Crystal Structure Determination and Mutagenesis Analysis of the Ene Reductase NCR

The crystal structure of the “ene” nicotinamide‐dependent cyclohexenone reductase (NCR) from Zymomonas mobilis (PDB ID: 4A3U) has been determined in complex with acetate ion, FMN, and nicotinamide, to a resolution of 1.95 Å. To study the activity and enantioselectivity of this enzyme in the bioreduction of activated α,β‐unsaturated alkenes, the rational design methods site‐ and loop‐directed mutagenesis were applied. Based on a multiple sequence alignment of various members of the Old Yellow Enzyme family, eight single‐residue variants were generated and investigated in asymmetric bioreduction. Furthermore, a structural alignment of various ene reductases predicted four surface loop regions that are located near the entrance of the active site. Four NCR loop variants, derived from loop‐swapping experiments with OYE1 from Saccharomyces pastorianus, were analysed for bioreduction. The three enzyme variants, P245Q, D337Y and F314Y, displayed increased activity compared to wild‐type NCR towards the set of substrates tested. The active‐site mutation Y177A demonstrated a clear influence on the enantioselectivity. The loop‐swapping variants retained reduction efficiency, but demonstrated decreased enzyme activity compared with the wild‐type NCR ene reductase enzyme.     

Underexposed polar residues and protein stabilization

Increasing protein stability is interesting for practical reasons and because it tests our understanding of protein energetics. We explore here the feasibility of stabilizing proteins by replacing underexposed polar residues by apolar ones of similar size and shape. We have compared the stability of wild-type apoflavodoxin with that of a few carefully selected mutants carrying Y → F, Q → L, T → V or K → M replacements. Although a clear inverse correlation between native solvent exposures of replaced polar residues and stability of mutants is observed, most mutations fail to stabilize the protein. The promising exceptions are the two Q → L mutations tested, which characteristically combine the greatest reduction in polar burial with the greatest increase in apolar burial relative to wild type. Analysis of published stability data corresponding to a variety of mutant proteins confirms that, unlike Y → F or T → V replacements, Q → L mutations tend to be stabilizing, and it suggests that N → L mutations might be stabilizing as well. On the other hand, we show that the stability changes associated to the apoflavodoxin mutations can be rationalized in terms of differential polar and apolar burials upon folding plus a generic destabilizing penalty term. Simple equations combining these contributions predict stability changes in a large data set of 113 mutants (Y → F, Q → L or T → V) similarly well as more complex algorithms available on the Internet.     

Connecting Active‐Site Loop Conformations and Catalysis in Triosephosphate Isomerase: Insights from a Rare Variation at Residue 96 in the Plasmodial Enzyme

Despite extensive research into triosephosphate isomerases (TIMs), there exists a gap in understanding of the remarkable conjunction between catalytic loop‐6 (residues 166–176) movement and the conformational flip of Glu165 (catalytic base) upon substrate binding that primes the active site for efficient catalysis. The overwhelming occurrence of serine at position 96 (98 % of the 6277 unique TIM sequences), spatially proximal to E165 and the loop‐6 residues, raises questions about its role in catalysis. Notably, Plasmodium falciparum TIM has an extremely rare residue—phenylalanine—at this position whereas, curiously, the mutant F96S was catalytically defective. We have obtained insights into the influence of residue 96 on the loop‐6 conformational flip and E165 positioning by combining kinetic and structural studies on the PfTIM F96 mutants F96Y, F96A, F96S/S73A, and F96S/L167V with sequence conservation analysis and comparative analysis of the available apo and holo structures of the enzyme from diverse organisms.     

Role of Conserved Residues and F322 in the Extracellular Vestibule of the Rat P2X7 Receptor in Its Expression,Function and Dye Uptake Ability

