Lanosterol synthase () is an oxidosqualene
cyclase A cyclase is an enzyme, almost always a lyase, that catalyzes a chemical reaction to form a cyclic compound. Important cyclase enzymes include:
* Adenylyl cyclase, which forms cyclic AMP from adenosine triphosphate (EC 4.6.1.1)
** ADCY1
** ADCY2 ...
(OSC)
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
that converts
(S)-2,3-oxidosqualene to a protosterol cation and finally to
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 ...
.
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 ...
is a key four-ringed intermediate in
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
biosynthesis.
In humans, lanosterol synthase is encoded by the LSS
gene
In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...
.
In
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, lanosterol synthase is an
integral monotopic protein
Integral monotopic proteins are permanently attached to the cell membrane from one side, and are a type of integral membrane protein (IMP).
Three-dimensional structures of the following integral monotopic proteins have been determined:
*prostagl ...
associated with the
cytosolic
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrio ...
side of the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
.
Some evidence suggests that the
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
is a soluble, non-
membrane bound 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, respo ...
in the few
prokaryotes
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
that produce it.
Due to the enzyme's role in
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
biosynthesis, there is interest in lanosterol synthase
inhibitors as potential cholesterol-reducing drugs, to complement existing
statins.
Mechanism
Though some data on the mechanism has been obtained by the use of
suicide inhibitors,
mutagenesis
Mutagenesis () is a process by which the genetic information of an organism is changed by the production of a mutation. It may occur spontaneously in nature, or as a result of exposure to mutagens. It can also be achieved experimentally using lab ...
studies, and
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 ...
, it is still not fully understood how the
enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
catalyzes the formation 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 ...
.
Initial epoxide protonation and ring opening
Before the acquisition of the protein's
X-ray crystal structure
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
,
site-directed mutagenesis
Site-directed mutagenesis is a molecular biology method that is used to make specific and intentional mutating changes to the DNA sequence of a gene and any gene products. Also called site-specific mutagenesis or oligonucleotide-directed mutagenesi ...
was used to determine residues key to the enzyme's catalytic activity. It was determined that an
aspartic acid
Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...
residue (D455) and two
histidine
Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an α-amino group (which is in the protonated –NH3+ form under biological conditions), a carboxylic acid group (which is in the de ...
residues (H146 and H234) were essential to enzyme function. ''Corey et al.'' hypothesized that the aspartic acid acts by protonating the substrate's
epoxide
In organic chemistry, an epoxide is a cyclic ether () with a three-atom ring. This ring approximates an equilateral triangle, which makes it strained, and hence highly reactive, more so than other ethers. They are produced on a large scale for ...
ring, thus increasing its susceptibility to
intramolecular attack by the nearest
double bond
In chemistry, a double bond is a covalent bond between two atoms involving four bonding electrons as opposed to two in a single bond. Double bonds occur most commonly between two carbon atoms, for example in alkenes. Many double bonds exist betwee ...
, with H146 possibly intensifying the proton donor ability of the aspartic acid through
hydrogen bonding
In chemistry, a hydrogen bond (or H-bond) is a primarily electrostatic force of attraction between a hydrogen (H) atom which is covalently bound to a more electronegative "donor" atom or group (Dn), and another electronegative atom bearing a l ...
.
After acquisition of the
X-ray crystal structure
X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
of the enzyme, the role of D455 as a proton donor to the substrate's epoxide was confirmed, though it was found that D455 is more likely stabilized by hydrogen bonding from two
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
residues (C456 and C533) than from the earlier suggested histidine.
Ring formation cascade
Epoxide protonation activates the substrate, setting off a cascade of ring forming reactions. Four rings in total (A through D) are formed, producing the
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
backbone.
Though the idea of a concerted formation of all four rings had been entertained in the past, kinetic studies with
(S)-2,3-oxidosqualene analogs showed that product formation is achieved through discrete
carbocation
A carbocation is an ion with a positively charged carbon atom. Among the simplest examples are the methenium , methanium and vinyl cations. Occasionally, carbocations that bear more than one positively charged carbon atom are also encountere ...
intermediates (see Figure 1). Isolation of monocyclic and bicyclic products from lanosterol synthase mutants has further weakened the hypothesis of a concerted mechanism.
Evidence suggests that epoxide ring opening and A ring formation is concerted, though.
