Isolation, characterization and inhibition by acarbose of the alpha-amylase from Lactobacillus fermentum: comparison with Lb. manihotivorans and Lb. plantarum amylases.

Extracellular alpha-amylase from Lactobacillus fermentum (FERMENTA) was purified by glycogen precipitation and ion exchange chromatography. The purification was approximately 28-fold with a 27% yield. The FERMENTA molecular mass (106,000 Da) is in the same range as the ones determined for L. amylovorus (AMYLOA), L. plantarum (PLANTAA) and L. manihotivorans (MANIHOA) alpha-amylases. The amino acid composition of FERMENTA differs from the other lactobacilli considered here, but however, indicates that the peptidic sequence contains two equal parts: the N-terminal catalytic part; and the C-terminal repeats. The isoelectric point of FERMENTA, PLANTAA, MANIHOA are approximately the same (3.6). The FERMENTA optimum pH (5.0) is slightly more acidic and the optimum temperature is lower (40 degrees C). Raw starch hydrolysis catalyzed by all three amylases liberates maltotriose and maltotretaose. Maltose is also produced by FERMENTA and MANIHOA. Maltohexaose FERMENTA catalyzed hydrolysis produces maltose and maltotriose. Finally, kinetics of FERMENTA, PLANTAA and MANIHOA using amylose as a substrate and acarbose as an inhibitor, were carried out. Statistical analysis of kinetic data, expressed using a general velocity equation and assuming rapid equilibrium, showed that: (1) in the absence of inhibitor k(cat)/Km are, respectively, 1x10(9), 12.6x10(9) and 3.2x10(9) s(-1) M(-1); and (2) the inhibition of FERMENTA is of the mixed non-competitive type (K(1i)=5.27 microM; L(1i)=1.73 microM) while the inhibition of PLANTAA and MANIHOA is of the uncompetitive type (L(1i)=1.93 microM and 1.52 microM, respectively). Whatever the inhibition type, acarbose is a strong inhibitor of these Lactobacillus amylases. These results indicate that, as found in porcine and barley amylases, Lactobacillus amylases contain in addition to the active site, a soluble carbohydrate (substrate or product) binding site.

Starch digestion in tropical fishes: isolation, structural studies and inhibition kinetics of alpha-amylases from two tilapias Oreochromis niloticus and Sarotherodon melanotheron.

alpha-Amylases from the intestinal cavity of two tilapia species, Oreochromis niloticus (ONI-AMY) and Sarotherodon melanotheron (SME-AMY), were purified using ammonium sulfate precipitation, affinity chromatography and chromatofocusing procedures. The purification was approximately 100-fold. The amylolytic activity, specific activity, product distribution, pH and temperature profile of ONI-AMY and SME-AMY are quite similar. The molecular mass differs slightly: 56600 (ONI-AMY) vs. 55500 (SME-AMY). As shown by isoelectric focusing analysis, both amylases contain two isoforms A and B with distinct pI: 7.2 (A) and 7.8 (B), vs. 8.3 (A) and 8.8 (B), respectively. It was not possible to isolate B, since B converts into A with time. The kinetics of the inhibition of ONI-AMY and SME-AMY activity by alpha-, beta- and gamma-cyclodextrin (alpha-, beta- and gamma-CD) were investigated using amylose as the substrate. Statistical analysis of the kinetic data expressed using a general velocity equation and assuming rapid equilibrium showed that the inhibition is of the mixed noncompetitive type. Similar results were obtained with ONI-AMY and SME-AMY. beta- and gamma-CD are stronger inhibitors than alpha-CD. ONI-AMY and SME-AMY are then closely related and show the general features common to the members of the alpha-amylase class (family 13). They enable ONI and SME tilapias to digest starch in food.

Mechanism of porcine pancreatic alpha-amylase. Inhibition of amylose and maltopentaose hydrolysis by alpha-, beta- and gamma-cyclodextrins.