Activation of the P2X7 receptor results in the opening of a large pore that plays a role in immune responses, apoptosis, and many other physiological and pathological processes. Here, we investigated the role of conserved and unique residues in the extracellular vestibule connecting the agonist-binding domain with the transmembrane domain of rat P2X7 receptor. We found that all residues that are conserved among the P2X receptor subtypes respond to alanine mutagenesis with an inhibition (Y51, Q52, and G323) or a significant decrease (K49, G326, K327, and F328) of 2′,3′-O-(benzoyl-4-benzoyl)-ATP (BzATP)-induced current and permeability to ethidium bromide, while the nonconserved residue (F322), which is also present in P2X4 receptor, responds with a 10-fold higher sensitivity to BzATP, much slower deactivation kinetics, and a higher propensity to form the large dye-permeable pore. We examined the membrane expression of conserved mutants and found that Y51, Q52, G323, and F328 play a role in the trafficking of the receptor to the plasma membrane, while K49 controls receptor responsiveness to agonists. Finally, we studied the importance of the physicochemical properties of these residues and observed that the K49R, F322Y, F322W, and F322L mutants significantly reversed the receptor function, indicating that positively charged and large hydrophobic residues are important at positions 49 and 322, respectively. These results show that clusters of conserved residues above the transmembrane domain 1 (K49–Y51–Q52) and transmembrane domain 2 (G326–K327–F328) are important for receptor structure, membrane expression, and channel gating and that the nonconserved residue (F322) at the top of the extracellular vestibule is involved in hydrophobic inter-subunit interaction which stabilizes the closed state of the P2X7 receptor channel.     

Effect of non-ionic detergents on apparent enzyme mechanism: V121A mutant of Streptomyces cholesterol oxidase endowed with enhanced sensitivity towards detergents

One of the mutants of Streptomyces cholesterol oxidase with the Val121Ala mutation (V121A) was kinetically analysed. Although the reaction rate-substrate concentration curve of wild type follows a simple Michaelis-Menten equation, that of V121A is sigmoidal. The cooperativity was apparent and caused by non-ionic detergents that were used as a solvent of cholesterol. The concentration dependence of V121A on detergents was more significant than that of wild type, although the reaction rates of both enzymes decrease as the concentrations of detergents increase. Further experiments suggested that less hydrophobic interactions between V121A and detergents should be responsible for the apparent cooperativity. Since Val121 is in a hydrophobic loop located near the active site, the mutational effect is structurally discussed.      

Characterization of functional residues in the interfacial recognition domain of lecithin cholesterol acyltransferase (LCAT)

Lecithin cholesterol acyltransferase (LCAT) is an interfacialenzyme active on both high-density (HDL) and low-density lipoproteins(LDL). Threading alignments of LCAT with lipases suggest thatresidues 50–74 form an interfacial recognition site andthis hypothesis was tested by site-directed mutagenesis. The(56–68) deletion mutant had no activity on any substrate.Substitution of W61 with F, Y, L or G suggested that an aromaticresidue is required for full enzymatic activity. The activityof the W61F and W61Y mutants was retained on HDL but decreasedon LDL, possibly owing to impaired accessibility to the LDLlipid substrate. The decreased activity of the single R52A andK53A mutants on HDL and LDL and the severer effect of the doublemutation suggested that these conserved residues contributeto the folding of the LCAT lid. The membrane-destabilizing propertiesof the LCAT 56–68 helical segment were demonstrated usingthe corresponding synthetic peptide. An M65N–N66M substitutiondecreased both the fusogenic properties of the peptide and theactivity of the mutant enzyme on all substrates. These resultssuggest that the putative interfacial recognition domain ofLCAT plays an important role in regulating the interaction ofthe enzyme with its organized lipoprotein substrates.     

Residue Asn277 Affects the Stability and Substrate Specificity of the SMG1 Lipase from Malassezia globosa

Thermostability and substrate specificity are important characteristics of enzymes for industrial application, which can be improved by protein engineering. SMG1 lipase from Malassezia globosa is a mono- and diacylglycerol lipase (MDL) that shows activity toward mono- and diacylglycerols, but no activity toward triacylglycerols. SMG1 lipase is considered a potential biocatalyst applied in oil/fat modification and its crystal structure revealed that an interesting residue-Asn277 may contribute to stabilize loop 273–278 and the 3₁₀4 helix which are important to enzyme characterization. In this study, to explore its role in affecting the stability and catalytic activity, mutagenesis of N277 with Asp (D), Val (V), Leu (L) and Phe (F) was conducted. Circular dichroism (CD) spectral analysis and half-life measurement showed that the N277D mutant has better thermostability. The melting temperature and half-life of the N277D mutant were 56.6 °C and 187 min, respectively, while that was 54.6 °C and 121 min for SMG1 wild type (WT). Biochemical characterization of SMG1 mutants were carried out to test whether catalytic properties were affected by mutagenesis. N277D had similar enzymatic properties as SMG1 WT, but N277F showed a different substrate selectivity profile as compared to other SMG1 mutants. Analysis of the SMG1 3D model suggested that N277D formed a salt bridge via its negative charged carboxyl group with a positively charged guanidino group of R227, which might contribute to confer N277D higher temperature stability. These findings not only provide some clues to understand the molecular basis of the lipase structure/function relationship but also lay the framework for engineering suitable MDL lipases for industrial applications.     