Structure
Lanosterol synthase is a two-domain monomeric protein
composed of two connected (α/α) barrel domains and three smaller
β-structures. The enzyme
active site
In biology and biochemistry, the active site is the region of an enzyme where substrate molecules bind and undergo a chemical reaction. The active site consists of amino acid residues that form temporary bonds with the substrate (binding site) a ...
is in the center of the protein, closed off by a constricted channel. Passage of the (S)-2,3-epoxysqualene substrate through the channel requires a change in
protein conformation
Protein structure is the three-dimensional arrangement of atoms in an amino acid-chain molecule. Proteins are polymers specifically polypeptides formed from sequences of amino acids, the monomers of the polymer. A single amino acid monomer ma ...
. In
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, a
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
surface (6% of the total enzyme surface area) is the
ER membrane-binding region (see Figure 2).
The enzyme contains five fingerprint regions containing
Gln-
Trp motifs, which are also present in the highly analogous bacterial enzyme squalene-hopene cyclase.
Residues of these fingerprint regions contain stacked sidechains which are thought to contribute to enzyme stability during the highly
exergonic
An exergonic process is one which there is a positive flow of energy from the system to the surroundings. This is in contrast with an endergonic process. Constant pressure, constant temperature reactions are exergonic if and only if the Gibbs fr ...
cyclization reactions catalyzed by the enzyme.
Function
Catalysis of lanosterol formation
Lanosterol synthase catalyzes the conversion of
(S)-2,3-epoxysqualene to
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 ...
, a key four-ringed intermediate in
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
biosynthesis.
Thus, it in turn provides the precursor to
estrogen
Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal acti ...
s,
androgen
An androgen (from Greek ''andr-'', the stem of the word meaning "man") is any natural or synthetic steroid hormone that regulates the development and maintenance of male characteristics in vertebrates by binding to androgen receptors. This inc ...
s,
progestogen
Progestogens, also sometimes written progestagens or gestagens, are a class of natural or synthetic steroid hormones that bind to and activate the progesterone receptors (PR). Progesterone is the major and most important progestogen in the body. ...
s,
glucocorticoids
Glucocorticoids (or, less commonly, glucocorticosteroids) are a class of corticosteroids, which are a class of steroid hormones. Glucocorticoids are corticosteroids that bind to the glucocorticoid receptor that is present in almost every vertebr ...
,
mineralocorticoid
Mineralocorticoids are a class of corticosteroids, which in turn are a class of steroid hormones. Mineralocorticoids are produced in the adrenal cortex and influence salt and water balances (electrolyte balance and fluid balance). The primary mi ...
s, and
neurosteroids. In
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
the enzyme is bound to the
cytosolic
The cytosol, also known as cytoplasmic matrix or groundplasm, is one of the liquids found inside cells (intracellular fluid (ICF)). It is separated into compartments by membranes. For example, the mitochondrial matrix separates the mitochondrio ...
side of the
endoplasmic reticulum
The endoplasmic reticulum (ER) is, in essence, the transportation system of the eukaryotic cell, and has many other important functions such as protein folding. It is a type of organelle made up of two subunits – rough endoplasmic reticulum ( ...
membrane.
While
cholesterol
Cholesterol is any of a class of certain organic molecules called lipids. It is a sterol (or modified steroid), a type of lipid. Cholesterol is biosynthesized by all animal cells and is an essential structural component of animal cell mem ...
synthesis is mostly associated with
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
, few
prokaryotes
A prokaryote () is a single-celled organism that lacks a nucleus and other membrane-bound organelles. The word ''prokaryote'' comes from the Greek πρό (, 'before') and κάρυον (, 'nut' or 'kernel').Campbell, N. "Biology:Concepts & Connec ...
have been found to express lanosterol synthase; it has been found as a soluble protein in ''
Methylococcus capsulatus
''Methylococcus capsulatus'' is an obligately methanotrophic gram-negative, non-motile coccoid bacterium. ''M. capsulatus'' are thermotolerant; their cells are encapsulated and tend to have a diplococcoid shape. In addition to methane, ''M. ...
''.
Catalysis of epoxylanosterol formation
Lanosterol synthase also catalyzes the cyclization of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol,
which is later converted to 24(S),25-epoxycholesterol.
