The MalT-dependent and malZ-encoded maltodextrin glucosidase of Escherichia coli can be converted into a dextrinyltransferase by a single mutation

Dimeric architecture of maltodextrin glucosidase (MalZ) provides insights into the substrate recognition and hydrolysis mechanism

Maltodextrin glucosidase (MalZ) is a key enzyme in the maltose utilization pathway in Escherichia coli that liberates glucose from the reducing end of the short malto-oligosaccharides. Unlike other enzymes in the GH13_21 subfamily, the hydrolytic activity of MalZ is limited to maltodextrin rather than long starch substrates, forming various transglycosylation products in α-1,3, α-1,4 or α-1,6 linkages. The mechanism for the substrate binding and hydrolysis of this enzyme is not well understood yet. Here, we present the dimeric crystal structure of MalZ, with the N-domain generating a unique substrate binding groove. The N-domain bears CBM34 architecture and forms a part of the active site in the catalytic domain of the adjacent molecule. The groove found between the N-domain and catalytic domain from the adjacent molecule, shapes active sites suitable for short malto-oligosaccharides, but hinders long stretches of oligosaccharides. The conserved residue of E44 protrudes at subsite +2, elucidating the hydrolysis pattern of the substrate by the glucose unit from the reducing end. The structural analysis provides a molecular basis for the substrate specificity and the enzymatic property, and has potential industrial application for protein engineering.

Role of N-terminal region of Escherichia coli maltodextrin glucosidase in folding and function of the protein

Maltodextrin glucosidase (MalZ) hydrolyses short malto-oligosaccharides from the reducing end releasing glucose and maltose in Escherichia coli. MalZ is a highly aggregation prone protein and molecular chaperonins GroEL and GroES assist in the folding of this protein to a substantial level. The N-terminal region of this enzyme appears to be a unique domain as seen in sequence comparison studies with other amylases as well as through homology modelling. The sequence and homology model analysis show a probability of disorder in the N-Terminal region of MalZ. The crystal structure of this enzyme has been reported in the present communication. Based on the crystallographic structure, it has been interpreted that the N-terminal region of the enzyme (Met1-Phe131) might be unstructured or flexible. To understand the role of the N-terminal region of MalZ in its enzymatic activity, and overall stability, a truncated version (Ala111-His616) of MalZ was created. The truncated version failed to fold into an active enzyme both in E. coli cytosol and in vitro even with the assistance of chaperonins GroEL and GroES. Furthermore, the refolding effort of N-truncated MalZ in the presence of isolated N-terminal domain didn't succeed. Our studies suggest that while the structural rigidity or orientation of the N-terminal region of the MalZ protein may not be essential for its stability and function, but the said domain is likely to play an important role in the formation of the native structure of the protein when present as an integral part of the protein.

Identification of an extracellular thermostable glycosyl hydrolase family 13 α-amylase from Thermotoga neapolitana

We cloned the gene for an extracellular α-amylase, AmyE, from the hyperthermophilic bacterium Thermotoga neapolitana and expressed it in Escherichia coli. The molecular mass of the enzyme was 92 kDa as a monomer. Maximum activity was observed at pH 6.5 and temperature 75°C and the enzyme was highly thermostable. AmyE hydrolyzed the typical substrates for α-amylase, including soluble starch, amylopectin, and maltooli-gosaccharides. The hydrolytic pattern of AmyE was similar to that of a typical α-amylase; however, unlike most of the calcium (Ca(2+))-dependent α-amylases, the activity of AmyE was unaffected by Ca(2+). The specific activities of AmyE towards various substrates indicated that the enzyme preferred maltooligosaccharides which have more than four glucose residues. AmyE could not hydrolyze maltose and maltotriose. When maltoheptaose was incubated with AmyE at the various time courses, the products consisting of maltose through maltopentaose was evenly formed indicating that the enzyme acts in an endo-fashion. The specific activity of AmyE (7.4 U/mg at 75° C, pH 6.5, with starch as the substrate) was extremely lower than that of other extracellular α-amylases, which indicates that AmyE may cooperate with other highly active extracellular α-amylases for the breakdown of the starch or α-glucans into maltose and maltotriose before transport into the cell in the members of Thermotoga sp.

