Crystal versus solution structures of enzymes: NMR spectroscopy of a crystalline serine protease

The hydrogen-bonding status of His57 in the catalytic triad (Asp-His-Ser) of serine protease has important mechanistic implications for this class of enzymes. Recent nitrogen-15 nuclear magnetic resonance (NMR) studies of alpha-lytic protease find His57 and Ser195 to be strongly hydrogen-bonded, a result that conflicts with the corresponding crystallographic studies, thereby suggesting that the crystal and solution structures may differ. This discrepancy is addressed and resolved in a nitrogen-15 NMR study of the enzyme in the crystalline state. The results show that the His-Ser and Asp-His interactions are identical in crystals and solutions, but that in crystals His57 titrates with a pKa of 7.9, nearly one pKa unit higher than in solution. This elevated pKa accounts for the absence of the His-Ser hydrogen bond in previous x-ray studies.     

Engineering enzyme specificity by "substrate-assisted catalysis"

A novel approach to engineering enzyme specificity is presented in which a catalytic group from an enzyme is first removed by site-directed mutagenesis causing inactivation. Activity is then partially restored by substrates containing the missing catalytic functional group. Replacement of the catalytic His with Ala in the Bacillus amyloliquefaciens subtilisin gene (the mutant is designated His64Ala) by site-directed mutagenesis reduces the catalytic efficiency (kcat/Km) by a factor of a million when assayed with N-succinyl-L-Phe-L-Ala-L-Ala-L-Phe-p-nitroanilide (sFAAF-pNA). Model building studies showed that a His side chain at the P2 position of a substrate bound at the active site of subtilisin could be virtually superimposed on the catalytic His side chain of this serine protease. Accordingly, the His64Ala mutant hydrolyzes a His P2 substrate (sFAHF-pNA) up to 400 times faster than a homologous Ala P2 or Gln P2 substrate (sFAAF-pNA or sFAQF-pNA) at pH 8.0. In contrast, the wild-type enzyme hydrolyzes these three substrates with similar catalytic efficiencies. Additional data from substrate-dependent pH profiles and hydrolysis of large polypeptides indicate that the His64Ala mutant enzyme can recover partially the function of the lost catalytic histidine from a His P2 side chain on the substrate. Such "substrate-assisted catalysis" provides a new basis for engineering enzymes with very narrow and potentially useful substrate specificities. These studies also suggest a possible functional intermediate in the evolution of the catalytic triad of serine proteases.     

棉铃虫羧酸酯酶基因cDNA的克隆、表达及活性分析

 【目的】克隆棉铃虫中肠羧酸酯酶基因,了解该酶的分子特性,为研究棉铃虫抗药性机理奠定基础。【方法】制备抗血清,对棉铃虫(Helicoverpa armigera (Hübner))中肠cDNA表达文库进行免疫筛选,得到编码棉铃虫羧酸酯酶基因的cDNA克隆,利用生物信息学软件和网站进行序列分析,构建原核表达载体进行表达,并以α-醋酸萘酯溶液为底物进行活性测定。【结果】测序分析表明,羧酸酯酶基因hc3(GenBank登录号：GU119888)全长2 896 bp,编码795个氨基酸。羧酸酯酶HC3的等电点pI为3.94,活性中心包括3个氨基酸残基(催化三联体)：Ser204, Glu331, His444;第545—773位包含大量高度重复的Gly,Asp、Glu(GDE区域)构成亲水区。该基因在大肠杆菌BL21中成功表达约100 kD的HC3蛋白,检测表达的羧酸酯酶活性为0.32 mmol/100 μL酶液。【结论】成功克隆、表达了棉铃虫羧酸酯酶蛋白HC3,并发现GDE亲水结构域。为进一步了解羧酸酯酶的三维结构及其水解作用并设计新型杀虫剂提供了可能。     

The three-dimensional structure of Asn102 mutant of trypsin: role of Asp102 in serine protease catalysis

The structure of the Asn102 mutant of trypsin was determined in order to distinguish whether the reduced activity of the mutant at neutral pH results from an altered active site conformation or from an inability to stabilize a positive charge on the active site histidine. The active site structure of the Asn102 mutant of trypsin is identical to the native enzyme with respect to the specificity pocket, the oxyanion hole, and the orientation of the nucleophilic serine. The observed decrease in rate results from the loss of nucleophilicity of the active site serine. This decreased nucleophilicity may result from stabilization of a His57 tautomer that is unable to accept the serine hydroxyl proton.     

