A strong carboxylate-arginine interaction is important in substrate orientation and recognition in lactate dehydrogenase

Using site-directed mutagenesis, Arginine-171 at the substrate-binding site of Bacillus stearothermophilus, lactate dehydrogenase has been replaced by lysine. In the closely homologous eukaryotic lactate dehydrogenase, this residue binds the carboxylate group of the substrate by forming a planar bifurcated bond. The mutation diminishes the binding energy of pyruvate, alpha-ketobutyrate and alpha-ketovalerate (measured by kcat/Km) by the same amount (about 6 kcal/mol). For each additional methylene group on the substrate, there is a loss of about 1.5 kcal/mol of binding energy in both mutant and wild-type enzymes. From these parallel trends in the two forms of enzyme, we infer that the mode of productive substrate binding is identical in each, the only difference being the loss of a strong carboxylate-guanidinium interaction in the mutant. In contrast to this simple pattern in kcat/Km, the Km alone increases with substrate-size in the wild-type enzyme, but decreases in the mutant. These results can be most simply explained by the occurrence of relatively tight unproductive enzyme-substrate complexes in the mutant enzyme as the substrate alkyl chain is extended. This does not occur in the wild-type enzyme, because the strong orienting effect of Arg-171 maximizes the frequency of substrates binding in the correct alignment.

The importance of arginine 171 in substrate binding by Bacillus stearothermophilus lactate dehydrogenase

A variant of lactate dehydrogenase from Bacillus stearothermophilus has been engineered by site-directed mutagenesis in which an active-site arginine residue at position 171 in the protein sequence is replaced by lysine. Replacement of this arginine by lysine has no effect on co-enzyme binding, a relatively small effect on the rate of turnover of the enzyme, but causes a 2000-fold increase in the Michaelis constant for pyruvate, a 6000-fold increase in the dissociation constant for oxamate and results in a Michaelis constant for lactate which is too high to measure. The decrease in binding energy for these carboxylate-containing substrates caused by this mutation is very large, around 5.5 kcal.mol-1 and in part, is explained by the small increase in the distance of a lysine-substrate carboxylate interaction at this site and the absence of the additional hydrogen bond from a two-point arginine-carboxylate interaction. Consistent with this last observation, the ability of this mutant enzyme to stabilize an NAD+-sulphite compound in its active site (an alternative enzyme-substrate complex which does not involve bifurcated bonding to arginine) is only reduced 14-fold.

Amino acid sequence of the L-lactate dehydrogenase of Bacillus caldotenax deduced from the nucleotide sequence of the cloned gene

The Bacillus caldotenax L-lactate dehydrogenase gene (lct) has been cloned into Escherichia coli, using the Bacillus stearothermophilus lct gene as a hybridisation probe, and its complete nucleotide sequence determined. The lct structural gene consists of an open reading frame of 951 base pairs commencing with an ATG start codon and followed by a TAA stop codon. Upstream of the gene are putative transcriptional promoter -35 and -10 regions; a ribosome binding site with a predicted delta G of -66.9 kJ/mol is also present six base pairs upstream of the ATG start codon. The B. caldotenax lct gene is highly homologous to the B. stearothermophilus lct gene displaying a DNA sequence homology of 89.7%. Examination of the DNA sequence 3' of the lct gene revealed the presence of two further open reading frames. This suggests that the lct gene may be the first gene of an operon. The deduced amino acid sequence of the L-lactate dehydrogenase (LDH) from B. caldotenax predicted a protein of 317 amino acid residues; comparison with the B. stearothermophilus enzyme revealed only 30 amino acid differences between the two enzymes; thus the enzymes are 90.4% homologous. These amino acid differences must account for the different thermostabilities of the two enzymes. The B. caldotenax lct gene was efficiently expressed in E. coli and the original lct-containing plasmid construct isolated (pKD1) induced the synthesis of LDH at a level of 4.5% of the E. coli soluble cell protein whilst a SmaI subfragment of this clone, (pKD2) produced LDH at a level of 6.9% of the E. coli soluble cell protein. LDH isolated from E. coli cells had the same thermal stability properties as LDH isolated from B. caldotenax cells.

The use of site-directed mutagenesis and time-resolved fluorescence spectroscopy to assign the fluorescence contributions of individual tryptophan residues in Bacillus stearothermophilus lactate dehydrogenase

Site-directed mutagenesis has been used to generate two mutant Bacillus stearothermophilus lactate dehydrogenases: in one, Trp-150 has been replaced with a tyrosine residue and, in the other, both Trp-150 and -80 are replaced with tyrosines. Both enzymes are fully catalytically active and their affinities for substrates and coenzymes, and thermal stabilities are very similar to those of the native enzyme. Time-resolved fluorescence measurements using a synchrotron source have shown that all three tryptophans in the native enzyme fluoresce. By comparing the mutant and native enzymes it was possible, for the first time, to assign, unambiguously, lifetimes to the individual tryptophans: Trp-203 (7.4 ns), Trp-80 (2.35 ns) and Trp-150 (less than 0.3 ns). Trp-203 is responsible for 75-80% of the steady-state fluorescence emission, Trp-80 for 20%, and Trp-150 for less than 2%.

