Gustavo Parisi asked:
"We can find in several biochemistry books, the classical ATP pushed
reactions:
"A---B DeltaG >0
"ATP----ADP + pi DeltaG <0
_______________
"A + ATP------B + ADP + pi DeltaG <0
"Should we ever suppose the existence of a phosphorylated intermediate
compound (enzyme or substrate) in order to explain the coupling?In that
case, is the formulation above correct?"
I think the usual representation is not quite as written above, but more
A + Pi = B + water DeltaG >0
ATP + water = ADP + Pi DeltaG <0
______________________
A + Pi + ATP + water = ADP + Pi + B + water DeltaG <0
where the irrelevant species Pi and water cancel to give
A + ATP = ADP + B DeltaG <0
Everything adds up right (deltaG values as well as reactants) regardless
of which irrelevant species are introduced (provided, of course, that
the concentrations of species that appear in both equations are the
same).
As they occur in the cell the first two lines do not represent separate
reactions; only the complete reaction A + ATP = ADP + B really occurs
and has an enzyme to catalyse it. Whether this complete reaction
requires a phosphorylated intermediate is a matter of opinion: Leonard
Spector would say yes, many others would say no. See the discussion on
pp. 135-136 of my book "Fundamentals of Enzyme Kinetics"
(http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/fek.html).
As long as one is content to confine manipulation of equations to
addition and subtraction errors should not arise, but one must resist
the temptation to divide deltaG0 values by one another, as is done in
calculating "metabolic efficiencies". The results in this case are
arbitrarily dependent on the choice of irrelevant species, as discussed
in an article Biochem. Soc. Trans. 11, 44-45 (1983).
For example, the reaction catalysed by 3-phosphoglycerate kinase has an
"efficiency" of 162% in gluconeogenesis if (as is usual) water and Pi
are chosen as irrelevant species. Hexokinase has an "efficiency" of 45%
when water and Pi are arbitrarily included in the reaction, but of -14%
if fructose and fructose 6-phosphate are included instead.
Dan Atkinson's discussion of similar matters on pp. 260-263 of "Cellular
Energy Metabolism and its Regulation" (Academic Press, 1977) is also
well worth a read.
Athel
Home page: http://ir2lcb.cnrs-mrs.fr/lcbpage/athel/home_page.html