大豆蚜羧酸酯酶基因AgCarE的克隆、表达及活性分析

【目的】克隆大豆蚜羧酸酯酶基因,了解该酶的分子特性,为研究大豆蚜抗药性机理奠定基础。【方法】利用RT-PCR和RACE技术,克隆了1个大豆蚜羧酸酯酶基因的全长cDNA序列,命名为AgCarE,利用生物信息学软件及网上工具进行序列分析,以pET-21b为载体构建原核表达系统进行表达,并以α-醋酸萘酯溶液为底物进行活性测定。【结果】测序分析表明,羧酸酯酶基因AgCarE（GenBank登录号：JF970181）全长1 946 bp,编码526个氨基酸。羧酸酯酶AgCarE的等电点（pI）为6.08,蛋白活性中心包括3个氨基酸残基（催化三联体）：Ser186、Glu313、His434,5个N-联糖基化位点,具备羧酸酯酶的结构特征,属羧酸酯酶家族（EC: 3.1.1.-）。将该基因编码序列与pET-21b载体重组,经IPTG诱导表达HIS标签融合蛋白、SDS-PAGE分析和Western-blot检测,在大肠杆菌BL21中成功表达约59 kD的AgCarE蛋白,以α-醋酸萘酯为底物,检测表达的羧酸酯酶活性为0.036 mmol/100 μL酶液。【结论】成功克隆、表达了大豆蚜羧酸酯酶蛋白AgCarE,以α-醋酸萘酯为底物,检测表达的羧酸酯酶活性为0.036 mmol/100 μL酶液,有助于进一步了解羧酸酯酶的结构特征及水解作用,为设计研发新型杀虫剂提供可能。     

Generation of a catalytic antibody by site-directed mutagenesis

A hybrid Fv fragment of the dinitrophenyl-binding immunoglobulin A (IgA), MPOC315, has been generated by reconstituting a recombinant variable light chain (VL) produced in Escherichia coli with a variable heavy chain (VH) derived from the antibody. The Tyr34 residue of VL was substituted by His in order to introduce a catalytic imidazole into the combining site for the ester hydrolysis. The His mutant Fv accelerated the hydrolysis of the 7-hydroxycoumarin ester of 5-(2,4-dinitrophenyl)-aminopentanoic acid 90,000-fold compared to the reaction with 4-methyl imidazole at pH 6.8 and had an initial rate that was 45 times as great as that for the wild-type Fv. The hydrolyses of aminopropanoic and aminohexanoic homologs were not significantly accelerated. Thus a single deliberate amino acid change can introduce significant catalytic activity into an antibody-combining site, and chemical modification data can be used to locate potential sites for the introduction of catalytic residues.     

Functional role of aspartic acid-27 in dihydrofolate reductase revealed by mutagenesis

The crystal structures and enzymic properties of two mutant dihydrofolate reductases (Escherichia coli) were studied in order to clarify the functional role of an invariant carboxylic acid (aspartic acid at position 27) at the substrate binding site. One mutation, constructed by oligonucleotide-directed mutagenesis, replaces Asp27 with asparagine; the other is a primary-site revertant to Ser27. The only structural perturbations involve two internally bound water molecules. Both mutants have low but readily measurable activity, which increases rapidly with decreasing pH. The mutant enzymes were also characterized with respect to relative folate: dihydrofolate activities and kinetic deuterium isotope effects. It is concluded that Asp27 participates in protonation of the substrate but not in electrostatic stabilization of a positively charged, protonated transition state.     

Redesigning trypsin: alteration of substrate specificity

A general method for modifying eukaryotic genes by site-specific mutagenesis and subsequent expression in mammalian cells was developed to study the relation between structure and function of the proteolytic enzyme trypsin. Glycine residues at positions 216 and 226 in the binding cavity of trypsin were replaced by alanine residues, resulting in three trypsin mutants. Computer graphic analysis suggested that these substitutions would differentially affect arginine and lysine substrate binding of the enzyme. Although the mutant enzymes were reduced in catalytic rate, they showed enhanced substrate specificity relative to the native enzyme. This increased specificity was achieved by the unexpected differential effects on the catalytic activity toward arginine and lysine substrates. Mutants containing alanine at position 226 exhibited an altered conformation that may be converted to a trypsin-like structure upon binding of a substrate analog.     