Enhancement of the substrate scope of transketolase

To enhance the activity of transketolase towards nonphosphorylated substrates and enlarge the scope of its substrates, notably to long polyol aldehyde acceptors (D-ribose or D-glucose), a rational design-supported evolution strategy was applied. By using docking experiments, an in silico library, and iterative mutagenesis, libraries of single- and double-point mutants were designed and generated. A double-screening approach was implemented, coupling a preselection activity assay (HPLC method) and a selective assay (GC method) to find the best enzymes. Several mutants (R526N, R526Q, R526Q/S525T, R526K/S525T) showed improved activities towards nonphosphorylated substrates as the coupled products of lithium hydroxypyruvate (HPA) with glycolaldehyde (GO), D-ribose or D-glucose. These mutated enzymes were further characterised. They were shown to be up to four times more active than the wild-type (mutant R526Q/S525T) for nonphosphorylated substrates LiHPA/GO (V(m) /K(m) for LiHPA = 92.4 instead of 28.8×10(-3) min(-1) for the wild-type) and 2.6 times more active for substrates LiHPA/rib.     

X-ray structures of two single-residue mutants of DNase I: H134Q and Y76A

The structures of the single-residue mutants H134Q and Y76Aof bovine pancreatic DNase I have been determined and refinedincluding data to 2.3 and 2.4 Å resolution respectively,by X-ray crystallography. H134 is an essential catalytic residue,while Y76 contributes to the binding of DNA by providing a largevan der Waals contact area that stabilizes the wide minor grooveseen in DNase I-DNA complexes. The mutant proteins, which showstrongly reduced activities of 0.001% (H134Q) and 0.3% (Y76A),were expressed in E.coli and both crystallize in space-groupC2 with almost identical unit cells. The crystal packing schemeis different from that found in wild type crystals grown undervery similar conditions, presumably due to the absence of thecarbohydrate moiety. In both mutants the conformation of theprotein is nearly identical to that of the wild type enzymeand changes are confined to surface loops involved in packing.The disruption of the hydrogen bonds between H134, E78 and Y76in both mutants leads to an increased mobility and positionalshifts in the DNA-binding loop, mainly around residue Y76. Thisin turn may further reduce DNA-binding affinity and, thus, contributeto the low activity. In contrast, symmetry contacts involvingresidues 97–108 lead to a stabilization of the flexibleloop compared to wild type DNase I.     

Effects of non-conservative changes to tyrosine 76, a key DNA binding residue of DNase I, on phosphodiester bond cleavage and DNA hydrolysis selectivity

Non-conservative changes, consisting of Y76E, Y76L, Y76Q and Y76W, have been made to tyrosine 76, one of the key DNA binding residues in DNase I. Normally Y76 inserts into the minor groove of DNA and makes an unusual, hydrophobic, stacking interaction with one of the sugars. All four mutants bind to DNA more tightly than the wild type, but cut it more slowly as assessed by Kunitz assays. This gives a rather small decrease in the specificity constants (Vmax/K(m)) for the hydrolysis of DNA, which is roughly paralleled by the loss of activity towards the non-DNA small chromophoric substrate, thymidine-3',5'-di(p- nitrophenyl)phosphate. These non-conservative mutants, therefore, show different behaviour to Y76A and Y76G, studied previously [Doherty A.J., Worrall A.F. and Connolly B.A. (1995) J: Mol. Biol., 251, 366-377]. These two mutants both bind to and cut DNA poorly, resulting in large decreases in Vmax/K(m) values. However, they showed little reduction in rates with the chromophoric substrate. It is likely that the altered side chains in the non-conservative mutants are still able to interact productively with the DNA and contribute to the observed DNA distortion that is essential for efficient catalysis. However, these mutations disrupt the active site, most probably by interference with the hydrogen bonded Y76-E78-H134 triad. H134 is a critical hydrolytic residue of DNase I that is essential for catalysis. The DNA cleavage selectivity of the Y76E, Y76L, Y76Q and Y76W mutants were little altered as compared with the wild-type enzyme as measured using the cutting patterns of a 160 base-pair Escherichia coli Tyr T promoter DNA fragment. This confirms earlier observations, with Y76F, Y76A and Y76G, that showed that this tyrosine has little role in DNA cleavage specificity.      

