Transition state analysis of model and enzymatic prenylation reactions

Protein Lipidation: Occurrence, Mechanisms, Biological Functions, and Enabling Technologies

Protein lipidation, including cysteine prenylation, N-terminal glycine myristoylation, cysteine palmitoylation, and serine and lysine fatty acylation, occurs in many proteins in eukaryotic cells and regulates numerous biological pathways, such as membrane trafficking, protein secretion, signal transduction, and apoptosis. We provide a comprehensive review of protein lipidation, including descriptions of proteins known to be modified and the functions of the modifications, the enzymes that control them, and the tools and technologies developed to study them. We also highlight key questions about protein lipidation that remain to be answered, the challenges associated with answering such questions, and possible solutions to overcome these challenges.

Unraveling the Catalytic Pathway of Metalloenzyme Farnesyltransferase through QM/MM Computation

The protein farnesyltransferase (FTase) is a Zn(2+)-metalloenzyme that catalyzes the farnesylation reaction, i.e., the transfer of the 15-carbon atom farnesyl group from farnesyl diphosphate (FPP) to a specific cysteine of protein substrates. Oncogenic Ras proteins, which are among the FTase substrates, are observed in about 20-30% of human cancer cells. Thus, FTase represents a target for anticancer drug design. Herein, we present a classical force-field-based and quantum mechanics/molecular mechanics (QM/MM) computational study of the FTase reaction mechanism. Our findings offer a detailed picture of the FTase catalytic pathway, describing structural features and the energetics of its saddle points. A moderate dissociation of the diphosphate group from the FPP is observed during the nucleophilic attack of the zinc-bound thiolate. At the transition state, a resonance structure is observed, which indicates the formation of a metastable carbocation. However, no stable intermediate is found along the reaction pathway. Thus, the reaction occurs via an associative mechanism with dissociative character, in agreement with the mechanism proposed by Fierke et al. ( Biochemistry 2000, 39, 2593-2602 and Biochemistry 2003, 42, 9741-9748 ). Moreover, a fluorine-substituted FPP analogue (CF3-FPP) is used to investigate the inhibitory effect of fluorine, which in turn provides additional agreement with experimental data.

Protein prenylation: enzymes, therapeutics, and biotechnology applications

Protein prenylation is a ubiquitous covalent post-translational modification found in all eukaryotic cells, comprising attachment of either a farnesyl or a geranylgeranyl isoprenoid. It is essential for the proper cellular activity of numerous proteins, including Ras family GTPases and heterotrimeric G-proteins. Inhibition of prenylation has been extensively investigated to suppress the activity of oncogenic Ras proteins to achieve antitumor activity. Here, we review the biochemistry of the prenyltransferase enzymes and numerous isoprenoid analogs synthesized to investigate various aspects of prenylation and prenyltransferases. We also give an account of the current status of prenyltransferase inhibitors as potential therapeutics against several diseases including cancers, progeria, aging, parasitic diseases, and bacterial and viral infections. Finally, we discuss recent progress in utilizing protein prenylation for site-specific protein labeling for various biotechnology applications.

δ-Deuterium isotope effects as probes for transition-state structures of isoprenoid substrates

The biosynthetic pathways to isoprenoid compounds involve transfer of the prenyl moiety in allylic diphosphates to electron-rich (nucleophilic) acceptors. The acceptors can be many types of nucleophiles, while the allylic diphosphates only differ in the number of isoprene units and stereochemistry of the double bonds in the hydrocarbon moieties. Because of the wide range of nucleophilicities of naturally occurring acceptors, the mechanism for prenyltransfer reactions may be dissociative or associative with early to late transition states. We have measured δ-secondary kinetic isotope effects operating through four bonds for substitution reactions with dimethylallyl derivatives bearing deuterated methyl groups at the distal (C3) carbon atom in the double bond under dissociative and associative conditions. Computational studies with density functional theory indicate that the magnitudes of the isotope effects correlate with the extent of bond formation between the allylic moiety and the electron-rich acceptor in the transition state for alkylation and provide insights into the structures of the transition states for associative and dissociative alkylation reactions.

Prenyltransferase Inhibitors: Treating Human Ailments from Cancer to Parasitic Infections

The posttranslational modification of protein prenylation is a covalent lipid modification on the C-terminus of substrate proteins that serves to enhance membrane affinity. Oncogenic proteins such as Ras have this modification and significant effort has been placed into developing inhibitors of the prenyltransferase enzymes for clinical therapy. In addition to cancer therapy, prenyltransferase inhibitors have begun to find important therapeutic uses in other diseases, including progeria, hepatitis C and D, parasitic infections, and other maladies. This review will trace the evolution of prenyltransferase inhibitors from their initial use as cancer therapeutics to their expanded applications for other diseases.

