Cloning, sequencing, and expression in Escherichia coli of the gene coding for phosphofructokinase in Lactobacillus bulgaricus

Global transcriptome response in Lactobacillus sakei during growth on ribose

BACKGROUND

Lactobacillus sakei is valuable in the fermentation of meat products and exhibits properties that allow for better preservation of meat and fish. On these substrates, glucose and ribose are the main carbon sources available for growth. We used a whole-genome microarray based on the genome sequence of L. sakei strain 23K to investigate the global transcriptome response of three L. sakei strains when grown on ribose compared with glucose.

RESULTS

The function of the common regulated genes was mostly related to carbohydrate metabolism and transport. Decreased transcription of genes encoding enzymes involved in glucose metabolism and the L-lactate dehydrogenase was observed, but most of the genes showing differential expression were up-regulated. Especially transcription of genes directly involved in ribose catabolism, the phosphoketolase pathway, and in alternative fates of pyruvate increased. Interestingly, the methylglyoxal synthase gene, which encodes an enzyme unique for L. sakei among lactobacilli, was up-regulated. Ribose catabolism seems closely linked with catabolism of nucleosides. The deoxyribonucleoside synthesis operon transcriptional regulator gene was strongly up-regulated, as well as two gene clusters involved in nucleoside catabolism. One of the clusters included a ribokinase gene. Moreover, hprK encoding the HPr kinase/phosphatase, which plays a major role in the regulation of carbon metabolism and sugar transport, was up-regulated, as were genes encoding the general PTS enzyme I and the mannose-specific enzyme II complex (EIIman). Putative catabolite-responsive element (cre) sites were found in proximity to the promoter of several genes and operons affected by the change of carbon source. This could indicate regulation by a catabolite control protein A (CcpA)-mediated carbon catabolite repression (CCR) mechanism, possibly with the EIIman being indirectly involved.

CONCLUSIONS

Our data shows that the ribose uptake and catabolic machinery in L. sakei is highly regulated at the transcription level. A global regulation mechanism seems to permit a fine tuning of the expression of enzymes that control efficient exploitation of available carbon sources.

Functional analysis, overexpression, and kinetic characterization of pyruvate kinase from methicillin-resistant Staphylococcus aureus

Novel antimicrobial targets are urgently needed to overcome rising antibiotic resistance of important human pathogens including methicillin-resistant Staphylococcus aureus (MRSA). Here we report the essentiality and kinetic properties of MRSA pyruvate kinase (PK). Targetron-mediated gene disruption demonstrated PK is essential for S. aureus growth and survival, suggesting that this protein may be a potential drug target. The presence of the pfk (6-phosphofructokinase)-pyk operon in MRSA252, and the nonessential nature of PFK shown by targetron, further emphasized the essential role of PK in cell viability. The importance of PK in bacterial growth was confirmed by showing that its enzymatic activity peaked during the logarithmic phase of S. aureus growth. PK from Staphylococcus and several other species of bacteria have an extra C-terminal domain (CT) containing a phosphoenolpyruvate (PEP) binding motif. To elucidate the possible structure and function of this sequence, the quaternary structures and kinetic properties of the full-length MRSA PK and truncated MRSA PK lacking the CT domain were characterized. Our results showed that (1) MRSA PK is an allosteric enzyme with homotetramer architecture activated by AMP or ribose 5-phosphate (R5P), but not by fructose 1,6-bisphosphate (FBP), which suggests a different mode of allosteric regulation when compared with human isozymes, (2) the CT domain is not required for the tetramerization of the enzyme; homotetramerization occurred in a truncated PK lacking the domain, (3) truncated enzyme exhibited high affinity toward both PEP and ADP and exhibited hyperbolic kinetics toward PEP in the presence of activators (AMP and R5P) consistent with kinetic properties of full-length enzyme, indicating that the CT domain is not required for substrate binding or allosteric regulation observed in the holoenzyme, (4) the kinetic efficiency (k(cat)/S(0.5)) of truncated enzyme was decreased by 24- and 16-fold, in ligand-free state, toward PEP and ADP, respectively, but was restored by 3-fold in AMP-bound state, suggesting that the sequence containing the CT domain (Gly(473)-Leu(585)) plays a substantial role in enzyme activity and comformational stability, and (5) full-length MRSA PK activity was stimulated at low concentrations of ATP (e.g., 1 mM) and inhibited by inorganic phosphate and high concentrations of FBP (10 mM) and ATP (e.g., >2.5 mM), whereas for truncated enzyme, stimulation at low concentrations of ATP was lost. These findings suggest that the CT domain is involved in maintaining the specificity of allosteric regulation of MRSA PK by AMP, R5P, and ATP. The CT extension also encodes a protein domain with homology to enzyme I of the Escherichia coli sugar-PTS system, suggesting that MRSA PK may also exert an important regulatory role in sugar transport metabolism. These findings yield new insights into MRSA PK function and mode of allosteric regulation which may aid in the development of clinically important drugs targeting this enzyme and further define the role of the extra C-terminal domain in modulating the enzyme's activity.

