Coenzyme A dithioesters: synthesis, characterization and reaction with citrate synthase and acetyl-CoA:choline O-acetyltransferase

The Novel Role of Mitochondrial Citrate Synthase and Citrate in the Pathophysiology of Alzheimer's Disease

Citrate synthase is a key mitochondrial enzyme that utilizes acetyl-CoA and oxaloacetate to form citrate in the mitochondrial membrane, which participates in energy production in the TCA cycle and linked to the electron transport chain. Citrate transports through a citrate malate pump and synthesizes acetyl-CoA and acetylcholine (ACh) in neuronal cytoplasm. In a mature brain, acetyl-CoA is mainly utilized for ACh synthesis and is responsible for memory and cognition. Studies have shown low citrate synthase in different regions of brain in Alzheimer's disease (AD) patients, which reduces mitochondrial citrate, cellular bioenergetics, neurocytoplasmic citrate, acetyl-CoA, and ACh synthesis. Reduced citrate mediated low energy favors amyloid-β (Aβ) aggregation. Citrate inhibits Aβ25-35 and Aβ1-40 aggregation in vitro. Hence, citrate can be a better therapeutic option for AD by improving cellular energy and ACh synthesis, and inhibiting Aβ aggregation, which prevents tau hyperphosphorylation and glycogen synthase kinase-3 beta. Therefore, we need clinical studies if citrate reverses Aβ deposition by balancing mitochondrial energy pathway and neurocytoplasmic ACh production. Furthermore, in AD's silent phase pathophysiology, when neuronal cells are highly active, they shift ATP utilization from oxidative phosphorylation to glycolysis and prevent excessive generation of hydrogen peroxide and reactive oxygen species (oxidative stress) as neuroprotective action, which upregulates glucose transporter-3 (GLUT3) and pyruvate dehydrogenase kinase-3 (PDK3). PDK3 inhibits pyruvate dehydrogenase, which decreases mitochondrial-acetyl-CoA, citrate, and cellular bioenergetics, and decreases neurocytoplasmic citrate, acetyl-CoA, and ACh formation, thus initiating AD pathophysiology. Therefore, GLUT3 and PDK3 can be biomarkers for silent phase of AD.

The Regulatory Effects of Acetyl-CoA Distribution in the Healthy and Diseased Brain

Brain neurons, to support their neurotransmitter functions, require a several times higher supply of glucose than non-excitable cells. Pyruvate, the end product of glycolysis, through pyruvate dehydrogenase complex reaction, is a principal source of acetyl-CoA, which is a direct energy substrate in all brain cells. Several neurodegenerative conditions result in the inhibition of pyruvate dehydrogenase and decrease of acetyl-CoA synthesis in mitochondria. This attenuates metabolic flux through TCA in the mitochondria, yielding energy deficits and inhibition of diverse synthetic acetylation reactions in all neuronal sub-compartments. The acetyl-CoA concentrations in neuronal mitochondrial and cytoplasmic compartments are in the range of 10 and 7 μmol/L, respectively. They appear to be from 2 to 20 times lower than acetyl-CoA Km values for carnitine acetyltransferase, acetyl-CoA carboxylase, aspartate acetyltransferase, choline acetyltransferase, sphingosine kinase 1 acetyltransferase, acetyl-CoA hydrolase, and acetyl-CoA acetyltransferase, respectively. Therefore, alterations in acetyl-CoA levels alone may significantly change the rates of metabolic fluxes through multiple acetylation reactions in brain cells in different physiologic and pathologic conditions. Such substrate-dependent alterations in cytoplasmic, endoplasmic reticulum or nuclear acetylations may directly affect ACh synthesis, protein acetylations, and gene expression. Thereby, acetyl-CoA may regulate the functional and adaptative properties of neuronal and non-neuronal brain cells. The excitotoxicity-evoked intracellular zinc excess hits several intracellular targets, yielding the collapse of energy balance and impairment of the functional and structural integrity of postsynaptic cholinergic neurons. Acute disruption of brain energy homeostasis activates slow accumulation of amyloid-β_1-42 (Aβ). Extra and intracellular oligomeric deposits of Aβ affect diverse transporting and signaling pathways in neuronal cells. It may combine with multiple neurotoxic signals, aggravating their detrimental effects on neuronal cells. This review presents evidences that changes of intraneuronal levels and compartmentation of acetyl-CoA may contribute significantly to neurotoxic pathomechanisms of different neurodegenerative brain disorders.

