Rapid purification of the potato ADP-glucose pyrophosphorylase by polyhistidine-mediated chromatography

In an attempt to obtain facile methods to purify the heterotetrameric ADP-glucose pyrophosphorylase (AGPase), polyhistidine tags were attached to either the large (LS) or small (SS) subunits of this oligomeric enzyme. The addition of polyhistidine tag to the N-terminus of the LS or SS and co-expression with its unmodified counterpart subunit resulted in substantial induction of enzyme activity. In contrast, attachment of a polyhistidine-containing peptide through the use of a commercially available pET vector or addition of polyhistidine tags to the C-terminal ends of either subunit resulted in poor expression and/or production of enzyme activity. Preliminary experiment showed that these polyhistidine N-terminal-tagged enzymes interacted with Ni-NTA-agarose, indicating that immobilized metal affinity chromatography (IMAC) would be useful for efficient purification of the heterotetrameric AGPases. When ion-exchange chromatography step was employed prior to the IMAC, the polyhistidine-tagged AGPases were purified to near homogeneity. Comparison of kinetic parameters between AGPases with and without the polyhistidine tags revealed that attachment of the polyhistidine did not alter the allosteric and catalytic properties of the enzymes. These results indicate that polyhistidine tags will be useful for the rapid purification of preparative amounts of AGPases for biochemical and physical studies.

Allosteric regulation of the higher plant ADP-glucose pyrophosphorylase is a product of synergy between the two subunits

The higher plant ADP-glucose pyrophosphorylase (AGPase) is a heterotetramer consisting of two regulatory large subunits (LSs) and two catalytic small subunits (SSs). To further characterize the roles of these subunits in determining enzyme function, different combinations of wildtype LS (LWT) and variant forms (LUpReg1, LM345) were co-expressed with wildtype SS (SWT) and variant forms (STG-15 and Sdevo330) and their enzyme properties compared to those measured for the heterotetrameric wildtype enzyme and SS homotetrameric enzymes. Analysis of the allosteric regulatory properties of the various enzymes indicates that although the LS is required for optimal activation by 3-phosphoglyceric acid and resistance to Pi, the overall allosteric regulatory and kinetic properties are specified by both subunits. Our results show that the regulatory and kinetic properties of AGPase are not simply due to the LS modulating the properties of the SS but, instead, are a product of synergistic interaction between the two subunits.

Generation, characterization, and heterologous expression of wild-type and up-regulated forms of Arabidopsis thaliana leaf ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase (AGPase), a key enzyme in starch biosynthesis of higher plants, consists of a pair of regulatory large (LS) and catalytically small (SS) subunits. In plants, these subunits are coded by multiple genes resulting in the formation of tissue-specific enzyme forms, which are differentially regulated during plant growth and development. Some AGPase isoforms differ in catalytic and regulatory properties as well as intracellular location. In an effort to gain a better understanding of the role of the leaf AGPase in carbon partitioning and its effect on plant productivity, the Arabidopsis leaf AGPase containing the mature forms of the SS and LS was expressed in a heterologous expression system and characterized enzymatically. The Arabidopsis recombinant AGPase had kinetic values for 3-phosphoglyceric acid, glucose-1-phosphate and Mg(2+) similar to those of the native enzyme. As the N-terminus of the LS has been suggested to be involved in enzyme function, the length of the N-terminal region was extended or shortened. Of the five modified LSs analyzed, only the T5 form lacking six residues of the mature N-terminus was able to form detectable levels of enzyme activity, indicating that the N-terminal region is critical for enzyme function. Two up-regulatory LS mutations that allosterically activate the potato enzyme, a stem isoform, were introduced into the corresponding Arabidopsis LS sequences and co-expressed with wild-type SS. Both modified enzymes showed up-regulatory properties, indicating that these specific residue changes were also operational in the leaf isoform.

Directed molecular evolution of ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase catalyzes a rate-limiting reaction in prokaryotic glycogen and plant starch biosynthesis. Despite sharing similar molecular size and catalytic and allosteric regulatory properties, the prokaryotic and higher plant enzymes differ in higher-order protein structure. The bacterial enzyme is encoded by a single gene whose product of ca. 50,000 Da assembles into a homotetrameric structure. Although the higher plant enzyme has a similar molecular size, it is made up of a pair of large subunits and a pair of small subunits, encoded by different genes. To identify the basis for the evolution of AGPase function and quaternary structure, a potato small subunit homotetrameric mutant, TG-15, was subjected to iterations of DNA shuffling and screened for enzyme variants with up-regulated catalytic and/or regulatory properties. A glycogen selection/screening regimen of buoyant density gradient centrifugation and iodine vapor colony staining on glucose-containing media was used to increase the stringency of selection. This approach led to the isolation of a population of AGPase small subunit homotetramer enzymes with enhanced affinity toward ATP and increased sensitivity to activator and/or greater resistance to inhibition than TG-15. Several enzymes displayed a shift in effector preference from 3-phosphoglycerate to fructose-6 phosphate or fructose-1,6-bis-phosphate, effectors used by specific bacterial AGPases. Our results suggest that evolution of AGPase, with regard to quaternary structure, allosteric effector selectivity, and effector sensitivity, can occur through the introduction of a few point mutations alone with low-level recombination hastening the process.