Primary metabolism in Lactobacillus sakei food isolates by proteomic analysis

BACKGROUND

Lactobacillus sakei is an important food-associated lactic acid bacterium commonly used as starter culture for industrial meat fermentation, and with great potential as a biopreservative in meat and fish products. Understanding the metabolic mechanisms underlying the growth performance of a strain to be used for food fermentations is important for obtaining high-quality and safe products. Proteomic analysis was used to study the primary metabolism in ten food isolates after growth on glucose and ribose, the main sugars available for L. sakei in meat and fish.

RESULTS

Proteins, the expression of which varied depending on the carbon source were identified, such as a ribokinase and a D-ribose pyranase directly involved in ribose catabolism, and enzymes involved in the phosphoketolase and glycolytic pathways. Expression of enzymes involved in pyruvate and glycerol/glycerolipid metabolism were also affected by the change of carbon source. Interestingly, a commercial starter culture and a protective culture strain down-regulated the glycolytic pathway more efficiently than the rest of the strains when grown on ribose. The overall two-dimensional gel electrophoresis (2-DE) protein expression pattern was similar for the different strains, though distinct differences were seen between the two subspecies (sakei and carnosus), and a variation of about 20% in the number of spots in the 2-DE gels was observed between strains. A strain isolated from fermented fish showed a higher expression of stress related proteins growing on both carbon sources.

CONCLUSIONS

It is obvious from the data obtained in this study that the proteomic approach efficiently identifies differentially expressed proteins caused by the change of carbon source. Despite the basic similarity in the strains metabolic routes when they ferment glucose and ribose, there were also interesting differences. From the application point of view, an understanding of regulatory mechanisms, actions of catabolic enzymes and proteins, and preference of carbon source is of great importance.

Engineering of primary carbohydrate metabolism for increased production of actinorhodin in Streptomyces coelicolor

The objectives of the current studies were to determine the roles of key enzymes in central carbon metabolism in the context of increased production of antibiotics in Streptomyces coelicolor. Genes for glucose-6-phosphate dehydrogenase and phosphoglucomutase (Pgm) were deleted and those for the acetyl coenzyme A carboxylase (ACCase) were overexpressed. Under the conditions tested, glucose-6-phosphate dehydrogenase encoded by zwf2 plays a more important role than that encoded by zwf1 in determining the carbon flux to actinorhodin (Act), while the function of Pgm encoded by SCO7443 is not clearly understood. The pgm-deleted mutant unexpectedly produced abundant glycogen but was impaired in Act production, the exact reverse of what had been anticipated. Overexpression of the ACCase resulted in more rapid utilization of glucose and sharply increased the efficiency of its conversion to Act. From the current experiments, it is concluded that carbon storage metabolism plays a significant role in precursor supply for Act production and that manipulation of central carbohydrate metabolism can lead to an increased production of Act in S. coelicolor.

Kinetic and structural characterization of phosphofructokinase from Lactobacillus bulgaricus

Phosphofructokinase from Lactobacillus delbrueckii subspecies bulgaricus (LbPFK) has been reported to be a nonallosteric analogue of phosphofructokinase from Escherichia coli at pH 8.2 [Le Bras et al. (1991) Eur. J. Biochem. 198, 683-687]. A reexamination of the kinetics of this enzyme shows LbPFK to have limited binding affinity toward the allosteric ligands, MgADP and PEP, with dissociation constants of approximately 20 mM for both. Their allosteric effects are observed only at high concentrations of these ligands, with both exhibiting inhibitory effects on substrate binding. No pH dependence was observed for the binding and the influence of MgADP and PEP on the enzyme. To attempt to explain these results, the crystal structure of LbPFK was solved using molecular replacement to 1.86 A resolution. A comparative study of the LbPFK structure with that of phosphofructokinases from E. coli (EcPFK) and Bacillus stearothermophilus (BsPFK) reveals a structure with conserved fold and substrate binding site. The effector binding site, however, shows many differences that could explain the observed decreases in binding affinity for MgADP and PEP in LbPFK as compared to the other two enzymes.

