Hydrolases are enzymes that catalyze cleavage reactions (or the reverse fragment
condensations); isomerases are enzymes that catalyze intramolecular rearrangements;
ligases split C-C, C-N, C-O, C-S, or C-halogen bonds without hydrolysis or oxidation;
lyases non-hydrolytically cleave groups from their substrates, concomitantly result-
ing in the formation of a double bond; oxidoreductases catalyze oxidation and reduc-
tion reactions involving the transfer of hydrogen atoms or electrons; and transferases
transfer functional groups containing C, N, P, or S atoms from one substrate to
another.
Since enzymes are key catalysts in many metabolic processes, they have traditionally
been (and will continue to be) among the most important targets for drug design.
8.1.3 Nucleic AcidsTwo nucleic acids, deoxyribonucleic acid (8.1, DNA) and ribonucleic acid (8.2, RNA),
constitute, respectively, the molecules that store hereditary information and those that
transcribe and translate such hereditary information, thus enabling the directed synthe-
sis of varied but specific proteins throughout the cell and the entire organism. These
nucleic acids are biopolymers, composed of monomeric building units called
nucleotides(figure 8.2). Just as proteins can be hydrolyzed to amino acids and poly-
saccharides can be hydrolyzed to monosaccharides, nucleic acids can be hydrolyzed to
nucleotides.
Mild hydrolysis of nucleic acids yields the monomeric nucleotides. Subsequent com-
plete hydrolysis of a nucleotide furnishes three structural subunits:
- A 5-carbon monosaccharide (either D-ribose [in RNA] or 2-deoxy-D-ribose [in DNA])
- A heterocyclic base (either a purine or a pyrimidine)
- A phosphate ion
A structural variant of a nucleotide is a nucleoside, which is a nucleotide with the phos-
phate group removed.
In nucleotide structure, the central component is the 5-carbon furanoside monosac-
charide, either D-ribose (in RNA) or 2-deoxy-D-ribose (in DNA). The four heterocyclic
bases found in DNA-based nucleotides are adenine, guanine, cytosine, and thymine:
ENDOGENOUS MACROMOLECULES 469