Purification and properties of citrate lyase ligase from Streptococcus diacetilactis

Characterization of CIM monoliths as enzyme reactors

The immobilization of the enzymes citrate lyase, malate dehydrogenase, isocitrate dehydrogenase and lactate dehydrogenase to CIM monolithic supports was performed. The long-term stability, reproducibility, and linear response range of the immobilized enzyme reactors were investigated along with the determination of the kinetic behavior of the enzymes immobilized on the CIM monoliths. The Michaelis-Menten constant K(m) and the turnover number k(3) of the immobilized enzymes were found to be flow-unaffected. Furthermore, the K(m) values of the soluble and immobilized enzyme were found to be comparable. Both facts indicate the absence of a diffusional limitation in immobilized CIM enzyme reactors.

Klebsiella pneumoniae genes for citrate lyase and citrate lyase ligase: localization, sequencing, and expression

In the course of studies on anaerobic citrate metabolism in Klebsiella pneumoniae, the DNA region upstream of the gene for the sodium-dependent citrate carrier (citS) was investigated. Nucleotide sequence analysis revealed a cluster of five new genes that were oriented inversely to citS and probably form an operon. The genes were named citCDEFG. Based on known protein sequence data, the gene products derived from citD, citE and citF could be identified as the gamma-, beta-, and alpha-subunits of citrate lyase, respectively. This enzyme catalyses the cleavage of citrate to oxaloacetate and acetate. The gene product derived from citC (calculated M(r) 38,476) exhibited no obvious similarity to other proteins. In the presence of acetate and ATP, cell extracts from a citC-expressing Escherichia coli strain were able to reactivate purified citrate lyase from K. pneumoniae that had been inactivated by chemical deacetylation of the prosthetic group. This represents 5-phosphoribosyl-dephospho-acetyl-coenzyme A which is covalently bound to serine-14 of the acyl carrier protein (gamma-subunit). CitC was thus identified as acetate:SH-citrate lyase ligase. The function of the gene product derived from citG (M(r) 32,645) has not yet been identified. Expression of the citCDEFG gene cluster in E. coli led to the formation of citrate lyase which was active only in the presence of acetyl-coenzyme A, a compound known to substitute for the prosthetic group. These and other data strongly indicated that the enzyme synthesized in E. coli lacked its prosthetic group. Thus, additional genes besides citCDEFG appear to be required for the formation of holo-citrate lyase.

Covalent modification of citrate lyase ligase from Clostridium sphenoides by phosphorylation/dephosphorylation

Citrate lyase ligase was shown to be present in Clostridium sphenoides actively degrading citrate. In contrast to citrate lyase ligase from C. sporosphaeroides and Streptococcus lactis, the enzyme from C. sphenoides was under stringent regulatory control. The alteration of the kinetic properties of the enzyme after depletion of citrate suggested the presence of two different enzyme species in different phases of growth: active and partially active citrate lyase ligase. These enzymes were purified from in vivo 32P-labeled C. sphenoides cells, which were grown on low-phosphate medium containing 40 mM citrate and 1 mCi [32]orthophosphate. During enzyme purification only the active form of citrate lyase ligase was shown to be radioactively labeled. Growth experiments with 14C-labeled precursors of purines and pyrimidines and subsequent purification of active citrate lyase ligase indicated that the 32P labeling of the enzyme was not due to the incorporation of a nucleotide. Inactivation of the ligase after its treatment with acid phosphatase also suggested that the active form of the enzyme is phosphorylated. Citrate lyase ligase, therefore, is the first known enzyme in an anaerobic bacterium whose activity is modulated by phosphorylation/dephosphorylation.

