Chemistry Reference
In-Depth Information
5.3
Catalysts Based on Hollow Lipid-binding Proteins
5.3.1
Lipid-binding Proteins
Lipid-binding proteins are a class of molecules found in eukaryotic cells involved in the
transport of fatty acids and other types of hydrophobic compounds. The protein struc-
ture consists of two orthogonal planes of
-sheets that form a cup-shaped cavity that is
capped off with a helix-turn-helix element. Considerable structural data, obtained from
X-ray and NMR analysis, exists for this family of proteins [42, 43]. Of particular interest
is the presence of a large, solvent sequestered, cavity whose overall volume varies be-
tween 500 and 1000 A
3
depending on the exact identity of the protein; with the excep-
tion of certain membrane-bound channel proteins, there are few other examples of
macromolecules that possess such a cavity. This structural feature serves as the ligand
binding site in the protein cavity for a diverse range of molecules. Such a large, solvent
sequestered, binding site also provides a useful scaffold for the design of catalysts.
Formally, this cavity can be viewed as a protein equivalent to the cyclodextrin template
used in much of the pioneering work of Breslow and co-workers in their development
of enzyme mimics [18].
b
5.3.2
Initial Work
In their initial work Distefano and co-workers used the protein ALBP (adipocyte lipid-
binding protein) for their protein scaffold. This protein contains a unique cysteine
residue a position 117 that can be selectively modified using reagents that capitalize
on the unique reactivity of the thiol side chain. Kuang and co-workers developed a
reagent, TP-PX (
5.6-1
) that contained an activated disulfide suitable for protein deri-
vatization. This molecule was used to incorporate a PX moiety into ALBP at position
117, resulting in the formation of a construct denoted ALBP-PX (Figure 5.6,
5.6-2
) [44].
This semisynthetic biocatalyst aminated reductively various
-keto acids (
5.7-2
) to ami-
no acids (
5.7-4
) with 0 to 94% ee (Figure 5.7). Because these reactions were performed
in the absence of any additional amine source, only single turnovers were obtained.
The reaction rates were not, however, significantly faster than those involving free
pyridoxamine (
5.7-1b
). This suggested that the protein cavity functions as a chiral en-
vironment that controls the facial selectivity of the protonation of the aldimine inter-
mediate without forming specific interactions with the bound pyridoxamine cofactor,
which could accelerate the reaction, as confirmed by a X-ray crystal structure [45]. Mod-
eling of the Schiff base complexes with several amino acids indicated that one face of
the putative aldimine intermediate was protected against the approach of the solvent or
buffer molecules that must be the proton source for the reaction given the lack of
suitable functional groups within the cavity. This structural data provided a rationale
for explaining the enantioselectivity observed in the ALBP-PX system.
a