The effects of alpha-, beta- and gamma-cyclodextrins on the amylose and maltopentaose hydrolysis catalysed by porcine pancreatic alpha-amylase (PPA) were investigated. The results of the statistical analysis performed on the kinetic data using the general initial velocity equation of a one-substrate reaction in the presence of one inhibitor indicate that the type of inhibition involved depends on the substrate used: the inhibition of amylose hydrolysis by alpha-, beta- and gamma-cyclodextrin is of the competitive type, while the inhibition of maltopentaose hydrolysis is of the mixed noncompetitive type. Consistently, the Lineweaver-Burk plots intersect on the vertical axis when amylose is used as the substrate, while in the case of maltopentaose, the intersection occurs at a point located in the second quadrant. The inhibition of the hydrolysis therefore involves only one abortive complex, PPA-cyclodextrin, when amylose is used as the substrate, while two abortive complexes, PPA-cyclodextrin and PPA-maltopentaose-cyclodextrin, are involved with maltopentaose. The mixed noncompetitive inhibition thus shows the existence of one accessory binding site. In any case, only one molecule of inhibitor binds to PPA. In line with these findings, the difference spectra of PPA produced by alpha-, beta- and gamma-cyclodextrin indicate that binding occurs at a tryptophan and a tyrosine residue. The corresponding dissociation constants and the inhibition constants obtained using the kinetic approach are in the same range (1.2-7 mM). The results obtained here on the inhibition of maltopentaose hydrolysis by cyclodextrin are similar to those previously obtained with acarbose as the inhibitor [Alkazaz, M., Desseaux, V., Marchis-Mouren, G., Prodanov, E. & Santimone, M. (1998) Eur. J. Biochem. 252, 100-107], but differ from those obtained with amylose as the substrate and acarbose as inhibitor [Alkazaz, M., Desseaux, V., Marchis-Mouren, G., Payan, F., Forest, E. & Santimone, M. (1996) Eur. J. Biochem. 241, 787-796]. It is concluded that the hydrolysis of both long and short chain substrates requires at least one secondary binding site, including a tryptophan residue.

Mechanism of porcine pancreatic alpha-amylase inhibition of amylose and maltopentaose hydrolysis by kidney bean (Phaseolus vulgaris) inhibitor and comparison with that by acarbose.

The effects of Phaseolus vulgaris inhibitor (alpha-AI) on the amylose and maltopentaose hydrolysis catalysed by porcine pancreatic alpha-amylase (PPA) were investigated. Based on a statistical analysis of the kinetic data and using the general velocity equation, which is valid at equilibrium for all types of inhibition in a single-substrate reaction, it was concluded that the inhibitory mode is of the mixed noncompetitive type involving two molecules of inhibitor. In line with this conclusion, the Lineweaver-Burk primary plots intersect in the second quadrant and the secondary plots of the slopes and the intercepts versus the inhibitor concentrations are parabolic curves, whether the substrate used was amylose or maltopentaose. A specific inhibition model of the mixed noncompetitive type applies here. This model differs from those previously proposed for acarbose [Al Kazaz, M., Desseaux, V., Marchis-Mouren, G., Payan, F., Forest, E. & Santimone, M. (1996) Eur. J. Biochem. 241, 787-796 and Al Kazaz, M., Desseaux, V., Marchis-Mouren, G., Prodanov, E. & Santimone, M. (1998) Eur. J. Biochem. 252, 100-107]. In particular, with alpha-AI, the inhibition takes place only when PPA and alpha-AI are preincubated together before the substrate is added. This shows that the inhibitory PPA-alphaAI complex is formed during the preincubation period. Secondly, other inhibitory complexes are formed, in which two molecules of inhibitor are bound to either the free enzyme or the enzyme-substrate complex. The catalytic efficiency was determined both with and without inhibitor. Using the same molar concentration of inhibitor, alpha-AI was found to be a much stronger inhibitor than acarbose. However, when the inhibitor amount is expressed on a weight basis (mg x L-1), the opposite conclusion is drawn. In addition, limited proteolysis was performed on PPA alone and on the alpha-AI-PPA complex. The results show that, in the complex, PPA is more sensitive to subtilisin attack, and shorter fragments are obtained. These data reflect the conformational changes undergone by PPA as the result of the protein inhibitor binding, which differ from those previously observed with acarbose.

