Steroid 21-hydroxylase (also known as steroid 21-monooxygenase, cytochrome P450
C21, 21α-hydroxylase and less commonly 21β-hydroxylase) is an
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 ...
that
hydroxylates steroids at the C21 position
and is involved in
biosynthesis of
aldosterone and
cortisol. The enzyme converts
progesterone and
17α-hydroxyprogesterone
17α-Hydroxyprogesterone (17α-OHP), also known as 17-OH progesterone (17-OHP), or hydroxyprogesterone (OHP), is an endogenous progestogen steroid hormone related to progesterone. It is also a chemical intermediate in the biosynthesis of many o ...
into
11-deoxycorticosterone and
11-deoxycortisol
11-Deoxycortisol, also known as cortodoxone (INN), cortexolone as well as 17α,21-dihydroxyprogesterone or 17α,21-dihydroxypregn-4-ene-3,20-dione, is an endogenous glucocorticoid steroid hormone, and a metabolic intermediate towards cortisol. It ...
, respectively,
within
metabolic pathway
In biochemistry, a metabolic pathway is a linked series of chemical reactions occurring within a cell. The reactants, products, and intermediates of an enzymatic reaction are known as metabolites, which are modified by a sequence of chemical reac ...
s that ultimately lead to
aldosterone and
cortisol. Deficiency in the enzyme may cause
congenital adrenal hyperplasia
Congenital adrenal hyperplasia (CAH) is a group of autosomal recessive disorders characterized by impaired cortisol synthesis. It results from the deficiency of one of the five enzymes required for the synthesis of cortisol in the adrenal cort ...
.
Steroid 21-hydroxylase is a member of 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 ...
family of
monooxygenase
Monooxygenases are enzymes that incorporate one hydroxyl group (−OH) into substrates in many metabolic pathways. In this reaction, the two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation o ...
enzymes that uses an iron containing
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 ...
cofactor to oxidize substrates. The enzyme is localized in
endoplasmic reticulum membranes of
adrenal cortex,
and is encoded by the
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 b ...
in humans, which is located near the ''CYP21A1P''
pseudogene with high degree of sequence similarity. This similarity makes it difficult to analyze the gene at the molecular level, and sometimes leads to
loss-of-function mutations of the gene due to intergenic exchange of
DNA.
Function
![Trimethyl steroid-nomenclature](https://upload.wikimedia.org/wikipedia/commons/f/fa/Trimethyl_steroid-nomenclature.svg)
The steroid 21-hydroxylase enzyme
hydroxylates steroids at the C21 position.
The enzyme
catalyzes the
chemical reaction
A chemical reaction is a process that leads to the IUPAC nomenclature for organic transformations, chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the pos ...
in which the
hydroxyl
In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydro ...
group (-OH) is added at the C21 position of the steroid
biomolecule.
The enzyme is a member of 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 ...
superfamily of
monooxygenase
Monooxygenases are enzymes that incorporate one hydroxyl group (−OH) into substrates in many metabolic pathways. In this reaction, the two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation o ...
enzymes. The cytochrome P450 enzymes catalyze many reactions involved in drug metabolism and synthesis of
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 ...
,
steroids and other
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.
21-hydroxylase is localized in
microsome
In cell biology, microsomes are heterogeneous vesicle-like artifacts (~20-200 nm diameter) re-formed from pieces of the endoplasmic reticulum (ER) when eukaryotic cells are broken-up in the laboratory; microsomes are not present in healthy, liv ...
s of
endoplasmic reticulum membranes within
adrenal cortex. It is one of three microsomal steroidogenic P450 enzymes, the others being
17-hydroxylase and
aromatase
Aromatase (), also called estrogen synthetase or estrogen synthase, is an enzyme responsible for a key step in the biosynthesis of estrogens. It is CYP19A1, a member of the cytochrome P450 superfamily, which are monooxygenases that catalyze many ...
.
21-hydroxylase is essential for the
biosynthesis of
cortisol and
aldosterone.
Structure
21-hydroxylase, as a member of 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 ...
family of
monooxygenase
Monooxygenases are enzymes that incorporate one hydroxyl group (−OH) into substrates in many metabolic pathways. In this reaction, the two atoms of dioxygen are reduced to one hydroxyl group and one H2O molecule by the concomitant oxidation o ...
enzymes, contains a conserved core of a four
α-helix
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ...
bundle, two additional alpha helices, two sets of
β-sheets, and 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 ...
cofactor binding loop.