Analysis of allosteric effector binding sites of potato ADP-glucose pyrophosphorylase through reverse genetics

ADP-glucose pyrophosphorylase (AGPase) is a key regulatory enzyme of bacterial glycogen and plant starch synthesis as it controls carbon flux via its allosteric regulatory behavior. Unlike the bacterial enzyme that is composed of a single subunit type, the plant AGPase is a heterotetrameric enzyme (alpha2beta2) with distinct roles for each subunit type. The large subunit (LS) is involved mainly in allosteric regulation through its interaction with the catalytic small subunit (SS). The LS modulates the catalytic activity of the SS by increasing the allosteric regulatory response of the hetero-oligomeric enzyme. To identify regions of the LS involved in binding of effector molecules, a reverse genetics approach was employed. A potato (Solanum tuberosum L.) AGPase LS down-regulatory mutant (E38A) was subjected to random mutagenesis using error-prone polymerase chain reaction and screened for the capacity to form an enzyme capable of restoring glycogen production in glgC(-) Escherichia coli. Dominant mutations were identified by their capacity to restore glycogen production when the LS containing only the second site mutations was co-expressed with the wild-type SS. Sequence analysis showed that most of the mutations were decidedly nonrandom and were clustered at conserved N- and C-terminal regions. Kinetic analysis of the dominant mutant enzymes indicated that the K(m) values for cofactor and substrates were comparable with the wild-type AGPase, whereas the affinities for activator and inhibitor were altered appreciably. These AGPase variants displayed increased resistance to P(i) inhibition and/or greater sensitivity toward 3-phosphoglyceric acid activation. Further studies of Lys-197, Pro-261, and Lys-420, residues conserved in AGPase sequences, by site-directed mutagenesis suggested that the effectors 3-phosphoglyceric acid and P(i) interact at two closely located binding sites.

Investigation of subunit function in ADP-glucose pyrophosphorylase

ADP-glucose pyrophosphorylase (AGPase), a key regulatory enzyme in higher plant starch biosynthesis, is composed of a pair of large and small subunits (alpha(2)beta(2)). Current evidence suggests that the large subunit has primarily a regulatory function, while the small subunit has both regulatory and catalytic roles. To define the structure-function relationship of the large subunit (LS), the LS of potato AGPase was subjected to chemical mutagenesis and coexpressed with the wild-type (WT) small subunit (SS) cDNA in an AGPase defective Escherichia coli strain. An LS mutant (M143) was isolated, which accumulated very low levels of glycogen compared to the WT recombinant AGPase, but maintained normal catalytic activity when assayed under saturating conditions. Sequence analysis revealed that M143 has a single amino acid change, V463I, which lies adjacent to the C-terminus. This single mutation had no effect on the Km for ATP and Mg(2+), which were similar to the WT enzyme. The K(m) for glucose 1-P, however, was sixfold higher than the WT enzyme. These results suggest that the LS plays a role in binding glucose 1-P through its interaction with the SS.

Isolation and characterization of a higher plant ADP-glucose pyrophosphorylase small subunit homotetramer

ADP-glucose pyrophosphorylase (AGPase) is the allosterically regulated gateway for carbon entry into transient and storage starch in plants as well as glycogen in bacteria. This enzyme plays a key role in the modulation of photosynthetic efficiency in source tissues and directly determines the level of storage starch in sink tissues, thus influencing overall crop yield potential. AGPase is a tetrameric enzyme; in higher plants it consists of two regulatory large subunits (LS) and two catalytic small subunits (SS), while in cyanobacteria and prokaryotes the enzyme is homotetrameric. The potato SS gene in pML10 was mutated by hydroxylamine and mutants were screened for elevated homotetrameric activity by iodine vapor staining. This search strategy led to the isolation of SS mutants (SUP-1, TG-15) that had pyrophosphorylase activity in the absence of the LS. TG-15 has a leucine to phenylalanine change at position 48 (L(48)F) that corresponds to a phenylalanine residue at the analogous position in the Escherichia coli homotetrameric AGPase as well as a valine to isoleucine change at position 59 (V(59)I). TG-15 was partially purified and kinetic analysis revealed substrate and effector affinities equal to wild type heterotetrameric enzyme with the exception of ATP binding.

Production, purification and characterization of a 50-kDa extracellular metalloprotease from Serratia marcescens

The extracellular metalloprotease (SMP 6.1) produced by a soil isolate of Serratia marcescens NRRL B-23112 was purified and characterized. SMP 6.1 was purified from the culture supernatant by ammonium sulfate precipitation, acetone fractional precipitation, and preparative isoelectric focusing. SMP 6.1 has a molecular mass of approximately 50,900 Da by sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE). The following substrates were hydrolyzed: casein, bovine serum albumin, and hide powder. SMP 6.1 has the characteristics of a metalloprotease, a pH optimum of 10.0, and a temperature optimum of 42 degrees C. The isoelectric point of the protease is 6.1. Restoration of proteolytic activity by in-gel renaturation after SDS-PAGE indicates a single polypeptide chain. SMP 6.1 is inhibited by EDTA (9 micrograms/ml) and not inhibited by antipain dihydrochloride (120 micrograms/ml), aprotinin (4 micrograms/ml), bestatin (80 micrograms/ml), chymostatin (50 micrograms/ml), E-64 (20 micrograms/ml), leupeptin (4 micrograms/ml), Pefabloc SC (2000 micrograms/ml), pepstatin (4 micrograms/ml), phosphoramidon (660 micrograms/ml), or phenylmethylsulfonyl fluoride (400 micrograms/ml). SMP 6.1 retains full activity in the presence of SDS (1% w/v), Tween-20 (1% w/v), Triton X-100 (1% w/v), ethanol (5% v/v), and 2-mercaptoethanol (0.5% v/v). The extracellular metalloprotease SMP 6.1 differs from the serratiopeptidase (Sigma) produced by S. marcescens ATCC 27117 in the following characteristics: isoelectric point, peptide mapping and nematolytic properties.