Understanding protein lids: structural analysis of active hinge mutants in triosephosphate isomerase

The conformational switch from open to closed of the flexible loop 6 of triosephosphate isomerase (TIM) is essential for the catalytic properties of TIM. Using a directed evolution approach, active variants of chicken TIM with a mutated C-terminal hinge tripeptide of loop 6 have been generated (Sun,J. and Sampson,N.S., Biochemistry, 1999, 38, 11474-11481). In chicken TIM, the wild-type C-terminal hinge tripeptide is KTA. Detailed enzymological characterization of six variants showed that some of these (LWA, NPN, YSL, KTK) have decreased catalytic efficiency, whereas others (KVA, NSS) are essentially identical with wild-type. The structural characterization of these six variants is reported. No significant structural differences compared with the wild-type are found for KVA, NSS and LWA, but substantial structural adaptations are seen for NPN, YSL and KTK. These structural differences can be understood from the buried position of the alanine side chain in the C-hinge position 3 in the open conformation of wild-type loop 6. Replacement of this alanine with a bulky side chain causes the closed conformation to be favored, which correlates with the decreased catalytic efficiency of these variants. The structural context of loop 6 and loop 7 and their sequence conservation in 133 wild-type sequences is also discussed.

Identification of neopterin as a potential indicator of infection in burned patients

Several fluorescent substances are present in the supernatants of acid-precipitated whole blood or plasma from burned patients. Perchloric acid supernatants of sera from infected, but not uninfected, burned rats contained a fluorescent substance with maximum emission at 420 nm at 355-nm excitation (355 ex/420 em). In this study of serum from burned human patients, several fluorescent substances were resolved by reverse-phase high-pressure liquid chromatography. One of these fluorescent components had an high-pressure liquid chromatography retention time and fluorescent characteristics identical to those of neopterin. The identification of this component as neopterin was verified by thermospray mass spectrometry. Serum neopterin concentrations were then determined in supernatants of patient serum samples having various levels of 355 ex/420 em fluorescence. A correlation was found between the concentrations of neopterin determined by high-pressure liquid chromatography and the presence of bacteremia in burned patients. These findings suggest that neopterin, which is a useful indicator of infection in other clinical settings, may also be an indicator of infection in burned patients.

Oxygen metabolism of catalase-negative and catalase-positive strains of Lactobacillus plantarum

Two catalase-negative strains of Lactobacillus plantarum and a strain producing the atypical, nonheme catalase were studied to determine if the ability to produce the atypical catalase conferred any growth advantage upon the producing strain. Both catalase-negative strains grew more rapidly than the catalase-positive strain under aerobic or anaerobic conditions in a glucose-containing, complex medium. Upon exhaustion of glucose from the medium, all three strains continued growth under aerobic but not under anaerobic conditions. The continued aerobic growth was accompanied by production of acetic acid in addition to the lactic acid produced during growth on glucose. Oxygen was taken up by exponential phase-cell suspensions grown on glucose when glucose or glycerol were used as substrates. Cells harvested from glucose-exhausted medium oxidized glucose, glycerol, and pyruvate. Oxygen utilization by a catalase-negative strain increased as did the specific activity of reduced nicotinamide adenine dinucleotide peroxidase during late growth in the glucose-exhausted medium. The catalase-positive strain and the catalase-negative strain tested both possessed low but readily detectable levels of superoxide dismutase throughout growth. The growth responses are discussed in terms of the presence of enzymes which would allow the cells to remove potentially damaging reduction products of O2.