<unk> s refers to molecules which are the precursors or<unk> es of biologically significant molecules.
Although these<unk> s are of relatively less direct importance to cellular function, they can play important roles both in the allosteric regulation of enzymes.
Some can be useful in measuring rates of<unk>  processes (for example, Dopac or 3-aminoisobutyrate).
Metabolic intermediate
Heterocyclic compound
Polycyclic compound
Cyclic compound
Substrate 
Neutron moderator
Biopolymer
Diradical