PubMed · 13682547
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R J ALLEN, R M GIBSON. 1961. Phenylketonuria with normal intelligence.. https://doi.org/10.1001/archpedi.1961.02080010117019
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The Briggs-Haldane approximation of the irreversible Michaelis-Menten scheme of enzyme kinetics is cited in virtually every biochemistry textbook and is widely considered the classic example of a quasi-steady-state approximation. Though of similar importance, the reversible Michaelis-Menten scheme is not as well characterized. This is a serious limitation since even enzymatic reactions that go to completion may be reversible. The current work derives a total quasi-steady-state approximation (tQSSA) for the reversible Michaelis-Menten and delineates its validity domain. The tQSSA allows the derivation of uniformly valid approximations for the limit of low enzyme concentrations, ET<<ST+KM, and under certain more restrictive conditions also for high enzyme concentrations such that ST<<ET+KM. Using these simple analytical approximations, a sequential experimental-theoretical method is suggested for unambiguously estimating all the kinetic parameters of the reversible Michaelis-Menten scheme.
In the yeast Saccharomyces cerevisiae, the most abundant phospholipid phosphatidylcholine is synthesized by the complementary CDP-diacylglycerol and Kennedy pathways. Using a cki1Delta eki1Delta mutant defective in choline kinase and ethanolamine kinase, we examined the consequences of a block in the Kennedy pathway on the regulation of phosphatidylcholine synthesis by the CDP-diacylglycerol pathway. The cki1Delta eki1Delta mutant exhibited increases in the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylcholine via the CDP-diacylglycerol pathway. The increase in phospholipid synthesis correlated with increased activity levels of the CDP-diacylglycerol pathway enzymes phosphatidylserine synthase, phosphatidylserine decarboxylase, phosphatidylethanolamine methyltransferase, and phospholipid methyltransferase. However, other enzyme activities, including phosphatidylinositol synthase and phosphatidate phosphatase, were not affected in the cki1Delta eki1Delta mutant. For phosphatidylserine synthase, the enzyme catalyzing the committed step in the pathway, activity was regulated by increases in the levels of mRNA and protein. Decay analysis of CHO1 mRNA indicated that a dramatic increase in transcript stability was a major component responsible for the elevated level of phosphatidylserine synthase. These results revealed a novel mechanism that controls phospholipid synthesis in yeast.
The MetaCyc database (see URL http://MetaCyc.org) is a collection of metabolic pathways and enzymes from a wide variety of organisms, primarily microorganisms and plants. The goal of MetaCyc is to contain a representative sample of each experimentally elucidated pathway, and thereby to catalog the universe of metabolism. MetaCyc also describes reactions, chemical compounds and genes. Many of the pathways and enzymes in MetaCyc contain extensive information, including comments and literature citations. SRI's Pathway Tools software supports querying, visualization and curation of MetaCyc. With its wide breadth and depth of metabolic information, MetaCyc is a valuable resource for a variety of applications. MetaCyc is the reference database of pathways and enzymes that is used in conjunction with SRI's metabolic pathway prediction program to create Pathway/Genome Databases that can be augmented with curation from the scientific literature and published on the world wide web. MetaCyc also serves as a readily accessible comprehensive resource on microbial and plant pathways for genome analysis, basic research, education, metabolic engineering and systems biology. In the past 2 years the data content and the Pathway Tools software used to query, visualize and edit MetaCyc have been expanded significantly. These enhancements are described in this paper.