Combined enzyme and substrate design: grafting of a cooperative two-histidine catalytic motif into a protein targeted at the scissile bond in a designed ester substrate

A histidine-based, two-residue reactive site for the catalysis of hydrolysis of designed sulfonamide-containing para-nitrophenyl esters has been engineered into a scaffold protein. A matching substrate was designed to exploit the natural active site of human carbonic anhydrase II (HCAII) for well-defined binding. In this we took advantage of the high affinity between the active site zinc atom and sulfonamides. The ester substrate was designed to position the scissile bond in close proximity to the His64 residue in the scaffold protein. Three potential sites for grafting the catalytic His-His pair were identified, and the corresponding N62H/H64, F131H/V135H and L198H/P202H mutants were constructed. The most efficient variant, F131H/V135H, has a maximum k(cat)/K(M) value of approximately 14 000 M(-1) s(-1), with a k(cat) value that is increased by a factor of 3 relative to that of the wild-type HCAII, and by a factor of over 13 relative to the H64A mutant. The results show that an esterase can be designed in a stepwise way by a combination of substrate design and grafting of a designed catalytic motif into a well-defined substrate binding site.     

Identification of amino acid residues of abrin-a A chain is essential for catalysis and reassociation with abrin-a B chain by site-directed mutagenesis

Abrin is a toxic protein consisting of two subunits, an enzymatic A chain (ABRaA) and a lectin-active B chain (ABRaB), linked by a disulfide bond. Site-directed mutagenesis was performed using PCR to study how the conserved amino acid residues, Tyr74, Tyr113, Glu164 and Trp198, around the active site of ABRaA are involved in enzyme catalysis, enzyme-substrate recognition and reassociation of ABRaA with ABRaB. The protein biosynthesis inhibitory activities of Y74F, Y113F and W198F were decreased moderately to that of wild type reABRaA, while that of E164Q decreased dramatically. Kinetic analysis showed that the kat of Y74F, Y113F and W198F resembled that of wild type, while the Km increased significantly. W198F did not reassociate with ABRaB to form heterodimers, while Y74F, Y113F and E164Q did. SDS-PAGE analysis of ABRaA treated with trypsin showed that reABRaA, Y74F, Y113F and E164Q survived digestion, whereas W198F was not protected from digestion. CD spectra revealed that W198F showed significant conformational changes. These observations suggest that E164 is directly involved in catalysis, and Tyr74, Tyr113 and Trp198 in substrate binding, while Trp198 also plays an important role in maintaining the conformation of ABRaA required for its reassociation with ABRaB.      

Modulation of enzyme activity by antibody binding to an alkaline phosphatase-epitope hybrid protein

An epitope from the HTV-1 gpl20 protein V3 loop has been insertedonto the surface of bacterial alkaline phosphatase at differentpositions in the vicinity of the enzyme active site, creatinghybrid proteins that can bind to an anti-gpl20 monoclonal antibody.One of the hybrid proteins, API1, has a 13 amino acid V3 loopsequence inserted between residues 407 and 408 of alkaline phosphatase.The enzymatic activity of this protein is modulated upon antibodybinding. API1 maintains the full activity of the wild type alkalinephosphatase but in the presence of the anti-gpl20 antibody,the enzyme activity is inhibited by 40–50%. Thus, thehybrid enzyme can be used to detect the presence of antibodyin solution. The concept of signalling proteins may have a wideapplication. Two models for the mechanism of modulation, sterichindrance and allosteric regulation, are discussed.     

