From owner-metabolic-reg@net.bio.net Tue Jan 18 22:00:00 1994 Path: biosci!ysics.physics.sunysb.edu!NewsWatcher!user From: sbbiovm.sunysb.edu (Ing-Nang Wang) Newsgroups: bionet.metabolic-reg Subject: Re: sources on various Keq values? Followup-To: bionet.metabolic-reg Date: Wed, 19 Jan 1994 09:56:30 -0500 Organization: Dept. of Ecology and Evolution Lines: 65 Message-ID: References: NNTP-Posting-Host: 129.49.19.31 Hello again, everyone: In article , schuetze@mailer.uni-marburg.de (Hartmut Schtze) wrote: > Hi Ing-Nang, > >1) gluconate + ATP -----> 6-phosphogluconate(6PG) + ADP phosphorylation > this one starts the Glycolysis and is totally irreversible. If u want to > undo it, u need another enzyme: gluconate-6-phosphatase. > So the equilibrium is somewhere very near to 6PG... QUESTION: Can we generalize from other available kinase data that the reaction shown above is most likely to be irreversible? > >2) 6PG -----> 2-keto-3-deoxy-phosphogluconate + H20 dehydration > don't know this one, but i'll have a look.. This reaction is basically irrversible. For the mechanism involved with the formation and ketonization of an enol. see: Kovachevich, R., and Wood, W. A. (1955) JBC 213: 745-756 Meloche, H. P., and Wood, W. A. (1964) JBC 239: 3505-3510 > >3) 6PG + NADP ------> ribulose 5-phosphate + CO2 + NADPH > again, u picked an irreversible reaction. I do not remember the very special > step which is irreversible, but it's the same as 1), i think. As far as I know, this reaction is not as irreversible as we think. I remember there is one experiment being done on this enzyme. The author(s) forced the reverse reaction by jacking up high concentrations of ribulose 5-P + NADPH + CO2. I can't find the reference right now. But I know it has been done. I was kind of surprised at the time. However, the concentration may be too high to be any significance in vivo. <<.....>> > Hartmut > ------------------------------------------------- > Hartmut Schuetze always > e-mail: schuetze@mailer.uni-marburg.de look > snail : H.Schuetze Chemnitzer Str.47 on the > 35039 Marburg / Germany bright > tube : germany 06421/43365 :-) side... > ------------------------------------------------- The reason I send out the first query about the sources on various Keq values is because I need those values to plug into enzyme kinetic rate equations (OR DO I NEED TO ?). At least 2 out of 3 reactions mentioned above are presumably irreversible, possibly all 3 of them. Also at the same time, I know from personal experience that both Km_ATP for the kinase and Km_NADP for the dehydrogenase are very small. That is, inside an E. coli cell both enzymes are likely to be saturated by these two cofactors ( assuming we adopt the published in vivo ATP and NADP concentration for E. coli). Since this is a branched pathway we are talking about, can we just simplify the potentially ugly looking explicit steady state rate equation into a simpler one? That is, using Michaelis-Menten equation to approximate each branch. The assumptions are :1) the reactiona are irrversible, and 2) the cofactors are saturating in vivo. To add a new twist on this. By adopting a simplified version of the steady state equation for the branched pathway I just mentioned, we are implying that the kinase step will determined the steady state flux, which, again from personal experience, I know is not true. So in a sense, I'm stuck. Any help or suggestion on this? -- Ing-Nang Wang ingnang@sbbiovm.sunysb.edu .