Studies on the β-galactosidase isolated from Escherichia coli ML 308. I. The effect of some ions on enzymic activity

LacZ β-galactosidase: structure and function of an enzyme of historical and molecular biological importance

This review provides an overview of the structure, function, and catalytic mechanism of lacZ β-galactosidase. The protein played a central role in Jacob and Monod's development of the operon model for the regulation of gene expression. Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino-terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. It was also possible to rationalize α-complementation, in which addition to the inactive dimers of peptides containing the "missing" N-terminal residues restored catalytic activity. The enzyme is well known to signal its presence by hydrolyzing X-gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X-ray structure represents an active conformation. Individual tetramers of β-galactosidase have been measured to catalyze 38,500 ± 900 reactions per minute. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action. Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step (called galactosylation) is a nucleophilic displacement by Glu537 to form a covalent bond with galactose. This is initiated by proton donation by Glu461. The second displacement (degalactosylation) by water or an acceptor is initiated by proton abstraction by Glu461. Both of these displacements occur via planar oxocarbenium ion-like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.

Photoreversible modulators of Escherichia coli beta-galactosidase. 1-Benzoyl-1-cyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene and 1,1-dicyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene

Beta-galactosidase (EC 3.2.1.23) is known to be inhibited by some thiol reagents. 1-Benzoyl-1-cyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene (1) was shown to be an irreversible inhibitor, while 1, 1-dicyano-2-(4,5-dimethoxy-2-nitrophenyl)-ethene (2) was demonstrated as a positive irreversible modulator causing a rise of up to 186% in beta-galactosidase activity. Compound 2 is, however, an irreversible inhibitor of the cysteine proteinase papain (preceding paper). Kinetic values of beta-galactosidase at pH 8.3 with o-nitrophenyl beta-D-galactopyranoside (ONPG) as the substrate and for compounds 1 and 2 were determined and in view of model experiments, it was assumed that both compounds possibly reacted with the thiol side chain of Cys in the active site inducing allosteric changes in the enzyme. Since the enzyme, modified by compound 1 or 2, was a 2-nitrobenzyl derivative, near-UV irradiation resulted in a recovery of up to 91% and a reduction of the enzyme's activity to 90%, respectively.

Differences in the hydrolysis of lactose and other substrates by beta-D-galactosidase from Kluyveromyces lactis

The hydrolysis of o-nitrophenyl galactopyranoside and lactose by beta-D-galactosidase from Kluyveromyces lactis was enhanced by the addition of Mg2+ and Mn2+, but the rates of activation by each metal on both substrates were not the same. The Co2+, Zn2+, and Ni2+ activated the o-nitrophenyl galactopyranoside-hydrolyzing activity of the enzyme, but these same metals inhibited the lactose-hydrolyzing activity. The addition of Mg2+ and EDTA to the assay buffer increased the hydrolysis of o-nitrophenyl galactopyranoside and lactose at different rates. The responses of o-nitrophenyl galactopyranoside and lactose to the enzyme activity were different as a function of pH. The hydrolyzing activity toward both substrates also was influenced by the concentration of the phosphate in the assay buffer. However, the profile of the enzyme activity toward each substrate was different as a function of concentration. Because the assay of beta-galactosidase using o-nitrophenyl galactopyranoside is fast and convenient, the estimation of lactose-hydrolyzing activity of the enzyme has frequently been made based on the assay of o-nitrophenyl galactopyranoside hydrolysis. As shown in this study, a slight change in the conditions of the assay system and the enzyme application may cause changes in the ability of the enzyme to hydrolyze both lactose and o-nitrophenyl galactopyranoside. The change in o-nitrophenyl galactopyranoside-hydrolyzing activity is not always consistent with that of the lactose-hydrolyzing activity under the given condition, which may cause an inaccurate estimation of the enzyme activity in the enzyme preparation as well as in actual applications of the enzyme.

