Lanosterol 14α-demethylase (CYP51A1) is the animal version of a cytochrome P450 enzyme that is involved in the conversion of

lanosterol Lanosterol is a tetracyclic triterpenoid and is the compound from which all animal and fungal steroids are derived. By contrast plant steroids are produced via cycloartenol. Role in biosynthesis of other steroids Elaboration of lanosterol under en ...

to 4,4-dimethylcholesta-8(9),14,24-trien-3β-ol. The

cytochrome P450 Cytochromes P450 (CYPs) are a superfamily of enzymes containing heme as a cofactor that functions as monooxygenases. In mammals, these proteins oxidize steroids, fatty acids, and xenobiotics, and are important for the clearance of various co ...

isoenzyme In biochemistry, isozymes (also known as isoenzymes or more generally as multiple forms of enzymes) are enzymes that differ in amino acid sequence but catalyze the same chemical reaction. Isozymes usually have different kinetic parameters (e.g. dif ...

s are a conserved group of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, res ...

s that serve as key players in the

metabolism Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run c ...

organic substance In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...

s and the biosynthesis of important steroids,

lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids includ ...

s, and

vitamin A vitamin is an organic molecule (or a set of molecules closely related chemically, i.e. vitamers) that is an essential micronutrient that an organism needs in small quantities for the proper functioning of its metabolism. Essential nutrie ...

s in eukaryotes. As a member of this family, lanosterol 14α-demethylase is responsible for an essential step in the biosynthesis of sterols. In particular, this protein catalyzes the removal of the C-14α-

methyl group In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula . In formulas, the group is often abbreviated as Me. This hydrocarbon group occurs in ma ...

from

. This demethylation step is regarded as the initial checkpoint in the transformation of

to other sterols that are widely used within the cell.

Evolution

The structural and functional properties of the

superfamily have been subject to extensive diversification over the course of evolution. Recent estimates indicate that there are currently 10 classes and 267

families Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Ideal ...

of CYP proteins. It is believed that 14α-demethylase or CYP51 diverged early in the cytochrome's

evolutionary history The history of life on Earth traces the processes by which living and fossil organisms evolved, from the earliest emergence of life to present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''gigaannum'') and ev ...

and has preserved its function ever since; namely, the removal of the 14α-methyl group from sterol substrates. Although CYP51's mode of action has been well conserved, the protein's sequence varies considerably between biological kingdoms. CYP51 sequence comparisons between kingdoms reveal only a 22-30% similarity in amino acid composition.

Structure

Although the structure of 14α-demethylase may vary substantially from one organism to the next, sequence alignment analysis reveals that there are six regions in the protein that are highly conserved in eukaryotes. These include residues in the B' helix, B'/C loop, C helix, I helix, K/β1-4 loop, and β-strand 1-4 that are responsible for forming the surface of the substrate binding cavity.

Homology modeling Homology modeling, also known as comparative modeling of protein, refers to constructing an atomic-resolution model of the "''target''" protein from its amino acid sequence and an experimental three-dimensional structure of a related homologous pr ...

reveals that substrates migrate from the surface of the protein to the enzyme's buried active site through a channel that is formed in part by the A' alpha helix and the β4 loop. Finally, the active site contains a

heme Heme, or haem (pronounced / hi:m/ ), is a precursor to hemoglobin, which is necessary to bind oxygen in the bloodstream. Heme is biosynthesized in both the bone marrow and the liver. In biochemical terms, heme is a coordination complex "consis ...

prosthetic group A prosthetic group is the non-amino acid component that is part of the structure of the heteroproteins or conjugated proteins, being tightly linked to the apoprotein. Not to be confused with the cofactor that binds to the enzyme apoenzyme (eith ...

in which the iron is tethered to the sulfur atom on a conserved cysteine residue. This group also binds diatomic oxygen at the sixth coordination site, which is eventually incorporated onto the substrate.

Mechanism

The enzyme-catalyzed

demethylation Demethylation is the chemical process resulting in the removal of a methyl group (CH3) from a molecule. A common way of demethylation is the replacement of a methyl group by a hydrogen atom, resulting in a net loss of one carbon and two hydrogen at ...

is believed to occur in three steps, each of which requires one molecule of diatomic oxygen and one molecule of NADPH (or some other

reducing equivalent Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

). During the first two steps, the 14α-methyl group undergoes typical

cytochrome Cytochromes are redox-active proteins containing a heme, with a central Fe atom at its core, as a cofactor. They are involved in electron transport chain and redox catalysis. They are classified according to the type of heme and its mode of bi ...

monooxygenation in which one oxygen atom is incorporated by the substrate and the other is reduced to water, resulting in the sterol's conversion to a carboxyalcohol and then a carboxyaldehyde. The aldehyde then departs as formic acid and a double bond is simultaneously introduced to yield the demethylated product.

References

External links

* {{Portal bar, Biology, border=no EC 1.14.14 51 NADPH-dependent enzymes Enzymes of known structure

Evolution

Structure

Mechanism

See also

References

Further reading

External links