Each subunit in the human enzyme consists of a total of 13
α-helices
The alpha helix (α-helix) is a common motif in the secondary structure of proteins and is a right hand-helix conformation in which every backbone N−H group hydrogen bonds to the backbone C=O group of the amino acid located four residues ear ...
and 9
β-strands that folds into a triangular prism-like
tertiary structure
Protein tertiary structure is the three dimensional shape of a protein. The tertiary structure will have a single polypeptide chain "backbone" with one or more protein secondary structures, the protein domains. Amino acid side chains may i ...
.
The iron(III) heme group that defines the
active site resides in the center of each subunit. The human enzyme binds one substrate at a time.
In contrast, the well-characterized bovine enzyme can bind two substrates. The human and bovine enzyme share 80%
amino acid sequence
Protein primary structure is the linear sequence of amino acids in a peptide or protein. By convention, the primary structure of a protein is reported starting from the amino-terminal (N) end to the carboxyl-terminal (C) end. Protein biosynthe ...
identity, but are structurally different, particularly in loop regions, and also evident in
secondary structure elements.
Mechanism
![Steroidogenesis](https://upload.wikimedia.org/wikipedia/commons/1/13/Steroidogenesis.svg)
![Corticosteroid-biosynthetic-pathway-rat](https://upload.wikimedia.org/wikipedia/commons/8/89/Corticosteroid-biosynthetic-pathway-rat.png)
21-Hydroxylase is a cytochrome P450 enzyme that is notable for its
substrate specificity
Chemical specificity is the ability of binding site of a macromolecule (such as a protein) to bind specific ligands. The fewer ligands a protein can bind, the greater its specificity.
Specificity describes the strength of binding between a giv ...
and relatively high
catalytic efficiency
In the field of biochemistry, the specificity constant (also called kinetic efficiency or k_/K_), is a measure of how efficiently an enzyme converts substrates into products. A comparison of specificity constants can also be used as a measure of t ...
. Variations of the 21-hydroxylase enzyme can be found in all
vertebrate
Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...
s.
Like other cytochrome P450 enzymes, 21-hydroxylase participates in the
cytochrome P450 catalytic cycle and engages in one-electron transfer with
NADPH-
P450 reductase. 21-Hydroxylase is highly specific for hydroxylation of progesterone and 17-hydroxyprogesterone. This is in marked contrast to the evolutionarily and functionally related P450 enzyme
17-hydroxylase, which has a broad range of substrates.
The chemical reaction in which 21-hydroxylase catalyzes the addition of hydroxyl (-OH) to the C21 position of
progesterone,
17α-hydroxyprogesterone
17α-Hydroxyprogesterone (17α-OHP), also known as 17-OH progesterone (17-OHP), or hydroxyprogesterone (OHP), is an endogenous progestogen steroid hormone related to progesterone. It is also a chemical intermediate in the biosynthesis of many o ...
and
21-desoxycortisone was first described in 1952.
Studies of the human enzyme expressed in
yeast
Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
initially classified 17-hydroxyprogesterone as the preferred substrate for 21-hydroxylase,
however, later analysis of the purified human enzyme found a lower
KM and greater catalytic efficiency for progesterone over 17-hydroxyprogesterone.
The
catalytic efficiency
In the field of biochemistry, the specificity constant (also called kinetic efficiency or k_/K_), is a measure of how efficiently an enzyme converts substrates into products. A comparison of specificity constants can also be used as a measure of t ...
of 21-hydroxylase for conversion of progesterone in humans is approximately 1.3 x 10
7 M
−1s
−1 at 37 °C. This makes it the most catalytically efficient P450 enzyme of those reported to date, and catalytically more efficient than the closely related
bovine
Bovines (subfamily Bovinae) comprise a diverse group of 10 genera of medium to large-sized ungulates, including cattle, bison, African buffalo, water buffalos, and the four-horned and spiral-horned antelopes. The evolutionary relationship betwe ...
21-hydroxylase enzyme.
C-H bond breaking to create a primary carbon
radical is thought to be the
rate-limiting step
In chemical kinetics, the overall rate of a reaction is often approximately determined by the slowest step, known as the rate-determining step (RDS or RD-step or r/d step) or rate-limiting step. For a given reaction mechanism, the prediction of th ...
in the hydroxylation.
Genetics
21-hydroxylase is a protein encoded by the ''CYP21A2'' gene in humans. A related
pseudogene, ''CYP21A1P'', is located nearby and retains 98%
exonic sequence identity with the functional gene ''CYP21A2''.
Both genes are located on
chromosome 6, in the
major histocompatibility complex III close to the
Complement component 4
Complement component 4 (C4), in humans, is a protein involved in the intricate complement system, originating from the human leukocyte antigen (HLA) system. It serves a number of critical functions in immunity, tolerance, and autoimmunity with ...
genes ''
C4A'' and ''
C4B'', the
Tenascin X
A member of the tenascin family, tenascin X (TN-X) also known as flexillin or hexabrachion-like protein is a 450kDa glycoprotein that is expressed in connective tissues. TN-X possesses a modular structure composed, from the N- to the C-terminal pa ...
gene ''TNXB'' and ''
STK19''.