Molecular characterization, enzyme properties and transcriptional regulation of phosphoenolpyruvate carboxykinase and pyruvate kinase in a ruminal bacterium, Selenomonas ruminantium

To elucidate the regulatory mechanism for propionate production in Selenomonas ruminantium, the molecular properties and gene expression of phosphoenolpyruvate carboxykinase (Pck) and pyruvate kinase (Pyk) were investigated. The Pck was deduced to consist of 538 aa with a molecular mass of 59.6 kDa, and appeared to exist as a monomer. The Pyk was revealed to consist of four identical subunits consisting of 469 aa with a molecular mass of 51.3 kDa. Both Mg(2+) and Mn(2+) were required for the maximal activity of Pck, and Pck utilized ADP, not GDP or IDP, as a substrate. Either Mg(2+) or Mn(2+) was required for Pyk activity, and the enzyme was activated by phosphoenolpyruvate (PEP) and fructose 1,6-bisphosphate (FBP). Pyk activity was severely inhibited by P(i), but restored by the addition of FBP. The K:(m) value of Pck for PEP (0.55 mM) was nearly equal to the K:(m) value of Pyk for PEP, suggesting that the partition of the flow from PEP in the fermentation pathways is determined by the activity ratio of Pck to Pyk. Both pck and pyk genes were monocistronic, although two transcriptional start sites were found in pyk. The level of pyk mRNA was not different whether glucose or lactate was the energy substrate. However, the pck mRNA level was 12-fold higher when grown on lactate than on glucose. The level of pck mRNA was inversely related to the sufficiency of energy, suggesting that Pck synthesis is regulated at the transcriptional level when energy supply is altered. It was conceivable that the transcription of pck in S. ruminantium is triggered by PEP and suppressed by ATP.

Thermotoga maritima phosphofructokinases: expression and characterization of two unique enzymes

A pyrophosphate-dependent phosphofructokinase (PP(i)-PFK) and an ATP-dependent phosphofructokinase (ATP-PFK) from Thermotoga maritima have been cloned and characterized. The PP(i)-PFK is unique in that the K(m) and V(max) values indicate that polyphosphate is the preferred substrate over pyrophosphate; the enzyme in reality is a polyphosphate-dependent PFK. The ATP-PFK was not significantly affected by common allosteric effectors (e.g., phosphoenolpyruvate) but was strongly inhibited by PP(i) and polyphosphate. The results suggest that the control of the Embden-Meyerhof pathway in this organism is likely to be modulated by pyrophosphate and/or polyphosphate.

"Early" protein synthesis of Lactobacillus delbrueckii ssp. bulgaricus in milk revealed by [35S] methionine labeling and two-dimensional gel electrophoresis

The proteomes of exponentially growing and stationary cells of Lactobacillus delbrueckii ssp. bulgaricus grown in rich medium (MRS) were separated by two-dimensional polyacrylamide gel electrophoresis (2-DE) and quantified after Coomassie staining. Stationary cells grown in MRS were inoculated in reconstituted skim milk, and "early" protein synthesis during the first 30 min of fermentation in milk was monitored by [35S]methionine labeling and 2-DE. In contrast to exponentially growing or stationary cells, the predominant "early" proteins were small (< 15 kDa) and of low pI (< 5.3). Quantification of the proteome of the "early" lag phase based on 47 "spots" revealed that only three "early" proteins accounted for more than 80% of the total label. They were identified as pI 4.7 and 4.9 isoforms of the heat-stable phosphoryl carrier protein (HPr) with 45.2 and 9.4% of total label, respectively, and an unknown protein called EPr1 ("early" protein 1) with 26.6% of total label. Although an N-terminal sequence of 19 amino acids was obtained, no homologs to EPr1 could be found. De novo synthesis of the 10 and 60 kDa heat shock proteins (GroES and GroEL) was considerably lower (0.04 and 0.9% of total label, respectively), indicating only low levels of stress. Synthesis of triosephosphate isomerase (Tpi) as marker for glycolytic enzymes reached only 0.08% of total label. Our results demonstrate that inoculation in milk, resulting in a change from glucose to lactose as carbon source, imposes only little need for synthesis of stress or glycolytic enzymes, as sufficient proteins are present in the stationary, MRS-grown cells. The high level of expression of the pI 4.7 isoform of HPr suggests a regulatory function of the presumed Ser-46 phosphorylated form of HPr.

