A classification scheme for b-lactamases based on functional characteristics is presented. Three major groups of enzymes are defined by their substrate and inhibitor profiles: group 1 cephalosporinases that are not well inhibited by clavulanic acid; group 2 penicillinases, cephalosporinases, and broadspectrum b-lactamases that are generally inhibited by active site-directed b-lactamase inhibitors; and the group 3 metallob-lactamases that hydrolyze penicillins, cephalosporins, and carbapenems and that are poorly inhibited by almost all b-lactam-containing molecules. Functional characteristics have been correlated with molecular structure in a dendrogram for those enzymes with known amino acid sequences. b-Lactamases (EC 3.5.2.6) have been designated by the Nomenclature Committee of the International Union of Biochemistry as ‘‘enzymes hydrolysing amides, amidines and other CON bonds . . . separated on the basis of the substrate: . . . cyclic amides’’ (323). These enzymes are the major cause of bacterial resistance to b-lactam antibiotics and have been the subject of extensive microbiological, biochemical, and genetic investigations. Investigators have described more than 190 unique bacterial proteins with the ability to interact with the variety of b-lactam-containing molecules that can serve as sub-

A functional classification scheme for beta-lactamases and its correlation with molecular structure.

Neoplastic cells simultaneously harbor widespread genomic hypomethylation, more regional areas of hypermethylation, and increased DNA-methyltransferase (DNA-MTase) activity. Each component of this "methylation imbalance" may fundamentally contribute to tumor progression. The precise role of the hypomethylation is unclear, but this change may well be involved in the widespread chromosomal alterations in tumor cells. A main target of the regional hypermethylation are normally unmethylated CpG islands located in gene promoter regions. This hypermethylation correlates with transcriptional repression that can serve as an alternative to coding region mutations for inactivation of tumor suppressor genes, including p16, p15, VHL, and E-cad. Each gene can be partially reactivated by demethylation, and the selective advantage for loss of gene function is identical to that seen for loss by classic mutations. How abnormal methylation, in general, and hypermethylation, in particular, evolve during tumorigenesis are just beginning to be defined. Normally, unmethylated CpG islands appear protected from dense methylation affecting immediate flanking regions. In neoplastic cells, this protection is lost, possibly by chronic exposure to increased DNA-MTase activity and/or disruption of local protective mechanisms. Hypermethylation of some genes appears to occur only after onset of neoplastic evolution, whereas others, including the estrogen receptor, become hypermethylated in normal cells during aging. This latter change may predispose to neoplasia because tumors frequently are hypermethylated for these same genes. A model is proposed wherein tumor progression results from episodic clonal expansion of heterogeneous cell populations driven by continuous interaction between these methylation abnormalities and classic genetic changes.

Alterations in dna methylation : a fundamental aspect of neoplasia

Genes constitute only a small proportion of the total mammalian genome, and the precise control of their expression in the presence of an overwhelming background of noncoding DNA presents a substantial problem for their regulation. Noncoding DNA, containing introns, repetitive elements, and potentially active transposable elements, requires effective mechanisms for its long-term silencing. Mammals appear to have taken advantage of the possibilities afforded by cytosine methylation to provide a heritable mechanism for altering DNA-protein interactions to assist in such silencing. Genes can be transcribed from methylation-free promoters even though adjacent transcribed and nontranscribed regions are extensively methylated. Gene promoters can be used and regulated while keeping noncoding DNA, including transposable elements, suppressed. Methylation is also used for long-term epigenetic silencing of X-linked and imprinted genes and can either increase or decrease the level of transcription, depending on whether the methylation inactivates a positive or negative regulatory element.

https://science.sciencemag.org/content/sci/293/5532/1068.full.pdf

The Role of DNA Methylation in Mammalian Epigenetics

beta-Lactamases are the commonest single cause of bacterial resistance to beta-lactam antibiotics. Numerous chromosomal and plasmid-mediated types are known and may be classified by their sequences or phenotypic properties. The ability of a beta-lactamase to cause resistance varies with its activity, quantity, and cellular location and, for gram-negative organisms, the permeability of the producer strain. beta-Lactamases sometimes cause obvious resistance to substrate drugs in routine tests; often, however, these enzymes reduce susceptibility without causing resistance at current, pharmacologically chosen breakpoints. This review considers the ability of the prevalent beta-lactamases to cause resistance to widely used beta-lactams, whether resistance is accurately reflected in routine tests, and the extent to which the antibiogram for an organism can be used to predict the type of beta-lactamase that it produces.

https://courses.washington.edu/pharm522/Week9/0139.pdf

beta-Lactamases in laboratory and clinical resistance.

Compositions of, genetic constructions coding for, and methods for producing multivalent antigen-binding proteins are described and claimed. The methods include purification of compositions containing both monomeric and multivalent forms of single polypeptide chain molecules, and production of multivalent proteins from purified monomers. Production of multivalent proteins may occur by a concentration-dependent association of monomeric proteins, or by rearrangement of regions involving dissociation followed by reassociation of different regions. Bivalent proteins, including homobivalent and heterobivalent proteins, are made in the present invention. Genetic sequences coding for bivalent single-chain antigen-binding proteins are disclosed. Uses include all those appropriate for monoclonal and polyclonal antibodies and fragments thereof, including use as a bispecific antigen-binding molecule.

Multivalent antigen-binding proteins

The catalytic properties of three class B beta-lactamases (from Pseudomonas maltophilia, Aeromonas hydrophila and Bacillus cereus) were studied and compared with those of the Bacteroides fragilis enzyme. The A. hydrophila beta-lactamase exhibited a unique specificity profile and could be considered as a rather specific 'carbapenemase'. No relationships were found between sequence similarities and catalytic properties. The problem of the repartition of class B beta-lactamases into sub-classes is discussed. Improved purification methods were devised for the P. maltophilia and A. hydrophila beta-lactamases including, for the latter enzyme, a very efficient affinity chromatography step on a Zn(2+)-chelate column.

/pdf/an-overview-of-the-kinetic-parameters-of-class-b-beta-4zoxv1974y.pdf

Roberto Strom

Papers

An overview of the kinetic parameters of class B beta-lactamases.

The biochemical mechanism of selective heat sensitivity of cancer cells. I. Studies on cellular respiration.

The biochemical mechanism of selective heat sensitivity of cancer cells. II. Studies on nucleic acids and protein synthesis.

The dynamics of myogenin site-specific demethylation is strongly correlated with its expression and with muscle differentiation.

Brain microvessels take up large neutral amino acids in exchange for glutamine. Cooperative role of Na+-dependent and Na+-independent systems.