Purification of L-glutamate-dependent citrate lyase from Clostridium sphenoides and electron microscopic analysis of citrate lyase isolated from Rhodopseudomonas gelatinosa, Streptococcus diacetilactis and C. sphenoides

Citrate lyase from Clostridium sphenoides was purified 72-fold with a yield of 11%. In contrast to citrate lyase from other sources the activity of this enzyme was strictly dependent on the presence of L-glutamate. The purified enzyme was only stable in the presence of 150 mM L-glutamate or 7 mM L-glutamate plus glycerol, sucrose or bovine serum albumin. Changes of the L-glutamate pool and of enzyme activity in growing cells of C. sphenoides indicated that citrate lyase activity in this organism was regulated by the intracellular L-glutamate concentration. Citrate lyase isolated from C. sphenoides, Rhodopseudomonas gelatinosa and Streptococcus diacetilactis was investigated by electron microscopy using the negative staining technique. Three different projections of enzyme molecules were observed: 'star' form, 'ring' form and 'triangle' form. In samples from R. gelatinosa and S. diacetilactis, star and ring forms occurred in a ratio of about 1:9. Using the enzyme from S. diacetilactis it was demonstrated that this ratio could be altered in favour of the star form by the addition of citrate or tricarballylate. The triangle form was observed in less than 1% of all evaluated molecules and may represent a transition form. In lyase samples from C. sphenoides there existed a correlation between enzyme activity and the proportion of stars and rings at varying concentrations of L-glutamate.

Copurification of citrate lyase and citrate lyase ligase from Rhodopseudomonas gelatinosa and subsequent separation of the two enzymes

A procedure has been worked out which allowed the purification and crystallization of a citrate lyase/citrate lyase ligase complex from Rhodopseudomonas gelatinosa. The complex was subsequently separated to yield two homogeneous enzymes. Citrate lyase ligase was purified 365-fold with a yield of 3.23%. The molecular weight of the enzyme was estimated to be 39500, the enzyme consisted of one polypeptide chain. The reaction rates for ATP, acetate and citrate lyase (sulfhydryl form) followed Michaelis-Menten kinetics (Km values: 0.14 mM, 5 mM and 37 nM respectively). Citrate lyase ligase exhibited a high substrate specificity and could not react with citrate lyases from nonphototrophic microorganisms. In contrast to the ligase from Streptococcus diacetilactis, the enzyme from R. gelatinosa was extremely labile; however, it could be stabilized by nucleotides, the most potent stabilizing one being ADP.

Citrate lyase deacetylase of Rhodopseudomonas gelatinosa. Isolation of the enzyme and studies on the inhibition by L-glutamate

Citrate lyase deacetylase or acetyl-S-(acyl-carrier protein) enzyme thioester hydrolase (acetate) (EC 3.1.2-), was purified 3100-fold with a yield of 3.8% from cell extracts of Rhodopseudomonas gelatinosa. The final enzyme preparation gave a single protein band upon polyacrylamide-gel electrophoresis in the absence or in the presence of sodium dodecylsulfate. The molecular weight of the native enzyme was estimated by gel filtration to be 14 300 +/- 1000. Sodium dodecylsulfate/polyacrylamide gel electrophoresis yielded a molecular weight of 7300 +/- 600 indicating that the enzyme consisted of two subunits. Citrate lyase deacetylase acted as an S-acetyl enzyme thioesterhydrolase because it catalyzed the conversion of citrate lyase (S-acetyl form) into citrate lyase (sulfhydryl form) and acetate. Citrate lyase deacetylase was strongly inhibited by L-glutamate. The half-maximal inhibitor concentration was 7.5 X 10(-4) M. and a Ki-value of 1.2 X 10(-4) M was determined. The mode of inhibition appeared to be of the linear mixed type. L-Glutamate was bound by citrate lyase deacetylase but not by citrate lyase. The pool concentrations of L-glutamate in R. gelatinosa were 10 mM when citrate was present in substrate amounts and 2.7 mM after total consumption of citrate. Simulation of conditions in vivo using homogeneous enzyme preparations of citrate lyase and citrate lyase deacetylase, and glutamate concentrations of 10 mM and 2.7 mM respectively, revealed that the rate of citrate lyase inactivation increased from 8% within 15 min during growth on citrate to 50% within 15 min in the absence of citrate.