Squalene-hopene Cyclase
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Squalene-hopene cyclase (SHC) () or ''hopan-22-ol hydro-lyase'' is an
enzyme An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
in the
terpene Terpenes () are a class of natural products consisting of compounds with the formula (C5H8)n for n ≥ 2. Terpenes are major biosynthetic building blocks. Comprising more than 30,000 compounds, these unsaturated hydrocarbons are produced predomi ...
cyclase/mutase family. It catalyzes the interconversion of squalene into a pentacyclic triterpenes, hopene and hopanol. This enzyme catalyses the following chemical reactions. :
squalene Squalene is an organic compound. It is a triterpene with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as '' Squalus'' is a genus of sharks). ...
\rightleftharpoons hop-22(29)-ene :
squalene Squalene is an organic compound. It is a triterpene with the formula C30H50. It is a colourless oil, although impure samples appear yellow. It was originally obtained from shark liver oil (hence its name, as '' Squalus'' is a genus of sharks). ...
+ H2O \rightleftharpoons hopan-22-ol SHC is important because its products, hopanoids, are very much like
sterol A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
s in eukaryotes in that they condense
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
s and reduce permeability. In fact, SHC and sterol-producing enzymes (
oxidosqualene cyclase Oxidosqualene cyclases (OSC) are enzymes involved in cyclization reactions of 2,3-oxidosqualene to form sterols or triterpenes. There are two major groups of sterol-producing OSC enzymes: * Cycloartenol synthase (CAS), found in all plants, which ...
) are evolutionarily related to each other. Hopanoids are inferred to provide stability in the face of high temperatures and extreme acidity due to the rigid ring structure. Indeed, up-regulation of SHC occurs in certain bacteria in the presence of hot or acidic environments. SHC is found mostly in bacteria, but some eukaryotes, such as
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
and
land plants The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophy ...
, are also known to possess the enzyme.


Introduction

SHC is found in a large number of bacteria but is most readily isolated from the thermophilic bacterium ''Alicyclobacillus acidocaldarius''. SHC does not require molecular oxygen for its reaction and is thought to be an evolutionary progenitor of oxygen-dependent
oxidosqualene cyclase Oxidosqualene cyclases (OSC) are enzymes involved in cyclization reactions of 2,3-oxidosqualene to form sterols or triterpenes. There are two major groups of sterol-producing OSC enzymes: * Cycloartenol synthase (CAS), found in all plants, which ...
(OSC), which produces tetracyclic
sterol A sterol is any organic compound with a Skeletal formula, skeleton closely related to Cholestanol, cholestan-3-ol. The simplest sterol is gonan-3-ol, which has a formula of , and is derived from that of gonane by replacement of a hydrogen atom on ...
s. OSC is a eukaryotic analog of SHC and requires molecular oxygen for its catalysis. This may suggest a later evolution of OSC relative to SHC, when the atmosphere began accumulating oxygen, although the distribution of SHC is also limited mostly to aerobic species.


Structure

Squalene-hopene cyclase is a membrane-associated 70-75kDa protein composed of 631 amino acids and seven PTFB repeats. It exists as a monotopic homodimer.


Mechanism

The formation of the hopene skeleton is one of the most complex single-step reactions in biochemistry. In a single step, 13 covalent bonds are broken or formed, 9 chiral centers are established, and 5 rings are produced. Squalene–hopene cyclase catalyses the conversion of the acyclic molecule of squalene into the pentacyclic triterpenes of hopene and hopanol. These products appear in the ratio 5:1. Hopene synthesis begins with binding squalene in an all pre-chair conformation and is followed by the formation of five C-C bonds. These sequential ring-forming reaction steps are initiated by an electrophilic attack of an acidic proton on one of the two terminal double bonds. The polycyclic formation is completed when a proton is eliminated from the alternative terminal methyl group of squalene via acceptance by a water molecule. This base is known as the ''front water''. Other water molecules work to enhance polarization (''back waters'') and construct
hydrogen bonds In chemistry, a hydrogen bond (H-bond) is a specific type of molecular interaction that exhibits partial covalent character and cannot be described as a purely electrostatic force. It occurs when a hydrogen (H) atom, covalently bonded to a mo ...
between seven residues—T41, E45, E93, R127, Q262, W133 and Y267. Front water also plays a role in determining the end product. If it stores the proton generated from either Methyl group 29 or 30 to form hopene. However, hopanol is produced in lesser quantities if instead of accepting the proton, water contributes a hydroxyl to the C-22 cation of the A-ring. During the formation of rings A through D, there is very little conformational change. The reaction therefore requires no intermediate and can take place in one step. However, ring E formation is hindered by an entropic barrier, which may explain its absence in the tetracyclic
steroids A steroid is an organic compound with four fused rings (designated A, B, C, and D) arranged in a specific molecular configuration. Steroids have two principal biological functions: as important components of cell membranes that alter mem ...
.


Active site

The SHC
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, the ''binding s ...
is located in a central cavity within the region of the protein adjacent to the membrane, and is accessed by the substrate via a non-polar channel. The active site is notably surrounded by aromatic residues forming a cavity that comfortably fits the squalene molecule when folded into a productive conformation. The
catalytic mechanism Enzyme catalysis is the increase in the rate of a process by an "enzyme", a biological molecule. Most enzymes are proteins, and most such processes are chemical reactions. Within the enzyme, generally catalysis occurs at a localized site, calle ...
uses coupled
aspartate 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. The L-isomer of aspartic acid is one of the 22 proteinogenic amino acids, i.e., the building blocks of protein ...
and
histidine Histidine (symbol His or H) is an essential amino acid that is used in the biosynthesis of proteins. It contains an Amine, α-amino group (which is in the protonated –NH3+ form under Physiological condition, biological conditions), a carboxylic ...
residues to initiate the
cyclization A cyclic compound (or ring compound) is a term for a compound in the field of chemistry in which one or more series of atoms in the compound is connected to form a ring. Rings may vary in size from three to many atoms, and include examples where ...
reaction by
protonating In chemistry, protonation (or hydronation) is the adding of a proton (or hydron, or hydrogen cation), usually denoted by H+, to an atom, molecule, or ion, forming a conjugate acid. (The complementary process, when a proton is removed from a Brø ...
at C3 and deprotonating at C29, proceeding through a discrete series of
carbocation Carbocation is a general term for ions with a positively charged carbon atom. In the present-day definition given by the IUPAC, a carbocation is any even-electron cation with significant partial positive charge on a carbon atom. They are further ...
intermediates. The enzyme can be inactivated by mutation of catalytic aspartates.


Thermodynamics

This enzyme is unusually
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
with an energy release of 40-50kcal/mol, well beyond the protein stabilization energy. This is thought to melt a lipid side channel through which the bulky product exits. In order to maintain its structural integrity, some scientists believe that the enzyme’s 7-8 non-tandem repeat QW motifs (Q is
glutamine Glutamine (symbol Gln or Q) is an α-amino acid that is used in the biosynthesis of proteins. Its side chain is similar to that of glutamic acid, except the carboxylic acid group is replaced by an amide. It is classified as a charge-neutral ...
and W is
tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromat ...
) that connect numerous surface
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' ...
tighten the protein structure and prevent denaturing.


References


External links

* {{DEFAULTSORT:Squalene-hopene cyclase EC 5.4.99