龙牙百合花氨基酸含量的柱前衍生OPA-HPLC法测定

[目的]测定龙牙百合花中的氨基酸含量。[方法]以邻苯二甲醛（OPA）与9-芴基羰酰氯（FMOC—C1）作衍生剂,利用VWD检测器可变波长检测程序,采用柱前衍生OPA—HPLC法测定邵阳龙牙百合花中的17种氨基酸含量。[结果]邵阳龙牙百合花中含有苏氨酸、丝氨酸、脯氨酸、蛋氨酸、谷氨酸、甘氨酸、亮氨酸、苯丙氨酸、酪氨酸、天门冬氨酸、丙氨酸、组氨酸、半胱氨酸、异亮氨酸等17种氨基酸。其中必需氨基酸的含量为49．7mg／g,占氨基酸总量的31．20％;儿童必需氨基酸的含量为30．6mg／g,占氨基酸总量的19．20％;含量最高的是谷氨酸（28．7mg／g,占氨基酸总量的18．03％）;其次是精氨酸（27．5mg／g,占氨基酸总量的17．30％）。[结论]该方法操作简便,灵敏度高,结果准确,可靠。龙牙百合花中氨基酸的种类齐全,含量丰富,具有很高的开发利用价值。     

珍珠薏米保健饮料的研制

以珍珠和薏米为主要原料,经过酶解、过滤、调配等工艺研制品味纯正的珍珠薏米饮料.通过正交试验,确定最佳调配组合为:蔗糖5％,果汁20％,珍珠、薏米的比例为1∶1.对饮料的可溶性固形物、酸度、还原糖、金属元素、氨基酸含量等指标进行测定,结果为还原糖含量:1.4 mg· mL-1;可溶性固形物:3.2％;pH值:5.48(1...     

酸酐修饰脂肪酶的研究

[目的]研究酸酐修饰对脂肪酶催化性能的作用。[方法]采用琥珀酸酐、丙酸酐和乙酸酐对Novozym 435脂肪酶进行了修饰,优化修饰条件,并对修饰后Novozym 435的催化性能进行测定。[结果]酸酐修饰可显著提高Novozym 435脂肪酶的水解活性。琥珀酸酐修饰Novozym 435酶活为79.3 U/g,相对于未修饰酶酶活（63.9 U/g）,提高了24%;丙酸酐修饰后酶活为85.6 U/g,提高了33.9%;而乙酸酐修饰后酶活为84.0 U/g,提高了31.4%。而且酸酐修饰后酶的热稳定性、pH稳定性和耐有机溶剂的能力也都有很大提高。[结论]酸酐修饰是一种有效的提高脂肪酶催化性能的重要方法。     

猪胰脏中胰酶的制备新工艺技术研究

以猪胰脏为原料,采用单因素试验研究胰酶制备的不同工艺路线、不同提取溶剂、提取激活温度、时间、pH和沉淀剂种类对三酶活力和收率及其他品质的影响,并通过正交试验优化得胰酶制备的工艺条件为:提取溶剂为25%乙醇(原料量的2倍,V·m-2),保护剂为1.0%氯化钠和1.0%甘油,激活剂为0.2%CaCl2和1.0%胰酶原粉;提...     

壳聚糖固定化胰蛋白酶研究

以自制的壳聚糖为载体,以戊二醛为交联剂,制备了壳聚糖固定化胰蛋白酶。经测定该酶活性回收率达到72.7%,操作半衰期为12 d,表观米氏常数为5.88 mg/ml,最适pH值为8.0,最适温度为40℃;与游离胰蛋白酶相比,酶的耐热性和操作半衰期有明显提高,但表观米氏常数也略有增大。     

