fungus Mucor rouxii protein kinase A were determined using 13 peptides derived from the peptide containing the basic consensus sequence RRASVA, plus kemptide, S6 peptide, and protamine. As a whole, although with a greater Km, the order of preference of the peptides by the M. rouxii catalytic subunit was similar to the one displayed by mammalian protein kinase A. Particularly significant is the replacement of serine by threonine in the basic peptide RRATVA, which impaired its role as a substrate of M. rouxii catalytic subunit. Mucor rouxii protein kinase A is a good model in which to study the mechanism of activation since cAMP alone is not enough to promote activation and dissociation. Four peptides were selected for the study of holoenzyme activation under conditions in which the enzymatic activity was not proportional to the holoenzyme concentration: RRASVA, RRRRASVA, KRRRLSSRA (S6 peptide), and LRRASLG (kemptide); protamine was used as reference. Differential activation degree was observed depending on the peptide used and on cAMP concentration. Ratios of activity between different substrates displayed by the holoenzyme under the above conditions did not reflect the one expected for the free catalytic subunit. The degree of inhibition of the holoenzyme activity by an active peptide derived from the thermostable protein kinase inhibitor was dependent on the substrate used and on the holoenzyme concentration, while it was found to be independent of these two parameters for free catalytic subunit. Polycation modulation of holoenzyme activation by cAMP was also dependent on the polycation itself and on the peptide used as substrate. The observed kinetic differences between holoenzyme and free catalytic subunit were decreased or almost abolished when working at low enzyme or at high cAMP concentrations. Two hypotheses compatible with the results are discussed: substrate participation in the dissociation process and/or holoenzyme activation without dissociation. 相似文献
11.
Purification and properties of a novel raw starch degrading cyclomaltodextrin glucanotransferase from Bacillus firmus 总被引:3,自引:0,他引:3
B. N. Gawande A. Goel A. Y. Patkar S. N. Nene 《Applied microbiology and biotechnology》1999,51(4):504-509
A novel raw starch degrading cyclomaltodextrin glucanotransferase (CGTase; E.C. 2.4.1.19), produced by Bacillus firmus, was purified to homogeneity by ultrafiltration, affinity and gel filtration chromatography. The molecular weight of the
pure protein was estimated to be 78 000 and 82 000 Da, by SDS-PAGE and gel filtration, respectively. The pure enzyme had a
pH optimum in the range 5.5–8.5. It was stable over the pH range 7–11 at 10 °C, and at pH 7.0 at 60 °C. The optimum temperature
for enzyme activity was 65 °C. In the absence of substrate, the enzyme rapidly lost its activity above 30 °C. K
m and k
cat for the pure enzyme were 1.21 mg/ml and 145.17 μM/mg per minute respectively, with soluble starch as the substrate. For cyclodextrin
production, tapioca starch was the best substrate used when gelatinized, while wheat starch was the best substrate used when
raw. This CGTase could degrade raw wheat starch very efficiently; up to 50% conversion to cyclodextrins was obtained from
150 g/l starch without using any additives. The enzyme produced α-, β- and γ-cyclodextrins in the ratio of 0.2:9.2:0.6 and
0.2:8.6:1.2 from gelatinized tapioca starch and raw wheat starch with 150 g/l concentration respectively, after 18 h incubation.
Received: 25 September 1998 / Received revision: 15 December 1998 / Accepted: 21 December 1998 相似文献
12.
We report studies on an L-asparaginase from Pyrococcus furiosus, cloned and expressed in Escherichia coli and purified to homogeneity. Protein stability and enzyme kinetic parameters were determined. The enzyme was found to be
thermostable, natively dimeric, and glutaminase-free, with optimum activity at pH 9.0. It showed a K
m of 12 mM and a substrate inhibition profile above 20 mM L-asparagine. Urea could not induce unfolding and enzyme inactivation;
however, with guanidine hydrochloride (GdnCl) a two-state unfolding pattern was observed. Reduced activity and an altered
near-UV-CD signal for protein at low GdnCl concentration (1 M) suggested tertiary structural changes at the enzyme active
site. A homology three-dimensional model was developed and the structural information was combined with activity and stability
data to give functional clues about the asparaginase. 相似文献
13.