Glucose catabolism of Escherichia coli strains with increased activity and altered regulation of key glycolytic enzymes

This study investigates the effect of overexpression of key glycolytic enzymes exhibiting either native or alternative allosteric regulation on glucose bioconversion by resting Escherichia coli cells previously engineered for ethanol production. Homologous and heterologous pyruvate kinases (Pyk) and phosphofructokinases (Pfk) were individually and simultaneously overexpressed. Overexpression of the E. coli Pfk led to a shift from ethanol to lactate formation (three-fold above the control level) while overexpression of Pyks accelerated lactate formation two-fold with less reduction in ethanol formation. Further increase in lactate formation (five-fold above the control level) resulted from overexpression of Pfk from Lactobacillus bulgaricus which, unlike the E. coli Pfk, is not allosterically regulated by either phosphoenolpyruvate or ADP. These effects on the carbon flux distribution were accompanied by significant changes in the intracellular concentrations of several glycolytic intermediates. Increased Pfk levels led primarily to reduced levels of hexose phosphates. Increased Pyk activity resulted in more complex changes which were different for overexpressed native Pyk and for overexpressed Bacillus stearothermophilus Pyk, which differs from E. coli Pyk in lacking activation by fructose 1,6-diphosphate, but is allosterically activated by AMP and ribose 5-phosphate. Simultaneous overexpression of native Pfk and Pyk caused a Pfk-overexpression-like phenotype with lower levels of hexose phosphates and further increased lactate formation (nine-fold above the control level). The flux data demonstrate that overexpression of even single enzymes early in a central pathway can increase the fluxes to a particular metabolic product, although it may not affect the glucose uptake rate.

Genetics of lactobacilli in food fermentations

Lactobacilli play a substantial role in food biotechnology and influence our quality of life by their fermentative and probiotic properties. Despite their obvious importance in fermentation ecology and biotechnology only recent years have brought some insight into the genetics of lactobacilli. These genetic investigations allow the elucidation of traits determinative for competitiveness and ecology and thus product safety and quality. They have concentrated only on a small selection of lactobacilli whereas others are hardly touched or remained recalcitrant to genetic analysis and manipulation. The knowledge gained on the biochemistry, physiology, ecology and especially genetics is a prerequisite for the deliberate application and improved handling of lactobacilli in traditional and novel applications. In this review, the achievements in the genetics of lactobacilli are described including detection systems, genetic elements, host vector systems, gene cloning and expression and risk assessment of genetically engineered lactobacilli.

Application of mathematical tools for metabolic design of microbial ethanol production

Many attempts to engineer cellular metabolism have failed due to the complexity of cellular functions. Mathematical and computational methods are needed that can organize the available experimental information, and provide insight and guidance for successful metabolic engineering. Two such methods are reviewed here. Both methods employ a (log)linear kinetic model of metabolism that is constructed based on enzyme kinetics characteristics. The first method allows the description of the dynamic responses of metabolic systems subject to spatiotemporal variations in their parameters. The second method considers the product-oriented, constrained optimization of metabolic reaction networks using mixed-integer linear programming methods. The optimization framework is used in order to identify the combinations of the metabolic characteristics of the glycolytic enzymes from yeast and bacteria that will maximize ethanol production. The methods are also applied to the design of microbial ethanol production metabolism. The results of the calculations are in qualitative agreement with experimental data presented here. Experiments and calculations suggest that, in resting Escherichia coli cells, ethanol production and glucose uptake rates can be increased by 30% and 20%, respectively, by overexpression of a deregulated pyruvate kinase, while increase in phosphofructokinase expression levels has no effect on ethanol production and glucose uptake rates.