Molecular cloning and primary structure analysis of porcine pancreatic alpha-amylase.

A cDNA library was constructed in a Uni-ZAP XR vector using mRNA isolated from porcine pancreas. A full-length alpha-amylase cDNA was obtained using a combination of library screening and nested polymerase chain reaction. Sequencing of the clone revealed a 1536-nucleotide (nt) open reading frame encoding a protein of 496 amino acid (aa) residues with a signal peptide of 15 aa. The calculated molecular mass of the enzyme was 55354 Da, in accordance with those of the purified porcine pancreatic alpha-amylase forms (PPAI and PPAII) as determined by mass spectrometry. A comparison of the deduced aa sequence with published peptidic sequences of PPAI identified a number of mismatches. The sequence of the cDNA reported here provides a sequence reference for PPA in excellent agreement with the refined three-dimensional structures of both PPAI and PPAII. No evidence for a second variant was found in the cDNA library and it is most likely that PPAI and PPAII are two forms of the same protein. The primary structure of PPA shows high homology with human, mouse and rat pancreatic alpha-amylases. The 304-310 region, corresponding to a mobile loop involved in substrate binding and processing near the active site, is fully conserved.

The human pancreatic alpha-amylase isoforms: isolation, structural studies and kinetics of inhibition by acarbose.

A rapid method is proposed for isolating the two main components of human pancreatic alpha-amylase (HPA I and HPA II). The isoelectric point of HPA I (7.2), the main component, was determined using an isoelectrofocusing method and found to differ from that of HPA II (6. 6). The molecular mass of HPA I (55862+/-5 Da) and that of HPA II (55786+/-5 Da) were determined by performing mass spectrometry and found to be quite similar to that of the protein moiety calculated from the amino acid sequence (55788 Da), which indicates that the human amylase is not glycosylated. The structure of both HPA I and HPA II was further investigated by performing limited proteolysis. Two fragments with an apparent molecular mass of 41 kDa and 14 kDa were obtained by digesting the isoforms with proteinase K and subtilisin, whereas digestion with papain yielded two cleaved fragments with molecular masses of 38 kDa and 17 kDa. Proteinase K and subtilisin susceptible bonds are located in the L8 loop (A domain), while the papain cut which occurs in the presence of the calcium chelator EDTA is in the L3 loop (B domain). The kinetics of the inhibition of HPA I and HPA II by acarbose, a drug used to treat diabetes and obesity, were studied using an amylose substrate. The Lineweaver-Burk primary plots of HPA I and HPA II, which did not differ significantly, indicated that the inhibition was of the mixed non-competitive type. The secondary plots gave parabolic curves. All in all, these data provide evidence that two acarbose molecules bind to HPA. In conclusion, apart from the pI, no significant differences were observed between HPA I and HPA II as regards either their molecular mass and limited proteolysis or their kinetic behavior. As was to be expected in view of the high degree of structural identity previously found to exist between human and porcine pancreatic amylases, the present data show that the inhibitory effects of acarbose on the kinetic behavior of these two amylases are quite comparable. In particular, the process of amylose hydrolysis catalyzed by HPA as well as by PPA in both cases requires two carbohydrate binding sites in addition to the catalytic site.

The mechanism of porcine pancreatic alpha-amylase. Inhibition of maltopentaose hydrolysis by acarbose, maltose and maltotriose.

A kinetic study was carried out on the inhibitory effects of acarbose, maltose, and maltotriose on porcine pancreatic alpha-amylase (PPA), using maltopentaose as the substrate. Lineweaver-Burk plots showed that the inhibitory action of acarbose is of the mixed non-competitive type. The secondary plots gave straight lines. A model involving abortive complexes accounts for these results. Dixon plot analysis led to the same conclusion. According to the proposed model, one molecule of acarbose per amylase molecule binds either directly to free enzyme at the active site or to the enzyme-substrate complex at a secondary carbohydrate-binding site, which becomes functional after the substrate has bound to the enzyme molecule at the active site. Kinetic analysis of the inhibition exerted by either the maltose or maltotriose reaction products of maltopentaose hydrolysis were then performed. The inhibitory effect of maltose was found to be of the non-competitive type, while that of maltotriose was competitive. It can therefore be concluded that the first reaction product to be released upon maltopentaose hydrolysis is maltose, and that the second product is maltotriose. This indicates that after hydrolysis of the maltopentaose chain, the reducing side fragment is released first.