E. E. Melnikov K. B. Tsirulnikov T. V. Rotanova 《Russian Journal of Bioorganic Chemistry》2000,26(7):474-481
Some aspects of theEscherichia coli Lon protease ATPase function were studied around the optimum pH value. It was revealed that in the absence of the protein
substrate the maximum ATPase activity of the enzyme is observed at an equimolar ratio of ATP and Mg2+ ions in the area of their millimolar concentrations. Free components of the substrate complex (ATP-Mg)2− inhibit the enzyme ATPase activity. It is hypothesized that the effector activity of free Mg2+ ions is caused by the formation of the “ADP-Mg-form” of ATPase centers. It was shown that the activation of ATP hydrolysis
in the presence of the protein substrate is accompanied by an increase in the affinity of the (ATP-Mg)2− complex to the enzyme, by an elimination of the inhibiting action of free Mg2+ ions without altering the efficiency of catalysis of ATP hydrolysis (based on thek
cat value), and by a change in the type of inhibition of ATP hydrolysis by the (ADP-Mg)− complex (without changing theK
i value). Interaction of the Lon protease protein substrate with the enzyme area located outside the peptide hydrolase center
was demonstrated by a direct experiment. 相似文献
14.
Yali Wang Jinyue Wu Xuejiao Ru Yucheng Jiang Mancheng Hu Shuni Li Quanguo Zhai 《Journal of industrial microbiology & biotechnology》2011,38(6):717-724
The catalytic performance of chloroperoxidase (CPO) in peroxidation of 2, 2′-azinobis-(-3 ethylbenzothiazoline-6-sulfononic
acid) diammonium salt (ABTS) and oxidation of indole in a reverse micelle composed of surfactant-water-isooctane-pentanol
was investigated and optimized in this work. Some positive results were obtained as follows: the peroxidation activity of
CPO was enhanced 248% and 263%, while oxidation activity was enhanced 215% and 222% in cetyltrimethylammonium bromide (CTABr)
reverse micelle medium and dodecyltrimethylammonium bromide (DTABr) medium, respectively. Thermostability was also greatly
improved in reverse micelle: at 40°C, CPO essentially lost all its activity after 5 h incubation, while 58–76% catalytic activity
was retained for both reactions in the two reverse micelle media. At 50°C, about 44–75% catalytic activity remained for both
reactions in reverse micelle after 2 h compared with no observed activity in pure buffer under the same conditions. The enhancement
of CPO activity was dependent mainly on the surfactant concentration and structure, organic solvent ratio (V
pentanol/V
isooctane), and water content in the reverse micelle. The obtained kinetic parameters showed that the catalytic turnover frequency
(k
cat) was increased in reverse micelle. Moreover, the lower K
m and higher k
cat/K
m demonstrated that both the affinity and specificity of CPO to substrates were improved in reverse micelle media. Fluorescence,
circular dichroism (CD) and UV–vis spectra assays indicated that a catalytically favorable conformation of enzyme was achieved
in reverse micelle, including the strengthening of the protein α-helix structure, and greater exposure of the heme prosthetic
group for easy access of the substrate in bulk solution. These results are promising in view of the industrial applications
of this versatile biological catalyst. 相似文献
15.
Hyeok-Jin Ko Eun Woo Lee Won-Gi Bang Cheol-Koo Lee Kyoung Heon Kim In-Geol Choi 《Molecules and cells》2010,29(5):485-492
In seeking aryl acylamidase (EC 3.5.1.13) acting on an amide bond in p-acetaminophenol (Tylenol™), we identified a novel gene encoding 496 residues of a protein. The gene revealed a conserved
amidase signature region with a canonical catalytic triad. The gene was expressed in E. coli and characterized for its biochemical properties. The optimum pH and temperature for the activity on p-acetaminophenol were 10 and 37°C, respectively. The half-life of enzyme activity at 37°C was 192 h and 90% of its activity
remained after 3 h incubation at 40°C. Divalent metals was found to inhibit the activity of enzyme. The K
m
values for various aryl acylamides such as 4-nitroacetanilide, p-acetaminophenol, phenacetin, 4-chloroacetanilide and acetanilide were 0.10, 0.32, 0.83, 1.9 and 19 mM, respectively. The
reverse reaction activity (amide synthesis) was also examined using various chain lengths (C1∼C4 and C10) of carboxylic donors and aniline as substrates. These kinetic parameters and substrate specificity in forward and reverse
reaction indicated that the aryl acylamidase in this study has a preference for aryl substrate having polar functional groups
and hydrophobic carboxylic donors. 相似文献
16.