Role of maltose enzymes in glycogen synthesis by Escherichia coli

Mutants with deletion mutations in the glg and mal gene clusters of Escherichia coli MC4100 were used to gain insight into glycogen and maltodextrin metabolism. Glycogen content, molecular mass, and branch chain distribution were analyzed in the wild type and in ΔmalP (encoding maltodextrin phosphorylase), ΔmalQ (encoding amylomaltase), ΔglgA (encoding glycogen synthase), and ΔglgA ΔmalP derivatives. The wild type showed increasing amounts of glycogen when grown on glucose, maltose, or maltodextrin. When strains were grown on maltose, the glycogen content was 20 times higher in the ΔmalP strain (0.97 mg/mg protein) than in the wild type (0.05 mg/mg protein). When strains were grown on glucose, the ΔmalP strain and the wild type had similar glycogen contents (0.04 mg/mg and 0.03 mg/mg protein, respectively). The ΔmalQ mutant did not grow on maltose but showed wild-type amounts of glycogen when grown on glucose, demonstrating the exclusive function of GlgA for glycogen synthesis in the absence of maltose metabolism. No glycogen was found in the ΔglgA and ΔglgA ΔmalP strains grown on glucose, but substantial amounts (0.18 and 1.0 mg/mg protein, respectively) were found when they were grown on maltodextrin. This demonstrates that the action of MalQ on maltose or maltodextrin can lead to the formation of glycogen and that MalP controls (inhibits) this pathway. In vitro, MalQ in the presence of GlgB (a branching enzyme) was able to form glycogen from maltose or linear maltodextrins. We propose a model of maltodextrin utilization for the formation of glycogen in the absence of glycogen synthase.

Characterization of an exo-acting intracellular alpha-amylase from the hyperthermophilic bacterium Thermotoga neapolitana

We cloned and expressed the gene for an intracellular alpha-amylase, designated AmyB, from the hyperthermophilic bacterium Thermotoga neapolitana in Escherichia coli. The putative intracellular amylolytic enzyme contained four regions that are highly conserved among glycoside hydrolase family (GH) 13 alpha-amylases. AmyB exhibited maximum activity at pH 6.5 and 75 degrees C, and its thermostability was slightly enhanced by Ca2+. However, Ca2+ was not required for the activity of AmyB as EDTA had no effect on enzyme activity. AmyB hydrolyzed the typical substrates for alpha-amylase, including soluble starch, amylose, amylopectin, and glycogen, to liberate maltose and minor amount of glucose. The hydrolytic pattern of AmyB is most similar to those of maltogenic amylases (EC 3.2.1.133) among GH 13 alpha-amylases; however, it can be distinguished by its inability to hydrolyze pullulan and beta-cyclodextrin. AmyB enzymatic activity was negligible when acarbose, a maltotetraose analog in which a maltose residue at the nonreducing end was replaced by acarviosine, was present, indicating that AmyB cleaves maltose units from the nonreducing end of maltooligosaccharides. These results indicate that AmyB is a new type exo-acting intracellular alpha-amylase possessing distinct characteristics that distinguish it from typical alpha-amylase and cyclodextrin-/pullulan-hydrolyzing enzymes.