Inhibition of reduced DP18-maltodextrin and amylose hydrolysis by acarbose: kinetic studies.

Kinetics of inhibition of porcine pancreatic alpha-amylase by acarbose were performed using maltodextrin and amylose as substrates. Similar Lineweaver-Burk primary plots were obtained. Two mixed non-competitive models are proposed. X-ray crystallographic data (Qian, M., Buisson, G., Duée, E., Haser, R. and Payan, F. Biochemistry, 1994; 33: 6284-6294) are in support of the mixed non-competitive inhibition model which involves abortive complexes. Secondary plots are different indicating that in the maltodextrin hydrolysis, one molecule of acarbose is bound per amylase molecule, while using amylose as substrate two molecules of acarbose are bound. These two kinetically determined binding sites might correspond to the two surface sites shown by X-ray crystallography (Qian, M., Haser, R. and Payan, F. Protein Science 1995; 4: 747-755).

Comparative quantitative analysis of sucrose and related compounds using ion exchange and reverse phase chromatographic methods.

Two analytical methods of sugar determination, namely ion exchange chromatography on an anionic resin coupled with electrochemical detection, and reverse phase chromatography on Nucleosil-NH2 resin equipped with a light scattering detector were tested and compared as regards their rapidity, sensitivity and accuracy with sucrose, fructose, glucose, raffinose, maltose, arabinose, fucose, rhamnose and xylose. Excellent resolution and highly reproducible results were obtained in both cases. Greater sensitivity up to the picomolar range was possible however only with ion exchange chromatography. Reverse phase chromatography was successfully applied to the time course of sucrose hydrolysis under chemical (acid) and enzymatic (invertase) conditions. The hydrolysis was monitored by determining sucrose degradation and the corresponding formation of glucose and fructose.

A new kinetic model for the mode of action of soluble and membrane-immobilized glutathione peroxidase from bovine erythrocytes--effects of selenium.

Kinetic studies on the oxidative reaction of glutathione by hydrogen peroxide were performed using soluble and membrane-bound ox erythrocyte glutathione peroxidase of various types. The effects of organic and inorganic selenium on the glutathione peroxidase activity were also examined. The kinetic behaviour of the enzyme was investigated using a coupled reaction within a relatively large range of hydrogen peroxide and glutathione concentrations. Non-parallel double-reciprocal plots were obtained which suggested that a sequential ordered rather than a ping-pong mechanism was involved. Similar results were obtained with soluble and membrane-bound enzyme, whatever the type of crosslinking used. Crosslinking was performed on a nylon support using various alkylating agents and bifunctional molecules. With all three types of immobilized enzyme thus obtained, a slight but significant increase in the Km was observed. The effects of selenium were then studied. Using soluble enzyme, a slight increase in the activity was observed in the presence of inorganic selenium (sodium selenite) but not with organic selenium (seleno-L-methionine). Inorganic selenium alone was also found to have a slight effect on the membrane-bound enzyme. An increase in the catalytic efficiency was observed when glutathione peroxidase was bound using lysine as the bifunctional agent and either glutaraldehyde or triethyloxonium tetrafluoroborate as the reticulation agent, after a three-month period of incubation.

The mechanism of porcine pancreatic alpha-amylase. Kinetic evidence for two additional carbohydrate-binding sites.