Somayeh Daneshjoo Neda Akbari Abbas Akhavan Sepahi Bijan Ranjbar Ramezan‐Ali Khavarinejad Khosro Khajeh 《Engineering in Life Science》2011,11(3):259-263
The activity of a lipase from a newly isolated Pseudomonas sp. was investigated in the presence of organic solvents and imidazolium chloride‐based ionic liquids (IL) such as BMIM[Cl] and HMIM[Cl]. The lipase activity in the presence of IL was higher compared to that in common organic solvents such as methanol and 2‐propanol. A possible explanation for the enzyme activation might be the structural changes induced in the protein in organic systems. Since IL quench the intensity of fluorescence emission, it was not possible to investigate the major factor that influences the enzyme behavior in these new organic salts. Furthermore, the enzyme exhibited excellent activity in buffer mixtures containing both organic solvent and IL. The stability of the lipase at 50°C was considerably increased in the presence of 20% BMIM[Cl] compared with the untreated lipase in aqueous medium. The light scattering method clearly showed that prevention of aggregation could be the reason for thermal stabilization at 50°C in reactions containing IL. Kinetic analysis of the enzyme in the presence of different concentrations of IL showed that the Km value increased from 0.45 mM in aqueous buffer to 2.4 mM in 50% v/v BMIM[Cl]/buffer. The increase in Km indicates that IL can significantly reduce the binding affinity of the substrate to the enzyme. Also, a linear correlation was observed between the BMIM[Cl] concentration and Vmax of the enzyme. As the concentration of BMIM[Cl] increased from 10 to 50% v/v, the Vmax value increased from 1.8 to 46 μM/min. 相似文献
17.
Yu. S. Kisrieva V. M. Serebrennikov N. A. Zagustina A. M. Bezborodov 《Applied Biochemistry and Microbiology》2000,36(2):109-114
Enzymes catalyzing the synthesis and subsequent transformation of α-acetolactate (AcL)—acetolactate synthase (AcLS) and acetolactate
decarboxylase (AcLDC)—were isolated and partially purified from the cells of lactic acid bacteriaLactococcus lactis ssp.lactis biovar.diacetylactis, strain 4. The preparation of AcLS, purified 560-fold, had a specific activity of 358 300 U/mg protein (9% yield). The preparation
of AcLDC., purified 4828-fold, had a specific activity of 140 U/mg protein (4.8% yield). The enzymes exhibited optimum activity
at pH 6.5 and 6.0, respectively (medium, phosphate buffer). The values of apparentK
m, determined for AcLS and AcLDC with pyruvate and AcL, respectively, were equal to 70 mM and 20 mM. AcLS appeared as an allosteric
enzyme with low affinity for the substrate and a sigmoid dependence of the activity on the substrate concentration. In the
case of AcLDC, this dependence was hyperbolic and the affinity of the enzyme for its substrate was high (K
m = 20 mM). Leucine, valine, and isoleucine were shown to be activators of AcDLC. 相似文献
18.