An operon encoding three glycolytic enzymes in Lactobacillus delbrueckii subsp. bulgaricus: glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase

The structural genes gap, pgk and tpi encoding three glycolytic enzymes, glyceraldehyde-3-phosphate dehydrogenase (GAPDH), 3-phosphoglycerate kinase (PGK) and triosephosphate isomerase (TPI), respectively, have been cloned and sequenced from Lactobacillus delbrueckii subsp. bulgaricus (L. bulgaricus). The genes were isolated after screening genomic sublibraries with specific gap and pgk probes obtained by PCR amplification of chromosomal DNA with degenerate primers corresponding to amino acid sequences highly conserved in GAPDHs and PGKs. Nucleotide sequencing revealed that the three genes were organized in the order gap-pgk-tpi. The translation start codons of the three genes were identified by alignment of the N-terminal sequences. These genes predicted polypeptide chains of 338, 403 and 252 amino acids for GAPDH, PGK and TPI, respectively, and they were separated by 96 bp between gap and pgk, and by only 18 bp between pgk and tpi. The codon usage in gap, pgk, tpi and three other glycolytic genes from L. bulgaricus differed, noticeably from that in other chromosomal genes. The site of transcriptional initiation was located by primer extension, and a probable promoter was identified for the gap-pgk-tpi operon. Northern hybridization of total RNA with specific probes showed two transcripts, an mRNA of 1.4 kb corresponding to the gap gene, and a less abundant mRNA of 3.4 kb corresponding to the gap-pgk-tpi cluster. The absence of a visible terminator in the 3'-end of the shorter transcript and the location of this 3'-end inside the pgk gene indicated that this shorter transcript was produced by degradation of the longer one, rather than by an early termination of transcription after the gap gene.

Identification of ATP-dependent phosphofructokinase as a regulatory step in the glycolytic pathway of the actinomycete Streptomyces coelicolor A3(2)

The ATP-dependent phosphofructokinase (ATP-PFK) of Streptomyces coelicolor A3(2) was purified to homogeneity (1,600-fold) and characterized (110 kDa, with a single type of subunit of 40 kDa); it is allosterically inhibited by phosphoenolpyruvate. Cloning of the pfk gene of S. coelicolor A3(2) and analysis of the deduced amino acid sequence (343 amino acids; 36,667 Da) revealed high similarities to the PPi-PFK enzyme from Amycolatopsis methanolica (tetramer, nonallosteric; 70%) and to the allosteric ATP-PFK enzymes from other bacteria, e.g., Escherichia coli (tetramer; 37%) and Bacillus stearothermophilus (tetramer, 41%). Further structural and functional analysis of the two actinomycete PFK enzymes should elucidate the features of these proteins that determine substrate specificity (ATP versus PPi) and allosteric (in)sensitivity.

The genes for phosphofructokinase and pyruvate kinase of Lactobacillus delbrueckii subsp. bulgaricus constitute an operon

In Lactobacillus delbrueckii subsp. bulgaricus, the pyk gene coding for pyruvate kinase and the pfk gene coding for phosphofructokinase formed a bicistronic operon transcribed into a 2.9-kb RNA. The nucleotide sequence of the pyk gene indicated that the encoded protein possessed an extra C-terminal domain with a potential phosphoenolpyruvate-dependent autophosphorylation site.

Cloning, sequence and expression in Escherichia coli of the gene encoding phosphofructokinase from Bacillus macquariensis

A chromosomal DNA fragment containing the Bacillus macquariensis (Bm) ATP-dependent phosphofructokinase-encoding gene (pfk) was cloned from a subgenomic library in pUC19 using a PCR-derived probe. The region containing pfk, including flanking sequences, was sequenced and the deduced amino acid sequence (aa) was found to be homologous to other PFK, but it contained two single-aa changes conserved in a range of other organisms from pro- and eukaryotic origins. Enzymatic studies with PFK purified from overproducing Escherichia coli (Ec) host cells showed that the Bm enzyme is similar to B. stearothermophilus (Bs) PFK in many respects and that it is relatively cold stable.

The potential of Lactobacillus as a carrier for oral immunization: development and preliminary characterization of vector systems for targeted delivery of antigens

Oral administration of lactobacilli evokes mucosal and systemic immune responses against epitopes associated with these organisms (Gerritse et al., 1990, 1991). The adjuvant function of different Lactobacillus species was investigated under the conditions of intraperitoneal (i.p.) injection or oral administration. After i.p. injection of trinitrophenylated chicken gamma-globulin, high DTH responses were observed with Lactobacillus casei and Lactobacillus plantarum, but low responses with Lactobacillus fermentum and Lactobacillus delbrueckii subsp. bulgaricus. In different experimental model systems L. casei and L. plantarum consistently showed significant adjuvanticity. A series of expression and expression-secretion vectors containing the strong constitutive promoter of the L. casei L-ldh gene or the regulatable promoter of the Lactobacillus amylovorus amy gene (Pouwels and Leer, 1995) was used for the intracellular, extracellular and surface-bound expression of an influenza virus antigenic determinant fused to Escherichia coli beta-glucuronidase. Intracellular expression of the fusion protein amounted to 1-2% of total soluble protein. Lactobacilli synthesizing the fusion protein intracellularly evoked an oral immune response after subcutaneous priming.