The maltodextrin system of Escherichia coli: metabolism and transport

The maltose/maltodextrin regulon of Escherichia coli consists of 10 genes which encode a binding protein-dependent ABC transporter and four enzymes acting on maltodextrins. All mal genes are controlled by MalT, a transcriptional activator that is exclusively activated by maltotriose. By the action of amylomaltase, we prepared uniformly labeled [(14)C]maltodextrins from maltose up to maltoheptaose with identical specific radioactivities with respect to their glucosyl residues, which made it possible to quantitatively follow the rate of transport for each maltodextrin. Isogenic malQ mutants lacking maltodextrin phosphorylase (MalP) or maltodextrin glucosidase (MalZ) or both were constructed. The resulting in vivo pattern of maltodextrin metabolism was determined by analyzing accumulated [(14)C]maltodextrins. MalP(-) MalZ(+) strains degraded all dextrins to maltose, whereas MalP(+) MalZ(-) strains degraded them to maltotriose. The labeled dextrins were used to measure the rate of transport in the absence of cytoplasmic metabolism. Irrespective of the length of the dextrin, the rates of transport at a submicromolar concentration were similar for the maltodextrins when the rate was calculated per glucosyl residue, suggesting a novel mode for substrate translocation. Strains lacking MalQ and maltose transacetylase were tested for their ability to accumulate maltose. At 1.8 nM external maltose, the ratio of internal to external maltose concentration under equilibrium conditions reached 10(6) to 1 but declined at higher external maltose concentrations. The maximal internal level of maltose at increasing external maltose concentrations was around 100 mM. A strain lacking malQ, malP, and malZ as well as glycogen synthesis and in which maltodextrins are not chemically altered could be induced by external maltose as well as by all other maltodextrins, demonstrating the role of transport per se for induction.

Hexose/Pentose and Hexitol/Pentitol Metabolism

Escherichia coli and Salmonella enterica serovar Typhimurium exhibit a remarkable versatility in the usage of different sugars as the sole source of carbon and energy, reflecting their ability to make use of the digested meals of mammalia and of the ample offerings in the wild. Degradation of sugars starts with their energy-dependent uptake through the cytoplasmic membrane and is carried on further by specific enzymes in the cytoplasm, destined finally for degradation in central metabolic pathways. As variant as the different sugars are, the biochemical strategies to act on them are few. They include phosphorylation, keto-enol isomerization, oxido/reductions, and aldol cleavage. The catabolic repertoire for using carbohydrate sources is largely the same in E. coli and in serovar Typhimurium. Nonetheless, significant differences are found, even among the strains and substrains of each species. We have grouped the sugars to be discussed according to their first step in metabolism, which is their active transport, and follow their path to glycolysis, catalyzed by the sugar-specific enzymes. We will first discuss the phosphotransferase system (PTS) sugars, then the sugars transported by ATP-binding cassette (ABC) transporters, followed by those that are taken up via proton motive force (PMF)-dependent transporters. We have focused on the catabolism and pathway regulation of hexose and pentose monosaccharides as well as the corresponding sugar alcohols but have also included disaccharides and simple glycosides while excluding polysaccharide catabolism, except for maltodextrins.

A novel amylolytic enzyme from Thermotoga maritima, resembling cyclodextrinase and alpha-glucosidase, that liberates glucose from the reducing end of the substrates

The gene previously designated as putative cyclodextrinase from Thermotoga maritima (TMG) was cloned and overexpressed in Escherichia coli. The recombinant TMG was partially purified and its enzymatic characteristics on various substrates were examined. The enzyme hydrolyzes various maltodextrins including maltotriose to maltoheptaose and cyclomaltodextrins (CDs) to mainly glucose and maltose. Although TMG could not degrade pullulan, it rapidly hydrolyzes acarbose, a strong amylase and glucosidase inhibitor, to acarviosine and glucose. Also, TMG initially hydrolyzes p-nitrophenyl-alpha-pentaoside to give maltopentaose and p-nitrophenol, implying that the enzyme specifically cleaves a glucose unit from the reducing end of maltooligosaccharides unlike to other glucosidases. Since its enzymatic activity is negligible if alpha-methylglucoside is present in the reducing end, the type of the residue at the reducing end of the substrate is important for the TMG activity. These results support the fact that TMG is a novel exo-acting glucosidase possessing the characteristics of both CD-/pullulan hydrolyzing enzyme and alpha-glucosidase.