A single amino acid substitution deregulates a bacterial lactate dehydrogenase and stabilizes its tetrameric structure

We have engineered a variant of the lactate dehydrogenase enzyme from Bacillus stearothermophilus in which arginine-173 at the proposed regulatory site has been replaced by glutamine. Like the wild-type enzyme, this mutant undergoes a reversible, protein-concentration-dependent subunit assembly, from dimer to tetramer. However, the mutant tetramer is much more stable (by a factor of 400) than the wild type and is destabilized rather than stabilized by binding the allosteric regulator, fructose 1,6-biphosphate (Fru-1,6-P2). The mutation has not significantly changed the catalytic properties of the dimer (Kd NADH, Km pyruvate, Ki oxamate and kcat), but has weakened the binding of Fru-1,6-P2 to both the dimeric and tetrameric forms of the enzyme and has almost abolished any stimulatory effect. We conclude that the Arg-173 residue in the wild-type enzyme is directly involved in the binding of Fru-1,6-P2, is important for allosteric communication with the active site, and, in part, regulates the state of quaternary structure through a charge-repulsion mechanism.

Site-directed mutagenesis reveals role of mobile arginine residue in lactate dehydrogenase catalysis

The binding of substrates to lactate dehydrogenases induces a marked rearrangement of the protein structure in which a 'loop' of polypeptide (residues 98-110) closes over the active site of the enzyme. In this rearrangement, arginine 109 (a basic residue conserved in all known lactate dehydrogenase sequences and in the homologous malate dehydrogenases) moves 0.8 nm from a position in the solvent to one in the active site where its guanidinium group resides within hydrogen bonding distance of both the reactive carbonyl of pyruvate and imidazole ring of the catalytic histidine 195 (see Fig. 1). Whilst this feature of the enzyme has been commented upon previously, the function of this mobile arginine residue during catalysis has not been tested experimentally. The advent of protein engineering has now enabled us to define the role of this basic residue by substituting it with the neutral glutamine. Transient kinetic and equilibrium studies of the mutant enzyme indicate that arginine 109 enhances the polarization of the pyruvate carbonyl group in the ground state and stabilizes the transition state. The gross active-site structure of the enzyme is not altered by the mutation since an alternative catalytic function of the enzyme (rate of addition of sulphite to NAD+), which does not require hydride transfer, is insensitive to the arginine----glutamine substitution.

Cloning, expression and complete nucleotide sequence of the Bacillus stearothermophilus L-lactate dehydrogenase gene

The structural gene for L-lactate dehydrogenase (LDH; EC 1.1.1.27) from Bacillus stearothermophilus NCA 1503 has been cloned in Escherichia coli and its complete nucleotide sequence determined. The predicted amino acid (aa) sequence of the LDH enzyme agrees with the previously determined aa sequence except to three positions: aa 125 and 126, Ser-Glu, are inverted whilst His at position 130 has been replaced by Ser in our sequence. The lct gene consists of an open reading frame (ORF) commencing from the ATG start codon of 951 bp followed by a TGA stop codon. Upstream from the start codon is a strong (delta G = -14.4 kcal) Shine-Dalgarno (SD) sequence, a feature typical of Gram-positive ribosome binding sites. Putative RNA polymerase recognition signals (-35 and -10 regions) have been identified upstream from the lct structural gene but there are no structures resembling Rho-independent transcription termination signals downstream from the TGA stop codon. Two further ORFs, preceded by SD sequences, are present downstream from the lct gene. Thus the lct gene may constitute the first gene of an operon. Subclones of the lct gene have been constructed in the expression plasmid pKK223-3 and the LDH enzyme produced in soluble form at levels of up to 36% of the E. coli soluble cell protein.

The rates of defined changes in protein structure during the catalytic cycle of lactate dehydrogenase

Rapid mixing, kinetic experiments were performed on native and modified [Tyr(3NO2)237)] porcine H4 lactate dehydrogenase at low temperatures in a medium containing 30% dimethyl sulphoxide. In the temperature range -16 to +8 degrees C, the modified enzyme-NADH complex, when mixed with 1 mM pyruvate, is converted to enzyme, NAD+ and lactate at two distinctly different rates. At -16 degrees C the more rapid process occurs at a rate of 40 s-1 and the slower at 3 s-1. The slower rate is identical to that assigned to the steady-state turnover of the enzyme in these conditions and therefore reflects the slow, rate-limiting rearrangement of protein structure which has been inferred from previous kinetic experiments. The fast phase of NADH oxidation, however, proceeds at a rate which coincides with that of the closure of a loop of polypeptide over the active site of the enzyme (sensed by the nitrotyrosine group, which protonates in response to the approach of glutamate 107, a residue situated on this mobile loop). We explain these results by proposing that: (i) both the slow and fast changes in protein structure must occur before the enzyme can accomplish the redox step, (ii) the enzyme-NADH (binary) complex exists in two, slowly interconverting forms, (iii) the structural change giving rise to this slow conformational equilibrium can also occur in the ternary (enzyme-NADH-pyruvate) complex and (iv) it is this step which limits the rate of the steady-state reaction. Both of the binary forms are able to bind pyruvate, but the rate of NADH oxidation in one of the forms is rapid, since it has already undergone this slow rearrangement. In this rapidly reacting form, it is the closure of the loop (not transfer of the hydride ion) which limits the rate at which the coenzyme is oxidized, while the slowly reacting form must undergo both loop-closure and the slow structural conversion before the redox reaction can occur.