Finding a needle in the haystack: computational modeling of Mg2+ binding in the active site of protein farnesyltransferase

Studies aimed at elucidating the unknown Mg2+ binding site in protein farnesyltransferase (FTase) are reported. FTase catalyzes the transfer of a farnesyl group to a conserved cysteine residue (Cys1p) on a target protein, an important step for proteins in the signal transduction pathways (e.g., Ras). Mg2+ ions accelerate the protein farnesylation reaction by up to 700-fold. The exact function of Mg2+ in catalysis and the structural characteristics of its binding remain unresolved to date. Molecular dynamics (MD) simulations addressing the role of magnesium ions in FTase are presented, and relevant octahedral binding motifs for Mg2+ in wild-type (WT) FTase and the Dβ352A mutant are explored. Our simulations suggest that the addition of Mg2+ ions causes a conformational change to occur in the FTase active site, breaking interactions known to keep FPP in its inactive conformation. Two relevant Mg2+ ion binding motifs were determined in WT FTase. In the first binding motif, WT1, the Mg2+ ion is coordinated to D352β, zinc-bound D297β, two water molecules, and one oxygen atom from the α- and β-phosphates of farnesyl diphosphate (FPP). The second binding motif, WT2, is identical with the exception of the zinc-bound D297β being replaced by a water molecule in the Mg2+ coordination complex. In the Dβ352A mutant Mg2+ binding motif, D297β, three water molecules, and one oxygen atom from the α- and β-phosphates of FPP complete the octahedral coordination sphere of Mg2+. Simulations of WT FTase, in which Mg2+ was replaced by water in the active site, recreated the salt bridges and hydrogen-bonding patterns around FPP, validating these simulations. In all Mg2+ binding motifs, a key hydrogen bond was identified between a magnesium-bound water and Cys1p, bridging the two metallic binding sites and, thereby, reducing the equilibrium distance between the reacting atoms of FPP Cys1p. The free energy profiles calculated for these systems provide a qualitative understanding of experimental results. They demonstrate that the two reactive atoms approach each other more readily in the presence of Mg2+ in WT FTase and mutant. The flexible WT2 model was found to possess the lowest barrier toward the conformational change, suggesting it is the preferred Mg2+ binding motif in WT FTase. In the mutant, the absence of D352β makes the transition toward a conformational change harder. Our calculations find support for the proposal that D352β performs a critical role in Mg2+ binding and Mg2+ plays an important role in the conformational transition step.

Biosynthesis via carbocations: theoretical studies on terpene formation

This review describes applications of quantum chemical calculations in the field of terpene biosynthesis, with a focus on insights into the mechanisms of terpene-forming carbocation rearrangements arising from theoretical studies.

Caged protein prenyltransferase substrates: tools for understanding protein prenylation

Originally designed to block the prenylation of oncogenic Ras, inhibitors of protein farnesyltransferase currently in preclinical and clinical trials are showing efficacy in cancers with normal Ras. Blocking protein prenylation has also shown promise in the treatment of malaria, Chagas disease and progeria syndrome. A better understanding of the mechanism, targets and in vivo consequences of protein prenylation are needed to elucidate the mode of action of current PFTase (Protein Farnesyltransferase) inhibitors and to create more potent and selective compounds. Caged enzyme substrates are useful tools for understanding enzyme mechanism and biological function. Reported here is the synthesis and characterization of caged substrates of PFTase. The caged isoprenoid diphosphates are poor substrates prior to photolysis. The caged CAAX peptide is a true catalytically caged substrate of PFTase in that it is to not a substrate, yet is able to bind to the enzyme as established by inhibition studies and X-ray crystallography. Irradiation of the caged molecules with 350 nm light readily releases their cognate substrate and their photolysis products are benign. These properties highlight the utility of those analogs towards a variety of in vitro and in vivo applications.

Computational studies of the farnesyltransferase ternary complex part II: the conformational activation of farnesyldiphosphate

Studies aimed at elucidating the reaction mechanism of farnesyltransferase (FTase), which catalyzes the prenylation of many cellular signaling proteins including Ras, has been an active area of research. Much is known regarding substrate binding and the impact of various catalytic site residues on catalysis. However, the molecular level details regarding the conformational rearrangement of farnesyldiphosphate (FPP), which has been proposed via structural analysis and mutagenesis studies to occur prior to the chemical step, is still poorly understood. Following on our previous computational characterization of the resting state of the FTase ternary complex, the thermodynamics of the conformational rearrangement step in the absence of magnesium was investigated for the wild type FTase and the Y300Fbeta mutant complexed with the peptide CVIM. In addition, we also explored the target dependence of the conformational activation step by perturbing isoleucine into a leucine (CVLM). The calculated free energy profiles of the proposed conformational transition confirm the presence of a stable intermediate state, which was identified only when the diphosphate is monoprotonated (FPP2-). The farnesyl group in the computed intermediate state assumes a conformation similar to that of the product complex, particularly for the first two isoprene units. We found that Y300beta can readily form hydrogen bonds with either of the phosphates of FPP. Removing the hydroxyl group on Y300beta does not significantly alter the thermodynamics of the conformational transition, but shifts the location of the intermediate farther away from the nucleophile by 0.5 A, which suggests that Y300beta facilitate the reaction by stabilizing the chemical step. Our results also showed an increased transition barrier height for CVLM (1.5 kcal/mol higher than that of CVIM). Although qualitatively consistent with the findings from the recent kinetic isotope experiments by Fierke and co-workers, the magnitude is not large enough to affect the rate-limiting step.