COMT

Catechol-O-methyltransferase (COMT, EC 2.1.1.6) is a two-phase metabolic enzyme expressed by endogenous gene and is involved in the inactivation of the catecholamine neurotransmitters (dopamine, epinephrine, and norepinephrine) and catechol containing compounds such as L-dopa, catecholestrogens, and flavonoids. This enzyme could introduce a methyl group donated by S-adenosyl-L-methionine (AdoMet) to the catecholamine. Metabolic disorder of COMT generally induces an increase in the risk of diseases like Parkinson's disease, schizophrenia and breast carcinoma since COMT is closely related to the metabolic degradation of compounds in vivo. The capacity of COMT in the peripheral tissues is probably so high that COMT is not easily induced or suppressed.

Distribution

COMT is a ubiquitously occurring enzyme present in plants, microorganisms, and animals. In mammals, COMT is widely distributed throughout the organs of the body and is indicated to be an intracellular enzyme. COMT behaves no significant activity in presynaptic dopaminergic neurons in the brain, while it shows some activity in postsynaptic neurons and substantial activity in glial cells. Furthermore, COMT-dependent dopamine degradation is of extremely importance in brain regions with low expression of the presynaptic dopamine transporter. Despite its significance in neurons, COMT is actually primarily expressed in the liver. The highest COMT activity in both rat and humans is in the liver, followed by the kidneys and gastrointestinal tract (both stomach and intestine). The sequences of COMT from different mammalian species possess high similarity.

Molecular Structure

COMT has a single domain α/β-folded structure where eight α-helices are arranged around the central mixed β-sheet. The active sites of COMT are composed of AdoMet-binding domain and the actual catalytic sites, which are formed by a few amino acids that are important in the binding of the substrate, water, and Mg²⁺ and the catalysis of O-methylation. The lysine residue (Lys144) that accepts the proton of one of the hydroxyls functions as a general catalytic base in the nucleophilic methyl transfer reaction.

Existing Forms

Two distinct forms of COMT have been confirmed: one is soluble (S-COMT) and the other is membrane-bound (MB-COMT), both of which are encoded by one single gene with the assistance of two separate promoters. The soluble COMT enzyme contains 221 amino acids, whereas the MB-COMT form has almost 50 (human) and 43 (rat) residues with a long amino-terminal extension containing a hydrophobic membrane-anchor region. It has been recently found that MB-COMT occurs intracellularly on the rough endoplasmic reticulum, and S-COMT is mainly cytoplasmic and to some extent nuclear. In vertebrates, COMT protein exists in a soluble form for the most part, and only a minor fraction is in the particular form. This distribution pattern of COMT in several cellular compartments recommends that the contribution of S- and MB-COMT to the methylation of different substrates may depend both on the kinetic properties of the enzymes and the intracellular distribution of the substrates.

Catalytic Mechanism

COMT catalyzes the transfer of the methyl group of AdoMet to one of the hydroxyl groups of the catechol substrate or substituted catechols in the presence of Mg²⁺, which is thought to be a direct nucleophilic attack by one of the hydroxyl groups of the catechol substrate on the methyl carbon of AdoMet through a tight S_N2-like transition state, where Mg²⁺ binds to both MB-COMT and S-COMT in a rapid equilibrium prior to the addition of AdoMet (Figure 1). Furthermore, a sequential ordered mechanism investigation suggests that coenzyme AdoMet is the first substrate to bind with enzyme, followed by Mg²⁺, whose binding is beneficial to improve the ionization of the hydroxyl groups, the subsequent catechol substrate and S-adenosyl-L-homocysteine (AdoHcy) is the last product to dissociate from the enzyme (Figure 2). The presence of Mg²⁺ is necessary for the catalysis and some other divalent cations such as Cd²⁺, Hg²⁺, Mn²⁺, Zn²⁺, and Cu²⁺ also promote the methylation. COMT is able to methylate only one of the two catechol hydroxyls and the O-methylation occurs primarily at the 3-hydroxyl (meta-position). At the same time, the O-methylation also depends on experimental conditions and the nature of the side chain of the catechol substrate and varying amounts of the 4-methylated (para-position) catechols are produced. The kinetic mechanisms of soluble and membrane bound COMT has been demonstrated to be identical, while S-COMT and MB-COMT are certainly different enzymes and MB-COMT is not a precursor of S-COMT.

Figure 1. A proposed catalytic mechanism of COMT. (Männistö P T; et al. 1999)

Figure 2 A sequential ordered mechanism of COMT. (E = enzyme; S = catechol substrate; P = methylated catechol product). (Lotta T; et al. 1995)

Functions

The general function of COMT is the degradation of bioactive or toxic catechols and some other hydroxylated metabolites. Some specific reactions catalyzed by COMT include: (1) Dopamine→3-Methoxytyramine; (2) DOPAC→HVA (homovanillic acid); (3) Norepinephrine→Normetanephrine; (4) Epinephrine→Metanephrine; (5) Dihydroxyphenylethylene glycol (DOPEG)→Methoxyhydroxyphenylglycol (MOPEG); (6) 3,4-Dihydroxymandelic acid (DOMA) →Vanillylmandelic acid (VMA). COMT existing in placenta protects the developing embryo from the harm of activated hydroxylated compounds during the first trimester of pregnancy. COMT also acts as an enzymatic detoxicating barrier between the blood and other tissues by shielding them against the detrimental effects of xenobiotics. COMT could modulate the dopaminergic tone and thus serves some unique or indirect functions in kidney and intestine tract, which may also be available in the brain for active COMT may regulate the amounts of active dopamine and norepinephrine in various part of the brain and therefore be connected with the mood and other mental processes.

References

1. Männistö P T, Kaakkola S. Catechol-O-methyltransferase (COMT): Biochemistry, molecular biology, pharmacology, and clinical efficacy of the new selective COMT inhibitors. Pharmacological Reviews, 1999, 51(4):593-628.
2. Lotta T, Vidgren J, Tilgmann C, Ulmanen I, Krister Melén, Julkunen I, Taskinen J. Kinetics of human soluble and membrane-bound catechol O-Methyltransferase: A revised mechanism and description of the thermolabile variant of the enzyme. Biochemistry, 1995, 34:4202-4210.