Non-covalent precedes covalent interaction between drug and
protein
Small molecules like drugs bind to proteins, e.g. human serum albumin
(HSA). The attachment of a drug to a protein occurs first via
non-covalent bonds (table 1 and figure 1) (14-16). These are the
dominant type of intermolecular forces in supra-molecular chemistry and
rely on van der Waals forces, electrostatic interactions, ion pairs, and
hydrogen pairs. Even though they are weak individually, their cumulative
energies of molecular interactions can be significant. The bindings are
formed very rapidly, are reversible and the
molar
concentrations of the protein [P], ligand [L], and complex
[LP] respectively are determined by the dissociation constant: Kd=
[L][P]/ [LP]. Thus, the affinity of interaction and the
concentrations of drug and protein determine the number of complexes
formed.
After an initial non-covalent binding the drug may bind by a
covalent bond to a certain amino acid within the protein (figure 1).
This feature depends on the chemical property of the drug. Other drugs
cannot bind directly, but gain this property by metabolism (13).
A drug or drug metabolite able to bind by covalent bonds to a carrier
molecule/protein is called a hapten. Covalent bonds involve the sharing
of electron pairs between atoms. The formation of such bonds may take
some time as studies with penicillin revealed (17-19). Under optimized
in-vitro conditions (pH 10.2), the first bond of a beta-lactam like
penicillin G (as penicilloyl or penicillenic acid) to lysine 199 of
human serum albumin (HSA) can be observed at 20 min. At physiological pH
7.4 bonds are seen after 60 min or later and the process continues in
the following hours. It is not readily reversible. The resulting
hapten-modified protein (“adduct”) represents a new antigen, to which
an immune response can be developed.
Of note, the drug-protein complexes or adducts based on non-covalent or
covalent bindings have a very similar “appearance”, as the location of
binding and the orientation of the drug versus protein may be the same
in non-covalent and covalent bindings (14). Consequently, an antibody
initiated by the covalent hapten-protein adduct may recognize both, the
complex formed by covalent bonds and the complex formed by non-covalent
bindings (figure 1).