Directed evolution of a β-glycosidase from Agrobacterium sp. to enhance its glycosynthase activity toward C3-modified donor sugars

Glycans bearing modified hydroxyl groups are common in biology but because these modifications are added after assembly, enzymes are not available for the transfer and coupling of hydroxyl-modified monosaccharide units. Access to such enzymes could be valuable, particularly if they can also introduce 'bio-orthogonal tags'. Glycosynthases, mutant glycosidases that synthesize glycosides using glycosyl fluoride donors, are a promising starting point for creation of such enzymes through directed evolution. Inspection of the active site of a homology model of the GH1 Agrobacterium sp. β-glycosidase, which has both glucosidase and galactosidase activity, identified Q24, H125, W126, W404, E411 and W412 as amino acids that constrain binding around the 3-OH group, suggesting these residues as targets for mutation to generate an enzyme capable of handling 3-O-methylated sugars. Site-directed saturation mutagenesis at these positions within the wild-type β-glycosidase gene and screening via an on-plate assay yielded two mutants (Q24S/W404L and Q24N/W404N) with an improved ability to hydrolyze 4-nitrophenyl 3-O-methyl-β-d-galactopyranoside (3-MeOGal-pNP). Translation of these mutations into the evolved glycosynthase derived from the same glucosidase (2F6) yielded glycosynthases (AbgSL-T and AbgNN-T, where T denotes transferase) capable of forming 3-O-methylated glucosides on multi-milligram scales at rates approximately 5 and 40 times greater, respectively, than the parent glycosynthase.     

The role of loop 7 in mediating calcineurin regulation

Calcineurin (CN) is a heterodimer protein consisting of a 61kDa catalytic subunit A and a 19 kDa regulatory subunit B. Itplays a critical role in T-cell activation and is involved inmany cellular processes. Regulation of CN is rather complex,including a number of factors such as divalent metal ions (primarilyCa²⁺ and Mn²⁺), calmodulin (CaM) and autoinhibition (AI) segment.Previously, we reported that a loop 7 deletion mutant (V314)in subunit A exhibited high phosphatase activity, although themechanism for the surprising activity enhancement and whetherthe activity change applies to other loop 7 residues were notknown. In order to probe the role of loop 7, we have carriedout extensive mutagenesis experiments, followed by systematicactivity assays under a number of regulatory conditions. Allmutants, including single deletion mutants Y315, N316 and doubledeletion mutant V314Y315, showed increased phosphatase activity.Significantly, activities of the mutants containing the V314deletion, namely V314 and V314Y315, were no longer regulatedby regulatory subunit B. These results, along with the structureanalysis, suggest that loop 7 as a whole plays an importantrole in mediating CN’s regulation through bridging theregulatory subunit and catalytic core and interaction with theAI segment of CN. Received April 30, 2003; revised September 9, 2003; accepted September 12, 2003.     

Biased mutagenesis in the N-terminal region by degenerate oligonucleotide gene shuffling enhances secretory expression of barley alpha-amylase 2 in yeast

Recombinant barley alpha-amylase 1 (rAMY1) and 2 (rAMY2), despite 80% sequence identity, are produced in very different amounts of 1.1 and <0.05 mg/l, respectively, by Saccharomyces cerevisiae strain S150-2B. The low yield of AMY2 practically excludes mutational analysis of structure-function relationships and protein engineering. Since different secretion levels of AMY1/AMY2 chimeras were previously ascribed to the N-terminal sequence, AMY1 residues were combinatorially introduced at the 10 non-conserved positions in His14-Gln49 of AMY2 using degenerate oligonucleotide gene shuffling (DOGS) coupled with homologous recombination in S.cerevisiae strain INVSc1. Activity screening of a partial library of 843 clones selected six having a large halo size on starch plates. Three mutants, F21M/Q44H, A42P/A47S and A42P rAMY2, also gave higher activity than wild-type in liquid culture. Only A42P showed wild-type stability and enzymatic properties. The replacement is located to a beta-->alpha loop 2 that interacts with domain B (beta-->alpha loop 3) protruding from the catalytic (beta/alpha)(8)-barrel. Most remarkably Pichia pastoris strain GS115 secreted 60 mg/l A42P compared with 3 mg/l of wild-type rAMY2. The crystal structure of A42P rAMY2 was solved and found to differ marginally from the AMY2 structure, suggesting that the high A42P yield stems from stabilization of the mature and/or intermediate form owing to the introduced proline residue. Moreover, the G to C substitution for the A42P mutation might have a positive impact on protein translation.     

