Farnesyltransferase inhibitors (FTIs) are a group of drugs that selectively inhibit the enzyme farnesyltransferase (FTase) that is responsible for the transfer of a farnesyl group to Ras and other proteins involved in signaling concerning cell transformation and survival.
Famesyltrasferase (FTase) is a heterodimeric enzyme consisting of 48 KD (α), and 46 KD (β) subunits. The α subunit also forms a component of the closely related enzyme, protein geranylgeranyltransferase type I (GGTase-I). Crystal structure of FTase reveals that both α and β-subunits are largely composed of α-helics. One zinc ion active site, involved in catalysis, is located at junction between the hydrophilic surface of β-subunit and the hydrophobic cleft of α subunit. FTase mediates the first step of the posttranslational modification of proteins with a CaaX motif (‘C’ is cysteine, ‘a’ stands for an aliphatic or aromatic amino acid, and ‘X’ is usually a methionine, cysteine, serine, alanine, or glutamine). It attaches a 15-C isoprenoid group to the cysteine of the motif. A peptidase then cleaves the three terminal amino acids, and lastly the carboxyl group of the cysteine group is methylated by a methyltransferase. A closely related enzyme, protein geranylgeranyltransferase type I (GGTase-I) recognizes the C-terminal CaaL motif (‘L’ is leucine or phenylalanine). Some famesylated proteins, such as H-Ras and N-Ras, can be further modified by palmitoyltransferase.
The reaction catalyzed by FTase has both associative (SN2) and dissociative (SN1) character. Zinc coordination of the cysteine thiolate, the essential nature of this ion for catalysis, and pre-steady state analysis of metal-substituted FTase suggest associative character involving activation of the cysteine thiolate nucleophile by the zinc ion. Evidence for a dissociative component comes from biochemical experiments using FPP substrate analogs with electronwithdrawing flourine-for-hydrogen substitutions at the C3 methyl position. These analogs, which would destabilize the Cl carbocation, reduce steady state and single turnover rates. Consistent with partial SNl character, these analogs do not slow catalysis as dramatically as in purely dissociative reactions such as that catalyzed by farnesyl diphosphate synthase. Therefore, it seems most likely that the FTase reaction mechanism lies in the continuum between the SNl and SN2 extremes. Accordingly, the reaction goes with complete inversion of stereochemistry at Cl. The combined nature of the reaction suggests that it proceeds through what has been called an exploded transition state. In this transition state it is thought that the bond to the attacking cysteine thiolate nucleophile is barely formed while the bond to the leaving diphosphate is almost broken. The chemical mechanism of FTase has been reviewed in.
Famesyltrasferase Inhibitors (FTIs)
A number of different types of FTIs have been developed. Early attempts to discover FTIs focus on the modifications of isoprenoid and CaaX polypeptide substrates or the screening from natural compounds libraries. For example, manumycin is one naturally occurring FTI compound that was identified from a microbial screen. These years, more inhibitors were designed to compete with the CaaX tetrapeptide, famesyl group, as well as the enzyme FPP-CAAX complex that is formed before catalysis begins. In addition, since zinc ion active site is directly involved in catalysis, metal-chelating inhibitors were also synthesized. Another successful trial was the identification of a general thiol replacement in imidazole. During the early period, FTI design was focused primarily on thio-containing peptidomimetics. Following that, wide varieties of non-thiol non-peptidic inhibitors have been developed. Modifications of the leading compounds have generated potent small molecule inhibitors that selectively inhibit FTase, but not geranylgeranyltransferase. Among them, BMS-225975 and BMS-214662 are shown to have improved potency to inhibit FTase.
Jiang, Chen. Investigation of farnesyltransferase inhibitors mechanism in human tumor cells. University of California, Los Angeles, 2006.
Long, Stephen Barstow. Structural biochemistry of the protein farnesyltransferase reaction path. Diss. Duke University, 2001.