Evaluation of 3-hydroxy-3-methylglutaryl-coenzyme A lyase arginine-41 as a catalytic residue: use of acetyldithio-coenzyme A to monitor product enolization

3-Hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) lyase catalyzes the divalent cation-dependent cleavage of HMG-CoA to produce acetyl-CoA and acetoacetate. Arginine-41 is an invariant residue in HMG-CoA lyases. Mutation of this residue (R41Q) correlates with human HMG-CoA lyase deficiency. To evaluate the functional importance of arginine-41, R41Q and R41M recombinant mutant human HMG-CoA lyase proteins have been constructed, expressed, and purified. These mutant proteins retain structural integrity based on Mn(2+) binding and affinity labeling stoichiometry. R41Q exhibits a 10(5)-fold decrease in V(max); R41M activity is >or=10-fold lower than the activity of R41Q. Acetyldithio-CoA, an analogue of the reaction product, acetyl-CoA, has been employed to test the function of arginine-41, as well as other residues (e.g., aspartate-42 and histidine-233) implicated in catalysis. Acetyldithio-CoA supports enzyme-catalyzed exchange of the methyl protons of the acetyl group with solvent; exchange is dependent on the presence of Mg(2+) and acetoacetate. In comparison with wild-type human enzyme, D42A and H233A mutant enzymes exhibit 4-fold and 10-fold decreases, respectively, in the proton exchange rate. In contrast, R41Q and R41M mutants do not catalyze any substantial enzyme-dependent proton exchange. These results suggest a role for arginine-41 in deprotonation or enolization of acetyldithio-CoA and implicate this residue in the HMG-CoA cleavage reaction chemistry that leads to acetyl-CoA product formation. Assignment of arginine-41 as an active site residue is also supported by a homology model for HMG-CoA lyase based on the structure of 4-hydroxy-2-ketovalerate aldolase. This model suggests the proximity of arginine-41 to other amino acids (aspartate-42, glutamate-72, histidine-235) implicated as active site residues based on their function as ligands to the activator cation.

Utility of acetyldithio-CoA in detecting the influence of active site residues on substrate enolization by 3-hydroxyl-3-methylglutaryl-CoA synthase

Hydroxymethylglutaryl-CoA synthase-catalyzed condensation of acetyl-CoA with acetoacetyl-CoA requires enolization/carbanion formation from the acetyl C-2 methyl group prior to formation of a new carbon-carbon bond. Acetyldithio-CoA, a readily enolizable analog of acetyl-CoA, was an effective competitive inhibitor of avian hydroxymethylglutaryl-CoA synthase (Ki = 28 microm). In the absence of cosubstrate, enzyme catalyzed the enolization/proton exchange from the C-2 methyl group of acetyldithio-CoA. Mutant enzymes that exhibited impaired formation of the covalent acetyl-S-enzyme reaction intermediate exhibited diminished (D159A and D203A) or undetectable (C129S) rates of enolization of acetyldithio-CoA. The results suggest that covalent thioacetylation of protein, which has not been detected previously for other enzymes that enolize this analog, occurs with hydroxymethylglutaryl-CoA synthase. Enzyme catalyzed the transfer of the thioacetyl group of this analog to 3'-dephospho-CoA suggesting the intermediacy of a covalent thioacetyl-S-enzyme species, which appears to be important for proton abstraction from C-2 of the thioacetyl group. Avian enzyme glutamate 95 is crucial to substrate condensation to form a new carboncarbon bond. Mutations of this invariant residue (avian enzyme E95A and E95Q; Staphylococcus aureus enzyme E79Q) correlated with diminished ability to catalyze enolization of acetyldithio-CoA. Enolization by E95Q was not stimulated in the presence of acetoacetyl-CoA. These observations suggest either a direct (proton abstraction) or indirect (solvent polarization) role for this active site glutamate.