Relationship of sequence and structure to specificity in the alpha-amylase family of enzymes

The hydrolases and transferases that constitute the alpha-amylase family are multidomain proteins, but each has a catalytic domain in the form of a (beta/alpha)(8)-barrel, with the active site being at the C-terminal end of the barrel beta-strands. Although the enzymes are believed to share the same catalytic acids and a common mechanism of action, they have been assigned to three separate families - 13, 70 and 77 - in the classification scheme for glycoside hydrolases and transferases that is based on amino acid sequence similarities. Each enzyme has one glutamic acid and two aspartic acid residues necessary for activity, while most enzymes of the family also contain two histidine residues critical for transition state stabilisation. These five residues occur in four short sequences conserved throughout the family, and within such sequences some key amino acid residues are related to enzyme specificity. A table is given showing motifs distinctive for each specificity as extracted from 316 sequences, which should aid in identifying the enzyme from primary structure information. Where appropriate, existing problems with identification of some enzymes of the family are pointed out. For enzymes of known three-dimensional structure, action is discussed in terms of molecular architecture. The sequence-specificity and structure-specificity relationships described may provide useful pointers for rational protein engineering.

The role of the trehalose system in regulating the maltose regulon of Escherichia coli

The maltose regulon consists of 10 genes encoding an ABC transporter for maltose and maltodextrins as well as enzymes necessary for their degradation. MalK, the energy-transducing subunit of the transport system, acts phenotypically as a repressor of MalT, the transcriptional activator of the mal genes. Using MacConkey maltose indicator plates we isolated an insertion mutation that strongly reduced the repressing effect of overproduced MalK. The insertion had occurred in treR encoding the repressor of the trehalose system. The loss of TreR function led to derepression of treB encoding an enzymeIITre of the PTS for trehalose and of treC encoding TreC, the cytoplasmic trehalose-6-phosphate hydrolase. Further analysis revealed that maltose can enter the cell by facilitated diffusion through enzymeIITre, thus causing induction of the maltose system. In addition, derepression of TreC by itself caused induction of the maltose system, and a mutant lacking TreC was reduced in the uninduced level of mal gene expression indicating synthesis of endogenous inducer by TreC. Extracts containing TreC transformed [14C]-maltose into another 14C-labelled compound (preliminarily identified as maltose 1-phosphate) that is likely to be an alternative inducer of the maltose system.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

The periplasmic cyclodextrin binding protein CymE from Klebsiella oxytoca and its role in maltodextrin and cyclodextrin transport

Klebsiella oxytoca M5a1 has the capacity to transport and to metabolize alpha-, beta- and gamma-cyclodextrins. Cyclodextrin transport is mediated by the products of the cymE, cymF, cymG, cymD, and cymA genes, which are functionally homologous to the malE, malF, malG, malK, and lamB gene products of Escherichia coli. CymE, which is the periplasmic binding protein, has been overproduced and purified. By substrate-induced fluorescence quenching, the binding of ligands was analyzed. CymE bound alpha-cyclodextrin, beta-cyclodextrin, and gamma-cyclodextrin, with dissociation constants (Kd) of 0.02, 0.14 and 0.30 microM, respectively, and linear maltoheptaose, with a Kd of 70 microM. In transport experiments, alpha-cyclodextrin was taken up by the cym system of K. oxytoca three to five times less efficiently than maltohexaose by the E. coli maltose system. Besides alpha-cyclodextrin, maltohexaose was also taken up by the K. oxytoca cym system, but because of the inability of maltodextrins to induce the cym system, growth of E. coli mal mutants on linear maltodextrin was not observed when the cells harbored only the cym uptake system. Strains which gained this capacity by mutation could easily be selected, however.

Maltose/maltodextrin system of Escherichia coli: transport, metabolism, and regulation