Kinetics of inhibition of the two porcine pancreatic alpha-amylase components (PPA I and PPA II) by acarbose were performed using reduced DP18-maltodextrin and amylose as substrates. Similar Line-weaver-Burk primary plots were obtained. Two mixed non-competitive models are proposed. X-ray crystallographic data [Qian, M., Buisson, G., Duée. E., Haser, R. & Payan, F. (1994) Biochemistry 33, 6284-6294] are in support of the mixed non-competitive inhibition model which involves abortive complexes. Secondary plots are different; inhibition of reduced DP18-maltodextrin hydrolysis gives straight-lines plots while amylose gives parabolic curves. These results, confirmed by Dixon-plot analyses, allow us to postulate that, in inhibition of reduced DP18-maltodextrin hydrolysis, one molecule of acarbose is bound/ amylase molecule. In contrast, using amylose as a substrate, two molecules of acarbose are bound. These kinetically determined binding sites might correspond to surface sites found by X-ray crystallography [Qian, M., Haser, R. & Payan, F. (1995) Protein Sci. 4, 747-755]; the glucose site close to the active site and the maltose site, 2 nm away. In conclusion, no significant difference between PPA I and PPA II has been observed, either from molecular mass or from kinetic behaviours; this suggests multiple forms of the enzyme. A general mechanism of PPA action is proposed; in addition to the active site, long-chain substrate hydrolysis requires the glucose-binding site and the maltose-binding site, while only one site is necessary for the hydrolysis of short chain substrate.

Crystal structure of pig pancreatic alpha-amylase isoenzyme II, in complex with the carbohydrate inhibitor acarbose.

Two different crystal forms of pig pancreatic alpha-amylase isoenzyme II (PPAII), free and complexed to a carbohydrate inhibitor (acarbose), have been compared together and to previously reported structures of PPAI. A crystal form obtained at 4 degrees C, containing nearly 72% solvent, made it possible to obtain a new complex with acarbose, different from a previous one obtained at 20 degrees C [Qian, M., Buisson, G., Duée, E., Haser, H. & Payan, F. (1994) Biochemistry 33, 6284-6294]. In the present form, six contiguous subsites of the enzyme active site are occupied by the carbohydrate ligand; the structural data indicate that the binding site is capable of holding more than the five glucose units of the scheme proposed through kinetic studies. A monosaccharide ring bridging two protein molecules related by the crystal packing is located on the surface, at a distance of 2.0 nm from the reducing end of the inhibitor ligand; the symmetry-related glucose ring in the crystal lattice is found 1.5 nm away from the non-reducing end of the inhibitor ligand.

Subsite mapping of porcine pancreatic alpha-amylase I and II using 4-nitrophenyl-alpha-maltooligosaccharides.

The catalytic efficiency (kcat/Km) and the cleaved bond distribution for the nitrophenylated maltooligosaccharides, p-NPGlcn (2 < or = n < or = 7) hydrolysed by porcine pancreatic alpha-amylase isozymes I and II were determined. The subsite affinities (Ai) were calculated from the p-NPGlcn (4 < or = n < or = 7) hydrolysis data. Five subsites (-3 to 2) bind glucosidic residues with a positive affinity. No additional subsites could be detected both at the reducing end (3, 4, 5) and at the nonreducing end (-4, -5, -6). The energetic profiles of both isozymes are similar. The energetic profile of PPA differs from other alpha-amylases by having both a small number of subsites, and a catalytic subsite with a high positive affinity. Excellent agreement was found between observed catalytic efficiency values and those calculated from the subsite affinities.

Differential chemical modification of substrate binding areas in porcine-pancreatic alpha-amylase by three regioisomeric photolabile ligands.