动物抗凝血酶Ⅲ和褐藻糖胶结合的结构模拟

褐藻糖胶是一种含有硫酸基的水溶性杂多糖,其抗凝血与抗血栓等生理机能与肝素类似。用分子动力学计算,模拟抗凝血酶Ⅲ结合褐藻糖胶的结构变化。抗凝血酶Ⅲ和褐藻糖胶结合时Arg393从抗凝血酶Ⅲ的内部激发到了分子表面,Arg393与凝血酶活性中心的Asp、His、Ser结合,继而进入到四面体的过渡状态。     

The structure of a complex of recombinant hirudin and human alpha-thrombin

The crystallographic structure of a recombinant hirudin-thrombin complex has been solved at 2.3 angstrom (A) resolution. Hirudin consists of an NH2-terminal globular domain and a long (39 A) COOH-terminal extended domain. Residues Ile1 to Tyr3 of hirudin form a parallel beta-strand with Ser214 to Glu217 of thrombin with the nitrogen atom of Ile1 making a hydrogen bond with Ser195 O gamma atom of the catalytic site, but the specificity pocket of thrombin is not involved in the interaction. The COOH-terminal segment makes numerous electrostatic interactions with an anion-binding exosite of thrombin, whereas the last five residues are in a helical loop that forms many hydrophobic contacts. In all, 27 of the 65 residues of hirudin have contacts less than 4.0 A with thrombin (10 ion pairs and 23 hydrogen bonds). Such abundant interactions may account for the high affinity and specificity of hirudin.     

Enhanced activity and altered specificity of phospholipase A2 by deletion of a surface loop

Protein engineering and x-ray crystallography have been used to study the role of a surface loop that is present in pancreatic phospholipases but is absent in snake venom phospholipases. Removal of residues 62 to 66 from porcine pancreatic phospholipase A2 does not change the binding constant for micelles significantly, but it improves catalytic activity up to 16 times on micellar (zwitterionic) lecithin substrates. In contrast, the decrease in activity on negatively charged substrates is greater than fourfold. A crystallographic study of the mutant enzyme shows that the region of the deletion has a well-defined structure that differs from the structure of the wild-type enzyme. No structural changes in the active site of the enzyme were detected.     

A mutation in the catalytic subunit of cAMP-dependent protein kinase that disrupts regulation

A mutant catalytic subunit of adenosine 3',5'-monophosphate (cAMP)-dependent protein kinase has been isolated from Saccharomyces cerevisiae that is no longer subject to regulation yet retains its catalytic activity. Biochemical analysis of the mutant subunit indicates a 100-fold decreased affinity for the regulatory subunit. The mutant catalytic subunit exhibits approximately a threefold increase in Michaelis constant for adenosine triphosphate and peptide cosubstrates, and is essentially unchanged in its catalytic rate. The nucleotide sequence of the mutant gene contains a single nucleotide change resulting in a threonine-to-alanine substitution at amino acid 241. This residue is conserved in other serine-threonine protein kinases. These results identify this threonine as an important contact between catalytic and regulatory subunits but only a minor contact in substrate recognition.     

Regulation of an enzyme by phosphorylation at the active site

The isocitrate dehydrogenase of Escherichia coli is an example of a ubiquitous class of enzymes that are regulated by covalent modification. In the three-dimensional structure of the enzyme-substrate complex, isocitrate forms a hydrogen bond with Ser113, the site of regulatory phosphorylation. The structures of Asp113 and Glu113 mutants, which mimic the inactivation of the enzyme by phosphorylation, show minimal conformational changes from wild type, as in the phosphorylated enzyme. Calculations based on observed structures suggest that the change in electrostatic potential when a negative charge is introduced either by phosporylation or site-directed mutagenesis is sufficient to inactivate the enzyme. Thus, direct interaction at a ligand binding site is an alternative mechanism to induced conformational changes from an allosteric site in the regulation of protein activity by phosphorylation.     

庄河产薏苡仁中氨基酸含量的测定

按照国标GB/T 5009.124-2003方法处理样品,对庄河产薏苡仁中氨基酸的含量进行了测定。结果表明,庄河产薏苡仁中含有16种氨基酸,Asp、Thr、Ser、Glu、Gly、Ala、Val、Met、Ile、Leu、Tyr、Phe、Lys、His、Arg、Pro的含量分别为1 350.0、252.0、349.0、1 480.0、214.0、699.0、297.0、84.3、181.0、544.0、169.0、215.0、208.0、170.0、456.0、846.0μg/kg。