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| A implausível Terra pré-biótica |
Cycles occur widely in all branches
of chemistry. The definition of a catalyst as an agent that facilitates the
conversion of reactants to products without itself being changed almost
guarantees that a catalyst can initiate successive “cycles” of the same
reaction. Metabolic cycles are different. Strictly, they are by definition
restricted to biochemistry. Like catalytic cycles, they too result in repeated
conversions of substrates into products, but they involve much more complex
sequences of chemical reactions. As far as I am aware, the formose reaction,
which converts formaldehyde to a complicated mixture of products, including
various sugars [1],
is the only known nonenzymatic reaction sequence that is at all similar to a
metabolic cycle, although the existence of one or two much simpler cycles has been
established or made probable in the literature of prebiotic chemistry [2,3].
The possibility that reactions of hydrogen cyanide (HCN) might form the basis
for a complex cyclic organization has been proposed [4],
but there is as yet no experimental evidence to support this proposal.
If
complex cycles analogous to metabolic cycles could have operated on the
primitive Earth, before the appearance of enzymes or other informational
polymers, many of the obstacles to the construction of a plausible scenario for
the origin of life would disappear. If, for example, a complex system of
nonenzymatic cycles could have made nucleotides available for RNA synthesis, many
of the problems of prebiotic chemistry would become irrelevant. Perhaps a
simpler polymer preceded RNA as the genetic material—for example, a polymer
based on a glycerol-phosphate backbone [5]
or a phosphoglyceric acid backbone. Could a nonenzymatic “metabolic cycle” have
made such compounds available in sufficient purity to facilitate the appearance
of a replicating informational polymer?
It
must be recognized that assessment of the feasibility of any particular
proposed prebiotic cycle must depend on arguments about chemical plausibility,
rather than on a decision about logical possibility. Any reaction sequence that
is allowed by thermodynamics could, in principle, be realized, given a
sufficiently active and specific family of catalysts. Plants synthesize complex
alkaloids, such as strychnine, from CO2, NH3, and reducing equivalents, so it
must, in principle, be possible to achieve these syntheses starting from CO2,
NH3, and H2, given a family of sufficiently active and specific prebiotic
catalysts. However, few would believe that any assembly of minerals on the
primitive Earth is likely to have promoted these syntheses in significant yield.
Each proposed metabolic cycle, therefore, must be evaluated in terms of the
efficiencies and specificities that would be required of its hypothetical
catalysts in order for the cycle to persist. Then arguments based on
experimental evidence or chemical plausibility can be used to assess the
likelihood that a family of catalysts that is adequate for maintaining the
cycle could have existed on the primitive Earth.
The
metabolic cycles that have been identified by biochemists are of two kinds:
simple cycles and autocatalytic cycles. The citric acid cycle, which brings
about the oxidation of acetate to CO2 with the concomitant synthesis of
ATP, and the urea cycle that results in the conversion of toxic NH3 to
relatively harmless urea, are both examples of simple cycles. The initial step
of the former cycle is the synthesis of citric acid from oxaloacetic acid and
acetyl-CoA. After one turn of the cycle, acetate is completely “burned” to
CO2 as one molecule of oxaloacetate is regenerated. The Calvin dark cycle and
the reverse citric acid cycle, both of which result in the fixation of
CO2 into important biochemical intermediates, are examples of
autocatalytic cycles. The reverse (reductive) citric acid cycle (Figure1) is initiated by the splitting of citric acid to give oxaloacetic acid
and acetyl-CoA. After one turn of the cycle, two molecules of citric acid are
formed, so long as no material is diverted from the cycle. That is why the
cycle is described as autocatalytic—each molecule of citric acid introduced
into the cycle results, after a turn of the cycle, in the generation of two
molecules of citric acid. The proposal that the reverse citric acid cycle
operated nonenzymatically on the primitive Earth has been a prominent feature
of some scenarios for the origin of life [6–8].
(PDF
GRATIS: PLoS Biology)
