PubMed · 7901524
The source did not provide an abstract. Follow the original record for more information.
Explore connections, maps & timelines
M Muers. 1993-11-13. Understanding breathlessness.. https://doi.org/10.1016/0140-6736(93)92180-2
Cite the original work for its findings. Save a collection to share your selection of sources.
KEEP EXPLORING
A mechanistic model is presented that describes the respiro-fermentative physiology of yeast. The model assumes the presence of multiple types of glucose carriers and multiple assimilation pathways. Respiro-fermentative physiology is explained by the mechanistic response of the different types of carriers and assimilation pathways on the substrate concentration. At low substrate concentrations, glucose is taken up mainly via a high affinity carrier with a low maximum uptake rate. At high substrate concentrations, this carrier becomes saturated and the main pathway for glucose uptake is via a low affinity carrier with a high maximum uptake rate. The price to pay for the high uptake rate is a lowered assimilation efficiency, resulting in a low biomass yield. Product formation occurs via the pathway with the high uptake rate. The model explains the link between substrate concentration and product formation generally observed in the literature on yeast and bacteria. Model parameter values are estimated by fitting data from the literature. The model distinguishes itself from other models in that it does not rely on the presence of switches, such as the 'critical dilution rate', or on the assumption that the respiratory capacity reaches its maximum during respiro-fermentative metabolism. The present theory is not designed exclusively for the phenomenon of respiro-fermentative physiology: it describes the degradation of substances by heterotrophic micro-organisms in general.
ROMK channels are inhibited by intracellular acidification. This pH sensitivity is related to several amino acid residues in the channel proteins such as Lys-61, Thr-51, and His-206 (in ROMK2). Unlike all other amino acids, histidine is titratable at pH 6-7 carrying a positive charge below pH 6. To test the hypothesis that certain histidine residues are engaged in CO(2) and pH sensing of ROMK1, we performed experiments by systematic mutations of all histidine residues in the channel using the site-directed mutagenesis. There are two histidine residues in the N terminus. Mutations of His-23, His-31, or both together did not affect channel sensitivity to CO(2). Six histidine residues are located in the C terminus. His-225, His-274, His-342, and His-354 were critical in CO(2) and pH sensing. Mutation of either of them reduced CO(2) and pH sensitivities by 20-50% and approximately 0.2 pH units, respectively. Simultaneous mutations of all of them eliminated the CO(2) sensitivity and caused this mutant channel to respond to only extremely acidic pH. Similar mutations of His-280 had no effect. The role of His-270 in CO(2) and pH sensing is unclear, because substitutions of this residue with either a neutral, negative, or positive amino acid did not produce any functional channel. These results therefore indicate that histidine residues contribute to the sensitivity of the ROMK1 channel to hypercapnia and intracellular acidosis.
The growth of a cocktail of spores from six nonproteolytic Clostridium botulinum type B and E isolates at 5 and 10 degrees C was used to assess the combined effect of NaCl (0.5-4.5% w/v), pH (5.5-6.5) and atmosphere (10% H2:90% N2, 5% CO2:10% H2:85% N2, or 100% CO2) in buffered peptone, yeast, glucose, starch broth with an Eh of approximately -350 mV. Under all atmospheres growth tended to be slower as the concentration of NaCl increased and with NaCl combined with pH levels below 6.0. Of the atmospheres tested, growth occurred at a slower rate and over a narrower range of conditions when C. botulinum was exposed to 100% CO2. This effect was enhanced when the incubation temperature was 5 degrees C. The results indicate that while CO2 decreased C. botulinum growth at chill temperatures, prevention of growth also depended on the NaCl concentration and the pH of the medium.