The maltose system of Escherichia coli offers an unusually rich set of enzymes, transporters, and regulators as objects of study. This system is responsible for the uptake and metabolism of glucose polymers (maltodextrins), which must be a preferred class of nutrients for E. coli in both mammalian hosts and in the environment. Because the metabolism of glucose polymers must be coordinated with both the anabolic and catabolic uses of glucose and glycogen, an intricate set of regulatory mechanisms controls the expression of mal genes, the activity of the maltose transporter, and the activities of the maltose/maltodextrin catabolic enzymes. The ease of isolating many of the mal gene products has contributed greatly to the understanding of the structures and functions of several classes of proteins. Not only was the outer membrane maltoporin, LamB, or the phage lambda receptor, the first virus receptor to be isolated, but also its three-dimensional structure, together with extensive knowledge of functional sites for ligand binding as well as for phage lambda binding, has led to a relatively complete description of this sugar-specific aqueous channel. The periplasmic maltose binding protein (MBP) has been studied with respect to its role in both maltose transport and maltose taxis. Again, the combination of structural and functional information has led to a significant understanding of how this soluble receptor participates in signaling the presence of sugar to the chemosensory apparatus as well as how it participates in sugar transport. The maltose transporter belongs to the ATP binding cassette family, and although its structure is not yet known at atomic resolution, there is some insight into the structures of several functional sites, including those that are involved in interactions with MBP and recognition of substrates and ATP. A particularly astonishing discovery is the direct participation of the transporter in transcriptional control of the mal regulon. The MalT protein activates transcription at all mal promoters. A subset also requires the cyclic AMP receptor protein for transcription. The MalT protein requires maltotriose and ATP as ligands for binding to a dodecanucleotide MalT box that appears in multiple copies upstream of all mal promoters. Recent data indicate that the ATP binding cassette transporter subunit MalK can directly inhibit MalT when the transporter is inactive due to the absence of substrate. Despite this wealth of knowledge, there are still basic issues that require clarification concerning the mechanism of MalT-mediated activation, repression by the transporter, biosynthesis and assembly of the outer membrane and inner membrane transporter proteins, and interrelationships between the mal enzymes and those of glucose and glycogen metabolism.

Enzymatic preparation of radiolabeled linear maltodextrins and cyclodextrins of high specific activity from [14C] maltose using amylomaltase, cyclodextrin glucosyltransferase and cyclodextrinase

Radiolabeled linear and cyclic maltodextrins of high specific radioactivity were prepared using enzymes involved in maltodextrin metabolism. 14C-Labeled maltose was the starting material yielding products of identical specific radioactivity with respect to glucosyl residues. The enzymatic steps involved: i) Formation of linear 14C-labeled maltodexrins (< maltooctaose) using amylomaltase from Escherischia coli; ii) Cyclisation to alpha-cyclodextrin using cyclodextrin-glucosyltransferase of Kiebsiella oxytoca M5a1; iii) Removal of the remaining linear dextrins by amyloglucosidase. The products were purified by paper chromatography, or maltohexaose was specifically obtained from purified alpha-cyclodextrin by the action of cyclodextrinase of K. oxytoca M5a1.

Characterization of the aes gene of Escherichia coli encoding an enzyme with esterase activity

malQ mutants of Escherichia coli lacking amylomaltase cannot grow on maltose. They express the maltose system constitutively and are sensitive to maltose when grown on another carbon source. In an attempt to isolate a multicopy suppressor that would result in growth on maltose, we transformed a malQ mutant with a gene bank of E. coli DNA which had been digested with Sau3a and cloned in pBR322. We screened the transformants on MacConkey maltose plates. A colony was isolated that appeared to be resistant to maltose and was pink on these plates, but it was still unable to grow on minimal medium with maltose as the carbon source. The plasmid was isolated, and the gene causing this phenotype was characterized. The deduced amino acid sequence of the encoded protein shows homology to that of lipases and esterases. We termed the gene aes, for acetyl esterase. Extracts of cells harboring plasmid-encoded aes under its own promoter exhibit a fivefold higher capacity to hydrolyze p-nitrophenyl acetate than do extracts of cells of plasmid-free strains. Similarly, strains harboring plasmid-encoded aes are able to grow on triacetyl glycerol (triacetin) whereas the plasmid-free strains are not. The expression of plasmid-encoded aes resulted in strong repression of the maltose transport genes in malT+ strains (10-fold reduction), but not in a malT(Con) strain which is independent of the inducer. Also, overproduction of MalT counteracted the Aes-dependent repression, indicating a direct interaction between MalT and Aes.