REACTIONS INVOLVING CARBAMYL-PHOSPHATE IN THE PRESENCE OF PRECIPITATED CALCIUM-PHOSPHATE WITH FORMATION OF PYROPHOSPHATE - A MODEL FOR PRIMITIVE ENERGY-CONSERVATION PATHWAYS
A. Vieyra et al., REACTIONS INVOLVING CARBAMYL-PHOSPHATE IN THE PRESENCE OF PRECIPITATED CALCIUM-PHOSPHATE WITH FORMATION OF PYROPHOSPHATE - A MODEL FOR PRIMITIVE ENERGY-CONSERVATION PATHWAYS, Origins of life and evolution of the biosphere, 25(4), 1995, pp. 335-350
The formation of carbamyl phosphate (CAP) in dilute solutions of cyana
te (NCO-) and orthophosphate (Pi) was measured both in the absence and
in the presence of a precipitated matrix of calcium phosphate (Pi.Ca)
. The second-order rate constant and the free energy of CAP synthesis
were not modified by the presence of the solid matrix, indicating that
synthesis occurs in the homogeneous Pi-containing solution. The elimi
nation reaction of CAP to form NCO- and Pi followed first-order kineti
cs and the rate constant was the same whether or not calcium phosphate
was present. Elimination was not complete, and the steady level of re
maining CAP was that expected from the free energy of synthesis. The f
ormation of pyrophosphate (PPi) was detected in CAP-containing medium
only in the presence of calcium, showing a close correlation with the
amount of precipitated Pi.Ca. Phosphorolysis of CAP followed a sigmoid
al time course, compatible with adsorption of CAP to the solid matrix
as a prelude to transphosphorylation. Addition of 5'-AMP and of short
linear polyphosphates inhibited phosphorolysis of CAP. It is proposed
that the presence of a solid phosphate matrix and the relative concent
rations of cyano compounds, as well as those of nucleotides and inorga
nic polyphosphates, could have played a crucial role in the conservati
on of chemical energy of CAP and in its use in prebiotic phosphorylati
on reactions.