Histochemical staining of clonal mammalian cell lines expressing E. coli beta galactosidase indicates heterogeneous expression of the bacterial gene

An evaluation has been made of the E. coli beta-galactosidase (beta-gal) gene for use as a reporter gene in mammalian cells in culture. We have adopted a histochemical procedure which enables identification of those cells within a population that express the introduced bacterial gene. Data is presented concerning the sensitivity of the histochemical method relative to an immunological method of detection. It has been found that several clonal cell lines generated after transfection of human 293 cells with a Rous sarcoma virus (RSV) long terminal repeat (LTR) promoter-beta-gal construction are mosaic for expression of the introduced mini-gene. Furthermore, after treatment of these clonal cell lines with the nucleoside analog 5-aza-cytidine (5-aza-C), an increase in production of beta-gal under control of this promoter element was observed.

Expression of a preproinsulin-beta-galactosidase gene fusion in mammalian cells

As an approach to the study of mammalian gene expression, the promoters and translation initiation regions of the rat preproinsulin II and the simian virus 40 early genes were fused to the structural gene of Escherichia coli beta-galactosidase, a sensitive probe for gene expression. These fusions were introduced into COS-7 cells, a simian virus 40 large tumor-antigen-producing monkey kidney cell line, where they directed the synthesis of enzymatically active hybrid beta-galactosidase proteins. Conditions for transfection were varied to optimize the expression of beta-galactosidase activity in the transfected cells. The pH optimum of this activity was found to be 7.0, the same as that of native E. coli beta-galactosidase and distinct from the major lysosomal "acid" beta-galactosidase. The fused preproinsulin-beta-galactosidase was further characterized by gel electrophoresis of nondenatured cell extracts stained by a fluorogenic substrate and by immunoprecipitation and gel electrophoresis of 3H-labeled cell proteins. These results all indicate that fully active tetrameric beta-galactosidase hybrids can be produced in mammalian cells. The expression of preproinsulin-beta-galactosidase activity was measured in the presence of high glucose, insulin, dexamethasone, or epidermal growth factor but no regulatory changes were observed.

Isolation and characterization of the newly evolved ebg beta-galactosidase of Escherichia coli K-12

The ebg beta-galactosidase of Escherichia coli K-12 strain LC110 has been purified and characterized. Strain LC110 is a Lac+ revertant of a mutant with a deletion of the lacZ beta-galactosidase gene. Its new ebg beta-galactosidase activity was shown to be due to a discrete protein, immunologically unrelated to lacZ beta-galactosidase. Its kinetics of action conformed to those of a simple conventional enzyme. With o-nitrophenyl-beta-D-galactoside as substrate, the Vmax was 11,200 nmol/min per mg of enzyme, the Km was 5 mM, and the activation energy was 12,400 cal/mol. Corresponding values for lacZ beta-galactosidase of wild-type E. coli K-12 were 350,000 nmol/min per mg of enzyme, 1.3 mM, and 8,000 cal/mol. A series of sugars has been examined as competitive inhibitors of ebg beta-galactosidase. Kinetic analyses suggest that ebg beta-galactosidase has a particularly high affinity for galactosamine and gamma-galactonolactone, binds galatose more tightly than lactose, and shows a general preference for monosaccharides rather than beta-galactosides. We conclude that the ebg beta-galactosidase may have arisen by modification of a gene involved with the metabolism of a monosaccharide, possibly a 2-amino sugar.

Studies on protein multimers. The association-dissociation behaviour of -galactosidase in glycerol

1. The effect of glycerol on the association-dissociation behaviour of beta-galactosidases from Escherichia coli is described. Two strains, K12 and ML308, were used as sources of enzyme. The conditions used, involving glycerol at a concentration of 90%, result in dissociation of the active 540000-dalton form to inactive structural subunits of 135000 daltons. 2. A pH-dependent process, assumed to be cyclic in mechanism, allows reassociation to an active form indistinguishable from the initial protein. 3. The apparently identical structural subunits, if produced in the presence of EDTA, were found to give rise to two electrophoretically distinguishable species. 4. Enzymes from both strains of E. coli can be distinguished electrophoretically but exhibit the same behaviour in glycerol. 5. A scheme of the association-dissociation is presented that is consistent with the behaviour observed and that has some predictive value.