Characterization and phylogeny of the pfp gene of Amycolatopsis methanolica encoding PPi-dependent phosphofructokinase

The actinomycete Amycolatopsis methanolica employs a PPi-dependent phosphofructokinase (PPi-PFK) (EC 2.7.1.90) with biochemical characteristics similar to those of both ATP- and PPi-dependent enzymes during growth on glucose. A 2.3-kb PvuII fragment hybridizing to two oligonucleotides based on the amino-terminal amino acid sequence of PPi-PFK was isolated from a genomic library of A. methanolica. Nucleotide sequence analysis of this fragment revealed the presence of an open reading frame encoding a protein of 340 amino acids with a high degree of similarity to PFK proteins. Heterologous expression of this open reading frame in Escherichia coli gave rise to a unique 45-kDa protein displaying a high level of PPi-PFK activity. The open reading frame was therefore designated pfp, encoding the PPi-PFK of A. methanolica. Upstream and transcribed divergently from pfp, a partial open reading frame (aroA) similar to 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase-encoding genes was identified. The partial open reading frame (chiA) downstream from pfp was similar to chitinase genes from Streptomyces species. A phylogenetic analysis of the ATP- and PPi-dependent proteins showed that PPi-PFK enzymes are monophyletic, suggesting that the two types of PFK evolved from a common ancestor.

The cooperativity and allosteric inhibition of Escherichia coli phosphofructokinase depend on the interaction between threonine-125 and ATP

During the reaction catalyzed by the phosphofructokinase (EC 2.7.1.11) from Escherichia coli, the phosphoryl group transferred from ATP interacts with Thr-125 [Shirakihara, Y. & Evans, P. R. (1988) J. Mol. Biol. 204, 973-994]. The replacement of Thr-125 by serine changes the saturation by fructose 6-phosphate from cooperative to hyperbolic and abolishes the allosteric inhibition by phosphoenolpyruvate. The same changes, a saturation by fructose 6-phosphate that is no longer cooperative and an activity that is no longer inhibited by phosphoenolpyruvate, are observed with wild-type phosphofructokinase when adenosine 5'-[gamma-thio]triphosphate is used instead of ATP as the phosphoryl donor. These two perturbations of the ATP-Thr-125 interaction lead to the suppression of both the allosteric inhibition by phosphoenolpyruvate and the cooperativity of fructose-6-phosphate saturation, as if replacing the neutral oxygen of ATP by sulfur or removing the methyl group of Thr-125 had "locked" phosphofructokinase in its active conformation. The geometry of this ATP-Thr-125 interaction and/or the presence of the methyl group on the beta-carbon of Thr-125 are crucial for the regulatory properties of phosphofructokinase. This interaction could be a hydrogen bond between the neutral oxygen of the gamma-phosphate of ATP and the hydroxyl group of Thr-125.

The role of Glu187 in the regulation of phosphofructokinase by phosphoenolpyruvate

In bacterial phosphofructokinases, either a glutamic or an aspartic residue is present at position 187, and the mechanism of inhibition by phosphoenolpyruvate seems to be correlated to the nature of residue 187. Upon binding phosphoenolpyruvate, only the enzymes with a Glu187 would undergo a major allosteric conformational change from an active into an inactive state, whereas the enzymes with an Asp187 would only show a simple upward shift in their pH-profile of activity. The phosphofructokinase from Spiroplasma citri, which has an Asp187, has been purified and its properties follow this pattern. The behaviour of mutants of the enzyme from Escherichia coli in which Glu187 is replaced by either aspartate or leucine confirms the importance of residue 187. The major allosteric transition of E. coli phosphofructokinase is abolished by the substitution Glu187-->Asp, suggesting that a glutamate at position 187 is necessary (but not sufficient) for the protein to undergo the change from the active into the inactive state induced by phosphenolpyruvate. In addition, the presence of an acidic residue, aspartate or glutamate, at position 187 is required (but not sufficient) for the binding of ADP (or GDP). This requirement of a negative charge for ADP binding could explain the striking conservation of an aspartate residue at position 187 in all the eukaryotic phosphofructokinases.