The effects of guanidinium chloride (GuHCl) on the activity of Penaeus vannamei β-N-acetyl-d-glucosaminidase (NAGase) have been studied. The results show that GuHCl, at appropriate concentrations, can lead to reversible
inactivation of the enzyme, and the IC50 is estimated to be 0.6 M. Changes of activity and conformation of the enzyme in different concentrations of GuHCl have been studied by measuring the
fluorescence spectra and its relative activity after denaturation. The fluorescence intensity of the enzyme decreases distinctly
with increasing GuHCl concentrations, and the emission peaks appear red-shifted (from 339.4 to 360 nm). Changes in the conformation
and catalytic activity of the enzyme are compared. The extent of inactivation is greater than that of conformational changes,
indicating that the active site of the enzyme is more flexible than the whole enzyme molecule. The kinetics of inactivation
has been studied using the kinetic method of the substrate reaction. The rate constants of inactivation have been determined.
The value of k+0 is larger than that of k+0′ which suggests that the enzyme is protected by substrate to a certain extent during guanidine denaturation. 相似文献
19.
Patrick Masson Marie-Thérèse Froment Sultan Darvesh Lawrence M. Schopfer Oksana Lockridge 《Journal of enzyme inhibition and medicinal chemistry》2013,28(4):463-469
Albumin is generally regarded as an inert protein with no enzyme activity. However, albumin has esterase activity as well as aryl acylamidase activity. A new acetanilide substrate, o-nitrotrifluoroacetanilide (o-NTFNAC), which is more reactive than the classical o-nitroacetanilide, made it possible to determine the catalytic parameters for hydrolysis by fatty-acid free human serum albumin. Owing to the low enzymatic activity of albumin, kinetic studies were performed at high albumin concentration (0.075 mM). The albumin behavior with this substrate was Michaelis-Menten like. Kinetic analysis was performed according to the formalism used for catalysis at high enzyme concentration. This approach provided values for the turnover and dissociation constant of the albumin-substrate complex: kcat = 0.13 ± 0.02 min ? 1 and Ks = 0.67 ± 0.04 mM. MALDI-TOF experiments showed that unlike the ester substrate p-nitrophenyl acetate, o-NTFNAC does not form a stable adduct (acetylated enzyme). Kinetic analysis and MALDI-TOF experiments demonstrated that hydrolysis of o-NTFNAC by albumin is fully rate-limited by the acylation step (kcat = k2). Though the aryl acylamidase activity of albumin is low (kcat/Ks = 195 M? 1min? 1), because of its high concentration in human plasma (0.6–1 mM), albumin may participate in hydrolysis of aryl acylamides through second-order kinetics. This suggests that albumin may have a role in the metabolism of endogenous and exogenous aromatic amides, including drugs and xenobiotics. 相似文献
20.
Tim Smilda Anne H. Kamminga Peter Reinders Wia Baron Johan E.T. van Hylckama Vlieg Jaap J. Beintema 《Journal of molecular evolution》2001,52(5):457-466
Enzymic and structural studies on Drosophila alcohol dehydrogenases and other short-chain dehydrogenases/reductases (SDRs) are presented. Like alcohol dehydrogenases
from other Drosophila species, the enzyme from D. simulans is more active on secondary than on primary alcohols, although ethanol is its only known physiological substrate. Several
secondary alcohols were used to determine the kinetic parameters kcat and Km. The results of these experiments indicate that the substrate-binding region of the enzyme allows optimal binding of a short
ethyl side-chain in a small binding pocket, and of a propyl or butyl side-chain in large binding pocket, with stereospecificity
for R(−) alcohols. At a high concentration of R(−) alcohols substrate activation occurs. The kcat and Km values determined under these conditions are about two-fold, and two orders of magnitude, respectively, higher than those
at low substrate concentrations.
Sequence alignment of several SDRs of known, and unknown three-dimensional structures, indicate the presence of several conserved
residues in addition to those involved in the catalyzed reactions. Structural roles of these conserved residues could be derived
from observations made on superpositioned structures of several SDRs with known structures. Several residues are conserved
in tetrameric SDRs, but not in dimeric ones. Two halohydrin-halide-lyases show significant homology with SDRs in the catalytic
domains of these enzymes, but they do not have the structural features required for binding NAD+. Probably these lyases descend from an SDR, which has lost the capability to bind NAD+, but the enzyme reaction mechanisms may still be similar.
Received: 23 May 2000 / Accepted: 4 January 2001 相似文献