Three regioisomeric radiolabelled spacer-modified oligosaccharides: methyl 4'-O-[4-S-(3-azi-4-alpha-D-glucopyranosyloxy-1-[3H]butyl)-6- deoxy- 4-thio-alpha-D-xylo-hex-5-enopyranosyl]-alpha-maltoside (12a, G1-G3*), methyl 4-O-[4-S-(3-azi-4-alpha-maltosyloxy-1-[3H]butyl)-6-deoxy-4-t hio- alpha-D-xylo-hex-5-enopyranosyl]-alpha-D-glucopyranoside (15a, G2-G2*) and methyl 4-S-(3-azi-4-alpha-maltotriosyloxy-1-[3H]butyl)-6-deoxy-4-th io-alpha- D-xylo-hex-5-enopyranoside (16a, G3-G1*) were synthesised and used as photoaffinity probes for the chemical modification of porcine-pancreatic alpha-amylase (PPA). Incorporation of covalently attached radioactivity amounted to 25-38% of the stoichiometric value. Tryptic digestion of the three labelled protein preparations PPA-G1-G3*, PPA-G2-G2*, and PPA-G3-G1* and the purification of the labelled peptides by fractional HPLC yielded altogether six pure components. On the basis of the published three-dimensional structure peptides G1-G3-II, G2-G2-II, and G2-G2-III were part of the catalytic site. G1-G3-I and G2-G2-I were part of the surface binding site. The major component derived from PPA, labelled by G3-G1*, corresponded to an area that is neither close to the active site nor to the surface starch-binding domain, which clearly indicates the presence of a third, hitherto undetected, substrate-binding site.

Comparison of barley malt alpha-amylase isozymes 1 and 2: construction of cDNA hybrids by in vivo recombination and their expression in yeast.

Germinating barley produces two alpha-amylase isozymes, AMY1 and AMY2, having 80% amino acid (aa) sequence identity and differing with respect to a number of functional properties. Recombinant AMY1 (re-AMY1) and AMY2 (re-AMY2) are produced in yeast, but whereas all re-AMY1 is secreted, re-AMY2 accumulates within the cell and only traces are secreted. Expression of AMY1::AMY2 hybrid cDNAs may provide a means of understanding the difference in secretion efficiency between the two isozymes. Here, the efficient homologous recombination system of the yeast, Saccharomyces cerevisiae, was used to generate hybrids of barley AMY with the N-terminal portion derived from AMY1, including the signal peptide (SP), and the C-terminal portion from AMY2. Hybrid cDNAs were thus generated that encode either the SP alone, or the SP followed by the N-terminal 21, 26, 53, 67 or 90 aa from AMY1 and the complementary C-terminal sequences from AMY2. Larger amounts of re-AMY are secreted by hybrids containing, in addition to the SP, 53 or more aa of AMY1. In contrast, only traces of re-AMY are secreted for hybrids having 26 or fewer aa of AMY1. In this case, re-AMY hybrid accumulates intracellularly. Transformants secreting hybrid enzymes also accumulated some re-AMY within the cell. The AMY1 SP, therefore, does not ensure re-AMY2 secretion and a certain portion of the N-terminal sequence of AMY1 is required for secretion of a re-AMY1::AMY2 hybrid.

Antigen specificity and cross-reactivity of fifteen monoclonal antibodies against porcine pancreatic alpha-amylase II and its AB and C domains.

Highly productive hybridoma secreting mabs specific for porcine alpha-pancreatic amylase II were established. Fifteen clones were selected. The mabs produced (KD = 1.68-11.2 nM) were checked for cross-reactivity with six heterologous antigens, namely porcine pancreatic alpha-amylase I, barley amylase, human pancreatic alpha-amylase, Taka amylase and triose phosphate isomerase, using direct ELISA assay; mabs were classified within seven groups: in a few groups mabs cross-reacted with a single heterologous antigen either porcine pancreatic amylase I (6 mabs) or barley amylase (2 mabs) or human pancreatic amylase (3 mabs). Two other groups cross-reacted with two heterologous antigen either porcine I and human or porcine I and barley. Only one mab out of fifteen cross-reacted in direct ELISA binding to all amylases and triose phosphate isomerase. Using sandwich ELISA test only three mabs were found to bind porcine amylase II present at high concentration. Results consistent with direct porcine amylase binding were obtained from binding inhibition assays. Analysis by the additivity test allowed to find that 3 mabs, B10.10, B1.11, C6.4 recognize distinct epitopes while the epitopes for the other pairs tested are either overlapping or at least close to each other. Finally mabs binding specifically either to the AB or to the C domain fragment or to both fragments have been obtained.

