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#bacterial enzymes

3 public questions tagged with this topic.

Microbial Physiology

Which bacterial enzyme neutralizes hydrogen peroxide?

Hydrogen peroxide is unavoidable byproduct of flavoprotein oxidases such as NADH oxidase and autoxidation of menaquinones generating reactive oxygen stress capable of hydroxyl radical formation via Fenton reaction damaging iron-sulfur clusters and DNA. Aerobic bacteria depend on extremely efficient enzymatic detoxification. Catalase is tetrameric heme enzyme containing heme b active site where first H2O2 oxidizes ferric heme to Compound I ferryl oxo porphyrin radical, second H2O2 reduces Compound I releasing water and dioxygen. Catalytic rate is among highest known about ten million molecules per second per active site, essentially diffusion limited. Genes encoding catalases katG catalase-peroxidase and katE are induced via OxyR peroxide sensor recognizing hydrogen peroxide and general stress sigmaS RpoS during stationary phase. DNA polymerase replicates chromosome, RNA helicase unwinds RNA secondary structures, topoisomerase decatenates chromosomes, none scavenge peroxide, leaving catalase together with peroxidases like alkyl hydroperoxide reductase AhpCF as primary peroxide defense system distinguishing them from central information processing enzymes and explaining positive bubble test for catalase-positive organisms.

Ref: Alberts et al., Molecular Biology of the Cell, 7th ed., Chapter 12: Catalase Function and Hydrogen Peroxide Neutralization.

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Microbial Physiology

Which bacterial enzyme hydrolyzes β-lactam antibiotics, leading to resistance?

Resistance to beta-lactam antibiotics including penicillins, cephalosporins, monobactams and carbapenems frequently arises through enzymatic destruction rather than target alteration. Beta-lactamases are hydrolases classified into Ambler molecular classes A to D based on amino acid sequence homology and catalytic mechanism. Serine beta-lactamases, including plasmid-encoded TEM-1, SHV-1, extended-spectrum beta-lactamases CTX-M and serine carbapenemases KPC, employ an active-site serine residue that performs nucleophilic attack on carbonyl carbon of the strained beta-lactam ring, forming a transient covalent acyl-enzyme intermediate that is rapidly hydrolyzed by activated water molecule, opening the ring irreversibly and rendering drug unable to acylate PBPs. Metallo-beta-lactamases like NDM, VIM and IMP are class B enzymes that use one or two Zn2+ ions to polarize water for direct nucleophilic attack, capable of hydrolyzing almost all beta-lactams except aztreonam. Production can be chromosomally encoded or plasmid-encoded, inducible or constitutive, often associated with porin loss compounding resistance. DNA gyrase is target of fluoroquinolones, catalase detoxifies hydrogen peroxide, transpeptidase is the drug target rather than resistance enzyme, distinguishing beta-lactamase as the specific deactivating enzyme responsible for clinical failure of beta-lactams.

Ref: Prescott's Microbiology, 11th ed., Chapter 36: Beta-Lactamase Enzymes and Antibiotic Resistance.

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Replication in Prokaryotes

DNA gyrase is a type of

Topoisomerases classified mechanistically by whether cleavage of one versus two DNA strands occurs during catalytic cycle. Type I cuts single strand allowing rotation to change linking number in steps of one. Type II cuts both strands, passes intact duplex through break, alters linking number by two. DNA gyrase and topoisomerase IV are bacterial type II enzymes requiring ATP and Mg2+ forming transient covalent phosphotyrosine intermediates. Gyrase uniquely introduces negative supercoils and removes positive supercoils ahead of replication, while topo IV primarily decatenates chromosomes. Double-strand mechanism distinguishes gyrase from type I enzymes.

Ref: Berg et al., Biochemistry, 9th ed., Chapter 27: Type II topoisomerases, DNA gyrase mechanism and classification

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