Since the enzyme affinity for this second
substrate is greater than for the monoepoxy (S)-2,3-epoxysqualene, under partial inhibition conversion of 2,3;22,23-diepoxysqualene to 24(S),25-epoxylanosterol is favored over
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 ...
synthesis.
This has relevance for disease prevention and treatment.
Clinical significance
Enzyme inhibitors as cholesterol-lowering drugs
Interest has grown in lanosterol synthase inhibitors as drugs to lower blood cholesterol and treat
atherosclerosis
Atherosclerosis is a pattern of the disease arteriosclerosis in which the wall of the artery develops abnormalities, called lesions. These lesions may lead to narrowing due to the buildup of atheroma, atheromatous plaque. At onset there are usu ...
. The widely popular
statin
Statins, also known as HMG-CoA reductase inhibitors, are a class of lipid-lowering medications that reduce illness and mortality in those who are at high risk of cardiovascular disease. They are the most common cholesterol-lowering drugs.
Low- ...
drugs currently used to lower
LDL (low-density lipoprotein) cholesterol function by inhibiting
HMG-CoA reductase activity.
Because this enzyme catalyzes the formation of precursors far upstream of
(S)-2,3-epoxysqualene and cholesterol,
statins may negatively influence amounts of intermediates required for other biosynthetic pathways (e.g. synthesis of
isoprenoids,
coenzyme Q
Coenzyme Q, also known as ubiquinone and marketed as CoQ10, is a coenzyme family that is ubiquitous in animals and most bacteria (hence the name ubiquinone). In humans, the most common form is coenzyme Q10 or ubiquinone-10.
It is a 1,4-benzoq ...
). Thus, lanosterol synthase, which is more closely tied to cholesterol biosynthesis than
HMG-CoA reductase, is an attractive drug target.
Lanosterol synthase inhibitors are thought to lower
LDL
Low-density lipoprotein (LDL) is one of the five major groups of lipoprotein that transport all fat molecules around the body in extracellular water. These groups, from least dense to most dense, are chylomicrons (aka ULDL by the overall densit ...
and
VLDL
Very-low-density lipoprotein (VLDL), density relative to extracellular water, is a type of lipoprotein made by the liver. VLDL is one of the five major groups of lipoproteins (chylomicrons, VLDL, intermediate-density lipoprotein, low-density lipo ...
cholesterol by a dual control mechanism. Studies in which lanosterol synthase is partially inhibited have shown both a direct decrease in
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 ...
formation and a decrease in
HMG-CoA reductase activity. The
oxysterol
An oxysterol is a derivative of cholesterol obtained by oxidation involving enzymes and / or pro-oxidants. Such compounds play important roles in various biological processes such as cholesterol homeostasis, lipid metabolism (sphingolipids, fatty ...
24(S),25-epoxylanosterol, which is preferentially formed over
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 ...
during partial lanosterol synthase inhibition, is believed to be responsible for this inhibition of
HMG-CoA reductase activity.
Evolution
It is believed that oxidosqualene cyclases (OSCs, the class to which lanosterol cyclase belongs) evolved from bacterial squalene-hopene cyclase (SHC), which is involved with the formation of
hopanoids
Hopanoids are a diverse subclass of triterpenoids with the same hydrocarbon skeleton as the compound hopane. This group of pentacyclic molecules therefore refers to simple hopenes, hopanols and hopanes, but also to extensively functionalized deri ...
.
Phylogenetic trees
A phylogenetic tree (also phylogeny or evolutionary tree Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA.) is a branching diagram or a tree showing the evolutionary relationships among various biological spec ...
constructed from the amino acid sequences of OSCs in diverse organisms suggest a single common ancestor, and that the synthesis pathway evolved only once.
The discovery of
steranes including
cholestane
Cholestane is a saturated tetracyclic triterpene. This 27-carbon biomarker is produced by diagenesis of cholesterol and is one of the most abundant biomarkers in the rock record. Presence of cholestane, its derivatives and related chemical compo ...
in 2.7-billion year-old shales from
Pilbara Craton
The Pilbara Craton is an old and stable part of the continental lithosphere located in the Pilbara region of Western Australia.
The Pilbara Craton is one of only two pristine Archaean 3.6–2.7 Ga (billion years ago) crusts identified on the ...
,
Australia
Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
, suggests that
eukaryotes
Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
with OSCs and complex steroid machinery were present early in earth's history.
References
Further reading
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External links
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