Barley malt-alpha-amylase. Purification, action pattern, and subsite mapping of isozyme 1 and two members of the isozyme 2 subfamily using p-nitrophenylated maltooligosaccharide substrates.

Isoforms AMY1, AMY2-1 and AMY2-2 of barley alpha-amylase were purified from malt. AMY2-1 and AMY2-2 are both susceptible to barley alpha-amylase/subtilisin inhibitor. The action of these isoforms is compared using substrates ranging from p-nitrophenylmaltoside through p-nitrophenylmaltoheptaoside. The kcat/Km values are calculated from the substrate consumption. The relative cleavage frequency of different substrate bonds is given by the product distribution. AMY2-1 is 3-8-fold more active than AMY1 toward p-nitrophenylmaltotrioside through p-nitrophenylmaltopentaoside. AMY2-2 is 10-50% more active than AMY2-1. The individual subsite affinities are obtained from these data. The resulting subsite maps of the isoforms are quite similar. They comprise four and six glucosyl-binding subsites towards the reducing and the non-reducing end, respectively. Towards the non-reducing end, the sixth and second subsites have a high affinity, the third has very low or even lack of affinity and the first (catalytic subsite) has a large negative affinity. The affinity declines from moderate to low for subsites 1 through 4 toward the reducing end. AMY1 has clearly a more negative affinity at the catalytic subsite, but larger affinities at both the fourth subsites, compared to AMY2. AMY2-1 has lower affinity than AMY2-2 at subsites adjacent to the catalytic site, and otherwise mostly higher affinities than AMY2-2. Theoretical kcat/Km values show excellent agreement with experimental values.

Effect of limited proteolysis in the 8th loop of the barrel and of antibodies on porcine pancreas amylase activity.

The porcine pancreatic alpha-amylase is a (beta/alpha)8-barrel protein, containing domains A and B (peptide sequence 1-403) and a distinct C-domain (peptide sequence 404-496). Separation of the terminal C-domain from the A and B domains has been attempted by limited proteolysis in the hinge region. Subtilisin was found to hydrolyse amylase between residues 369 and 370 situated in the loop between the eighth beta-strand and alpha-helix. The cleaved amylase was isolated by chromatofocusing and found to retain about 60% of the activity of the native enzyme, while the isolated fragments were inactive. Antigen binding fragments prepared from polyclonal antibodies to native amylase and the CNBr-fragment P1 (peptide sequence 395-496) respectively, were tested for influence on the enzyme activity. Antibodies directed against P1 had no effect whereas antibodies against the peptide sequence 1-394 and amylase respectively inhibited hydrolysis of substrates having four or more glucose residues but not of shorter oligomaltosides. Crystallographic analysis revealed that changes in the region of residue 369 might affect the conformation of the active site as well as of a second binding site. This site, located on the enzyme surface, is proposed to be required for the hydrolysis of larger substrates.

Porcine pancreatic alpha-amylase: a model for structure--function studies of homodepolymerases.

The amino acid sequence of the porcine pancreatic alpha-amylase chain (496 residues) contains four regions (96-101, 193-201, 233-236 and 295-300) which are highly homologous in amylases of different origins. These regions all belong to the N-terminal domain of the enzyme. Limited proteolysis by subtilisin allows a cut to be made at bond 369-370. Purified fragments indicate that both N- and C-terminal domains are required for amylolytic activity. Kinetic studies and reaction product analysis using oligomaltosides, their nitrophenylated derivatives and amylose as the substrate allowed us to establish: 1) the energy profile of the 5 subsites and, especially, that subsite number 3 is catalytic; 2) that 2 molecules of either maltotriose or its o-nitrophenylated analog or maltose bind to the active site at high substrate concentration. Such a subsite occupancy was confirmed by fluorescence quenching studies. Finally the hydrolysis of p-nitrophenylmaltoside was studied as a function of pH. In contrast to starch hydrolysis, the initial velocity plots for nitrophenol and p-nitrophenylglucoside liberation both gave a narrow pH-activity peak with a maximum value around pH 5.5. All data provide strong evidence for the participation of 2 carboxylic residues in the catalysis.