Inactivation of 3-hydroxy-3-methylglutaryl-CoA synthase and other Acyl-CoA-utilizing enzymes by 3-Oxobutylsulfoxyl-CoA

3-Oxobutylsulfoxyl-CoA has been produced by oxidation of S-3-oxobutyl-CoA, the thioether analog of acetoacetyl-CoA. Avian hydroxymethylglutaryl-CoA (HMG-CoA) synthase is inactivated by oxobutylsulfoxyl-CoA in a time-dependent fashion. Protection against inactivation is afforded by the substrate, acetyl-CoA, suggesting that inactivation involves modification of the enzyme's active site. Pretreatment of HMG-CoA synthase with the inactivator blocks the enzyme's ability to form Michaelis and acetyl-S-enzyme intermediates, supporting the hypothesis that modification is active-site directed. Incubation of enzyme with oxobutylsulfoxyl-[32P]CoA followed by precipitation with trichloroacetic acid indicates that inactivation correlates with stoichiometric formation of a covalent adduct between enzyme and a portion of the inactivator that includes the CoA nucleotide. The observation of reagent partitioning suggests that HMG-CoA synthase catalyzes conversion of oxobutylsulfoxyl-CoA into a reactive species that modifies the protein. Treatment of inactivated enzyme with DTT or other mercaptans restores enzyme activity and reverses the covalent modification with release of CoASH. Oxobutylsulfoxyl-CoA inactivates beta-ketothiolase and HMG-CoA lyase in a process that is also reversed by DTT. These three enzymes all contain active site cysteines, suggesting that inactivation results from disulfide formation between a cysteine and the CoA moiety of the inhibitor. The data are consistent with the hypothesis that enzymatic cleavage of oxobutylsulfoxyl-CoA results in the transient formation of a sulfenic acid derivative of CoA which subsequently reacts to form a stable disulfide linkage to protein.

3-Hydroxy-3-methylglutaryldithio-CoA: utility of an alternative substrate in elucidation of a role for HMG-CoA lyase's cation activator

(S)-(3-Hydroxy-3-methyl-1-thionoglutaryl)-Coenzyme A (HMG[= S]CoA), a dithioester analog of (S)-(3-hydroxy-3-methylglutaryl)-CoA (HMG-CoA), acts as an efficient alternative substrate for avian HMG-CoA lyase. Detection of product formation by HPLC, UV absorbance and coupled enzyme assays indicates that HMG[= S]CoA cleavage yields acetyl[= S]CoA and acetoacetate. HMG[= S]CoA binds to the lyase with a Km of 13 microM and undergoes the cleavage reaction at a maximal rate which is 20% of that observed with HMG-CoA. The enzyme-catalyzed cleavage of both HMG-CoA and HMG[= S]CoA is stimulated by the divalent cations Mg2+ and Mn2+. Mg2+ produces a 2-fold higher stimulation of HMG-CoA cleavage than that observed with Mn2+. In contrast, stimulation of HMG[= S]CoA cleavage is nearly seven times higher with Mn2+ than with Mg2+. Not only is the stimulation of enzymatic activity dependent on the cation, but also the Km values for Mg2+ and Mn2+ are dependent upon the substrate used. In contrast, the Km values for HMG-CoA and HMG[= S]CoA are not markedly dependent on the identity of the divalent cation. These results are compatible with the initial formation of a binary enzyme-substrate complex prior to binding of the divalent cation to produce a catalytically active enzyme-substrate-metal ternary complex.

3-Hydroxy-3-methylglutaryldithio-coenzyme A: a potent inhibitor of Pseudomonas mevalonii HMG-CoA reductase

3-Hydroxy-3-methyl-1-thionoglutaryl-coenzyme A, a dithioester analog of 3-hydroxy-3-methylglutaryl-CoA, has been enzymatically synthesized using the HMG-CoA synthase catalyzed condensation of acetyl-CoA with 3-oxo-1-thionobutyryl-CoA. HMGdithio-CoA is a potent inhibitor of Pseudomonas mevalonii HMG-CoA reductase. Inhibition was mainly competitive with respect to HMG-CoA with a Kis of 0.086 +/- .01 microM and noncompetitive with respect to NADH with a Kis of 3.7 +/- 1.5 microM and a Kii of 0.65 +/- .05 microM in the presence of 110 microM (R.S)-HMG-CoA.