细胞色素P450 BM-3羟基化吲哚能力的半理性改造

为进一步改造细胞色素P450 BM-3酶对吲哚的羟基化能力,以P450 BM-3结构与功能关系的推测为指导,选择突变酶P450 BM-3 （A74G/F87V/L188Q/E435T）为父本,在可能影响P450 BM-3催化吲哚羟基化区域选择性的D168位点进行定点饱和突变,根据全细胞催化产物颜色及组成进行筛选,得到了产物组成、酶动力学性质与父本不同的两个突变酶。突变酶D168W的吲哚羟基化产物中90%是靛玉红,而另一个突变酶D168R的产物中87%是靛蓝,产物组成均不同于亲本。在催化吲哚羟基化时,D168W的k_cat与父本相当,但K_m却是父本的4.8倍,催化活力只有父本的20%;而D168R的k_cat是父本的1.9倍,K_m是父本的82%,催化活力比父本提高了1.37倍。结果表明,在E435T突变上叠加D168位氨基酸残基突变对酶的催化性质产生了单一位点突变所不具有的协同效应,对酶催化的区域选择性和催化活力都有显著影响,以致改变了催化产物组成。这种基于知识的半理性定向进化方法由于是在关键位点进行突变,因此突变目的性强、突变效果显著。     

Changes in activity of porcine phospholipase A2 brought about by charge engineering of a major structural element to alter stability

We have modified the stability of porcine phospholipase A₂ bycharge engineering. The mutations are situated at the N-terminalof a major helix and are N89D and N89D/E92Q. This engineeringhas significantly altered the activity of the enzyme to aggregatedand monomeric substrates. A N89D/E92K mutant is more stablebut considerably less active than wild type. An N89D mutantis more stable and of similar activity to wild type. The substantialchange in activity may be due to direct interaction of residue92 with aggregated substrate or may be via second calcium binding.Second calcium binding may be more probable as activity againstmonomers is also affected. Additional calcium binding may thereforebe an important way of manipulating the activity of phospholipaseA₂.     

Identification of four amino acid residues essential for catalysis in human cytidine deaminase by site-directed mutagenesis and chemical modifications

By site-directed mutagenesis on human cytidine deaminase (CDA), five mutant proteins were obtained: C65A, C99A, C102A, E67D and E67Q. The three cysteine mutants were completely inactive, whereas E67D and E67Q showed a specific activity about 200- and 200000-fold lower, respectively, than the wild-type CDA. Zinc analysis revealed that only E67D, E67Q and C65A contained 1 mol Zn2+/mol subunit as in the wild- type CDA. Kinetic measurements with the specific carboxylic group reagent N-ethoxy-carbonyl-2-ethoxy-1,2-dihydroquinoline performed on wild-type CDA suggest that Glu67 is essential for the catalytic process. Furthermore, when both native and denatured CDA was titrated with 5,5'-dithiobis(2-nitrobenzoic acid) six sulfhydryl groups were detected, whereas in the denatured and reduced enzyme nine such groups were found, according to the sequence data. When p-hydroxymercuriphenyl sulfonate was used, nine sulfhydryl groups were detectable and the release of 1 mol of zinc per mole of CDA subunit was revealed by the metal indicator dye 4-(2-pyridylazo)resorcinol. It seems plausible that the limiting step for the maintenance of zinc in the active site is the formation of coordination between Cys99 and Cys102, whereas Cys65 could lead the zinc to the correct position and orientation within the active site.      

Enhancing the Activity of Glutamate Decarboxylase from Lactobacillus brevis by Directed Evolution

Glutamate decarboxylase (GAD, EC4.1.1.15) can catalyze the decarboxylation of l-glutamate to form γ-aminobutyrate (GABA), which is in great demand in some foods and pharmaceuticals. In our previous study, gad, the gene coding glutamate decarboxylase from Lactobacillus brevis CGMCC 1306, was cloned and its soluble expression was realized. In this study, error-prone PCR was conducted to improve its activity, followed by a screening. Mutant Q51H with high activity [55.4 mmol·L^− 1·min^− 1·(mg protein)^− 1, 120% higher than that of the wild type at pH 4.8] was screened out from the mutant library. In order to investigate the potential role of this site in the regulation of enzymatic activity, site-directed saturation mutagenesis at site 51 was carried out, and three specific mutants, N-terminal truncated GAD, Q51P, and Q51L, were identified. The kinetic parameters of the three mutants and Q51H were characterized. The results reveal that aspartic acid at site 88 and N-terminal domain are essential to the activity as well as correct folding of GAD. This study not only improves the activity of GAD, but also sheds new light on the structure–function relationship of GAD.