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D M Zhu

Publications and source records attributed to D M Zhu.

27 records · Page 2Linked to original sources
PubMed1995

The tryptophan synthase alpha 2 beta 2 complex: kinetic studies with a mutant enzyme (beta K87T) to provide evidence for allosteric activation by an aminoacrylate intermediate.

To investigate the mechanism by which the tryptophan synthase beta subunit accelerates the cleavage of the indole-3-glycerol phosphate catalyzed by the alpha subunit (alpha reaction), kinetic experiments were carried out with wild-type and mutant forms of the alpha 2 beta 2 complex. Previous studies indicate that this activation can be attributed to the conformational changes associated with the formation of a Schiff base between aminoacrylate and pyridoxal phosphate at the beta site. To test this hypothesis, we investigated a mutant form of the alpha 2 beta 2 complex having the lysine-87 of its beta subunits replaced by threonine. The mutant alpha 2 beta 2 complex (K87T) exhibits normal activity for the alpha reaction but fails to catalyze formation of L-tryptophan from L-serine and indole (beta reaction). However, the mutant enzyme can form a Schiff base intermediate with L-serine at the beta site. Using a "chemical rescue" method, we converted K87T L-serine intermediate to an aminoacrylate intermediate. Steady-state kinetic studies reveal that the aminoacrylate derivative exhibits a 7-fold enhancement in kcat/Km for the alpha reaction relative to that of the L-serine derivative of the mutant or the wild-type enzyme in the absence of L-serine. Rapid kinetic data show that the aminoacrylate derivative of the mutant enzyme exhibits a 6-fold increase in the rate constant for the indole-3-glycerol phosphate cleavage reaction. In addition, rate constants for the reverse reaction and product release steps are also altered. Together, these changes lead to a decrease in Km and an increase in kcat.(ABSTRACT TRUNCATED AT 250 WORDS)

Acrylates↗
PubMed1995

[Bacterial resistance in clinical isolates of Klebsiella pneumoniae].

Ninety-four percent of 336 strains of Klebsiella pneumoniae were resistant to ampicillin, while less than 10% were resistant to cefotaxine and ceftazidine. Among all the aminoglycosides tested, amikacin had the strongest activity on K. pneumoniae. The resistant rate of nosocomial strains was higher than that of community strains. The results of susceptibility test showed that 42% (119/280) of K. pneumoniae strains were gentamicin-resistant. Based on the resistant pattern of K. pneumoniae, the aminoglycoside-modifying enzymes produced by the organisms were inferred. Among them 90% of 119 strains produced aminoglycosides-modifying enzymes AAC (3)-V, 8% (10/119) AAC (3)-V together with AAC (6')-Ib, AAC (6')-V or ANT (2"). The resistance of two strains was assumed to be permeability change. Besides, 58% (161/280) were sensitive strains without producing any aminoglycoside-modifying enzymes. Bacteria typing done by phage typing or Klebcin in 27 strains showed that most strains were from different origins with few exceptions. The results show the significance for controlling the nosocomial infections and reasonable use of antibiotics.

Acetyltransferases↗
PubMed1992

[Survey of bacterial resistance in Shanghai district].

The data on bacterial resistance monitoring of 5,457 clinical isolates in 1990 against 21 antimicrobial agents showed that most isolates were highly resistant to commonly used antimicrobial agents, such as penicillin, ampicillin, erythromycin, chloramphenicol, tetracycline and gentamicin, with a resistant rate of around 50% or more. Strains resistant to some newer agents, such as cefotaxime, ceftazidime, amikacin and norfloxacin, have appeared and are increasing in number. Multi-resistant strains, i.e. strains resistant to five more antimicrobial agents existed in various bacterial species. Vancomycin was the most active antibiotic against methicillin-resistant staphylococci and no resistant strains were found. Strains of Salmonella and Shigella spp. were still very sensitive to the antimicrobial agents tested, with a sensitive rate of more than 90%. 80% of 765 gentamicin-resistant strains of Enterobacteriaceae were still sensitive to amikacin.

Ampicillin Resistance↗