synthesis is described in Chapter 6 of MCAT Biochemistry Review.Nucleotides, such ATP, GTP, and those in DNA, are referred to as organic phosphates due to the
presence of the phosphate group bonded to a carbon-containing molecule.
PROPERTIES
Phosphoric acid is unique in that it has three acidic hydrogens, each with its own pKa. Phosphoric
acid most properly refers to the form that predominates in strongly acidic conditions, H 3 PO 4 . In
mildly acidic conditions, it loses a proton to become dihydrogen phosphate, it will readily
lose a second proton to become hydrogen phosphate, in weakly basic solutions. The form
that exists in strongly basic solutions is phosphate, The pKa for the loss of the first hydrogen
is 2.15; for the second, 7.20; and for the third, 12.32. At a physiological pH of 7.4, this means that
dihydrogen phosphate and hydrogen phosphate predominate in nearly equal proportions. This
variety of pKa values also makes phosphates good buffers because they can pick up or give off
protons depending on the pH of the solution.
KEY CONCEPT
Phosphoric acid is an excellent buffer because it has three hydrogens with pKa values that
span nearly the entire pH scale.Adjacent phosphate groups on a nucleotide triphosphate experience a large amount of repulsion
because they are negatively charged. This, combined with the ability of phosphate to stabilize up to
three negative charges by resonance, means that the energy released when a phosphate or
pyrophosphate is cleaved is quite high.
MCAT Concept Check 10.3:
Before you move on, assess your understanding of the material with these questions.