Changes in the state of subunit association of lactate dehydrogenase from Bacillus stearothermophilus

Time-resolved measurements of the fluorescence anisotropy of an extrinsic dye-group attached to lactate dehydrogenase from B. stearothermophilus revealed that the rotational correlation time of the enzyme at low concentrations is 55 ns, while at high enzyme concentrations or in the presence of fructose 1,6-bisphosphate (Fru-1,6-P2) the correlation time increases to 95 ns. These correlation times are consistent with a change in Mr from 85 000 +/- 12 000 (dimer) to 150 000 +/- 22 000 (tetramer) and show that the tetrameric state can be induced either by raising the protein concentration or by the addition of the ligand. We have confirmed this change in molecular weight by gel-filtration experiments. In the ligand-induced tetramer, two Fru-1,6-P2 molecules are bound.

The mechanism of activation of lipoprotein lipase by apolipoprotein C-II. The formation of a protein-protein complex in free solution and at a triacylglycerol/water interface

The binding of lipoprotein lipase to a fluorescently labelled apolipoprotein C-II in free solution has been followed by measuring fluorescence anisotropy. The formation of a weak, binary complex in which a single apolipoprotein C-II molecule associates non-cooperatively with each subunit of the dimeric enzyme was observed. The dissociation constant for this complex in 0.05 M NaCl is 0.2 X 10(-6) M and it is weakened markedly by raising the salt concentration and by the binding of heparin to the enzyme. The assembly of the same protein-protein complex on the surface of glycerol trioleate globules has been monitored by steady-state and pre-steady-state kinetics. In these circumstances the lipoprotein lipase-apolipoprotein C-II interaction is much tighter (Kd = (7-10) X 10(-9) M) and is insensitive to salt and heparin. The mechanism of activation of the enzyme at low concentrations of apolipoprotein C-II is described by a kinetic model in which apolipoprotein C-II binds preferentially to the form of the enzyme which is associated with the triacylglycerol substrate. This preference leads to a stabilization of the enzyme-substrate complex, thus reducing the apparent Ks.

The effect of the chain length of heparin on its interaction with lipoprotein lipase

The interactions of bovine milk lipoprotein lipase (triacylglycero-protein acylhydrolase, EC 3.1.1.34) with the glycosaminoglycans heparin and heparan sulphate were investigated using the technique of fluorescence polarization spectroscopy. The type of complex formed with the enzyme depends on the chain length of the heparin. In 0.05 M NaCl and when the heparin was in molar excess, one heparin chain of Mr 10000-18400 formed a very stable complex with the dimeric protein molecule (the 1:1 complex). With excess protein, weaker interactions produced complexes with higher molecular weights. These two classes of complex were also detected with shorter heparins (Mr 6600-8000), although in these circumstances the more stable complex possessed a heparin:protein dimer ratio of 2:1. In higher salt (0.2 M NaCl) and lower heparin concentrations (less than 6 . 10(-8) M) the weaker class of compound was undetectable and Kd values of 4 . 10(-8) M and 6 . 10(-9) M were assigned to the 2:1 and 1:1 complexes, respectively. Heparan sulphate of Mr 17000 could only form one class of complex. This had a 1:1 stoichiometry and with Kd values of 3 . 10(-8) M and 1.6 . 10(-7) M at 0.05 and 0.2 M NaCl, respectively. The results could be explained if there is a distinct binding region for glycosaminoglycans on each subunit of the dimeric enzyme and a single heparin chain of Mr greater than 10000 can satisfy both sites to form a 1:1 complex. Smaller heparin chains are unable to span the sites and, in order to occupy them, two chains must interact with each enzyme molecule.

Absorption of sodium benzylpenicillin from the equine uterus after local Lugol's lodine treatment, compared with absorption after intramuscular injection

Plasma concentrations of sodium benzylpenicillin following intrauterine infusion were increased by reducing the volume of solution and expelling air from the vagina after infusion. Instillation of 10 per cent Lugol's iodine solution into the uterus before penicillin infusion further increased the absorption rate, although peak plasma levels of penicillin were less than half those which resulted from intramuscular injection of the same dose.