The enzyme polyneuridine-aldehyde esterase (EC 3.1.1.78)

catalyzes Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...

the following reaction: :polyneuridine aldehyde + H₂O

\rightleftharpoons

16-epivellosimine + CO₂ + methanol This enzyme participates in indole alkaloid biosynthesis.

Nomenclature

This enzyme belongs to the family of

hydrolase In biochemistry, hydrolases constitute a class of enzymes that commonly function as biochemical catalysts that use water to break a chemical bond: :\ce \quad \xrightarrowtext\quad \ce This typically results in dividing a larger molecule into s ...

s, specifically those acting on carboxylic

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

bonds. The

systematic name A systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance, out of a specific population or collection. Systematic names are usually part of a nomenclature. A semisystematic name or semitrivi ...

is polyneuridine aldehyde hydrolase (decarboxylating). Other names in common use include: * polyneuridine aldehyde esterase * PNAE

Homologues

This enzyme is found in various forms in plant species such as ''

Arabidopsis thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small plant from the mustard family (Brassicaceae), native to Eurasia and Africa. Commonly found along the shoulders of roads and in disturbed land, it is generally ...

'', ''

Glycine max The soybean, soy bean, or soya bean (''Glycine max'') is a species of legume native to East Asia, widely grown for its edible bean. Soy is a staple crop, the world's most grown legume, and an important animal feed. Soy is a key source of f ...

'' (soybean), ''

Vitis vinifera ''Vitis vinifera'', the common grape vine, is a species of flowering plant, native to the Mediterranean Basin, Mediterranean region, Central Europe, and southwestern Asia, from Morocco and Portugal north to southern Germany and east to northern ...

'' (wine grape), and ''

Solanum lycopersicum The tomato (, ), ''Solanum lycopersicum'', is a plant whose fruit is an edible berry that is eaten as a vegetable. The tomato is a member of the nightshade family that includes tobacco, potato, and chili peppers. It originated from and was d ...

'' (tomato) among others. Polyneuridine-aldehyde esterase also appears in select bacteria including ''

Enterobacter cloacae ''Enterobacter cloacae'' is a clinically significant Gram-negative, facultatively-anaerobic, rod-shaped bacterium. Microbiology In microbiology laboratories, ''E. cloacae'' is frequently grown at 30 °C on nutrient agar or at 35 ...

''.

Structure

The secondary structure of this enzyme consists mainly of

α helices Alpha (uppercase , lowercase ) is the first letter of the Greek alphabet. In the system of Greek numerals, it has a value of one. Alpha is derived from the Phoenician letter ''aleph'' , whose name comes from the West Semitic word for ' ox' ...

. In its native form, this enzyme has a tertiary structure that includes two main lobes (as depicted above in the blue 3D representation on the top right).

Reaction

Polyneuridine-aldehyde esterase catalyzes the

hydrolysis Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...

of the

methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...

in polyneuridine aldehyde to form polyneuridine β-aldehydoacid and methanol. The

carboxylic acid In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...

in the product spontaneously undergoes

decarboxylation Decarboxylation is a chemical reaction that removes a carboxyl group and releases carbon dioxide (CO2). Usually, decarboxylation refers to a reaction of carboxylic acids, removing a carbon atom from a carbon chain. The reverse process, which is ...

, yielding 16-epivellosimine and carbon dioxide.

Mechanism

The mechanism of hydrolysis performed by polyneuridine-aldehyde esterase is not known. It has been suggested that the enzyme utilizes a

catalytic triad A catalytic triad is a set of three coordinated amino acid residues that can be found in the active site of some enzymes. Catalytic triads are most commonly found in hydrolase and transferase enzymes (e.g. proteases, amidases, esterases, aminoac ...

composed of Ser-87, Asp-216 and His-244. The catalytic amino acid order is the same as the order of enzymes that are part of the α/β hydrolase family. Thus polyneuridine-aldehyde esterase may be a novel member of the α/β hydrolase group.

Broader significance

This enzyme is a part of the pathway of

indole alkaloid Indole alkaloids are a class of alkaloids containing a structural moiety of indole; many indole alkaloids also include isoprene groups and are thus called terpene indole or secologanin tryptamine alkaloids. Containing more than 4100 known differ ...

biosynthesis. The indole alkaloids that result from this metabolic pathway are used by many plant species as a defense against herbivores and parasites.

Open questions

The precise mechanisms by which this enzyme performs its function is still unknown. As noted above, researchers are formulating suggestions as to how polyneuridine-aldehyde esterase catalyses the decomposition of polyneuridine-aldehyde, but a mechanism has not yet been affirmed with absolute certainty. Due to the lack of complete understanding of polyneuridine-aldehyde esterase's precise mechanism, this enzyme cannot be grouped into a family of enzymes. Based on mechanism theories, suggestions can be made as to how this enzyme should be categorized, and some parallels can be drawn between polyneuridine-aldehyde esterase and other enzymes.