![CYP21Aevolution](https://upload.wikimedia.org/wikipedia/commons/7/7d/CYP21Aevolution.png)
In the
mouse genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
, the ''CYP21A2'' is a pseudogene and the ''CYP21A1'' is a functional gene.
In the
chicken
The chicken (''Gallus gallus domesticus'') is a domesticated junglefowl species, with attributes of wild species such as the grey and the Ceylon junglefowl that are originally from Southeastern Asia. Rooster or cock is a term for an adu ...
and
quail
Quail is a collective name for several genera of mid-sized birds generally placed in the order Galliformes. The collective noun for a group of quail is a flock, covey, or bevy.
Old World quail are placed in the family Phasianidae, and New ...
, there is only a single CYP21 gene, which locus is located between complement component C4 and TNX gene, along with
CENPA.
The ''CYP21A2'' gene travels in tandem with a pseudogene, ''CYP21P1'', and the high degree of sequence similarity between them indicates that these two genes are evolving in tandem through intergenic exchange of DNA.
The ''CYP21A2'' gene is located within the
RCCX cluster (an abbreviation composed of the names of the genes ''RP'' (a former name for ''
STK19'' serine/threonine kinase 19),
''
C4'', ''CYP21'' and ''
TNX
is a Japanese holding company for various entertainment companies. Its subsidiaries include the talent agency Up-Front Promotion and Up-Front Works, a music production and sales company that manages such record labels as Zetima, Piccolo Town, ...
''),
which is the most complex gene cluster in the human genome.
It is part of the major histocompatibility complex class III (MHC class III),
which is the most gene-dense region of the human genome, containing many genes that yet have unknown function or structure.
Due to the high degree of homology between the ''CYP21A2'' gene and the ''CYP21P1'' pseudogene and the complexity of the locus, it is difficult to study the ''CYP21A2'' gene at the molecular level.
Clinical significance
Congenital adrenal hyperplasia
Genetic variants in the ''CYP21A2'' gene cause a disturbance in the development of the enzyme, leading to congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency.
Gene conversion
Gene conversion is the process by which one DNA sequence replaces a homologous sequence such that the sequences become identical after the conversion event. Gene conversion can be either allelic, meaning that one allele of the same gene replaces a ...
events involving the functional gene and the pseudogene account for many cases of steroid 21-hydroxylase deficiency.
CAH is an autosomal recessive disorder. There are multiple forms of CAH, broken down into classical and nonclassical forms based on the amount of enzyme function retained.
The classical forms occur in approximately 1 in to 1 in births globally,
and include the salt-wasting and simple-virilizing forms. Complete loss of enzymatic activity causes the salt-wasting form. Variations in the structure of 21-hydroxylase are related to the clinical severity of congenital adrenal hyperplasia. Cortisol and aldosterone deficits are associated with life-threatening salt-loss (hence salt-wasting), as the steroids play roles in regulating
sodium
Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...
homeostasis
In biology, homeostasis (British also homoeostasis) (/hɒmɪə(ʊ)ˈsteɪsɪs/) is the state of steady internal, physical, and chemical conditions maintained by living systems. This is the condition of optimal functioning for the organism and ...
. Simple-virilizing CAH patients (~1-2% enzyme function)
maintain adequate sodium homeostasis, but exhibit other symptoms shared by the salt-wasting form, including accelerated growth in childhood and ambiguous
genitalia
A sex organ (or reproductive organ) is any part of an animal or plant that is involved in sexual reproduction. The reproductive organs together constitute the reproductive system. In animals, the testis in the male, and the ovary in the female, a ...
in female neonates.
The nonclassical form is the mildest one, retaining about 20% to 50% of enzyme function.
This form is associated with mild and clinically silent cortisol impairment,
but an excess of androgens post-puberty.
Non-classic congenital adrenal hyperplasia
Non-classical congenital adrenal hyperplasia caused by 21-hydroxylase deficiency (NCCAH) is a milder and late-onset congenital adrenal hyperplasia. Its prevalence rate in different ethnic groups varies from 1 in to 1 in .
Some people affected by the condition have no relevant signs and symptoms, while others experience symptoms of
hyperandrogenism
Hyperandrogenism is a medical condition characterized by high levels of androgens. It is more common in women than men. Symptoms of hyperandrogenism may include acne, seborrhea (inflamed skin), hair loss on the scalp, increased body or facia ...
.
Women with NCCAH usually have normal female genitalia at birth. In later life, the signs and symptoms of the condition may include
acne
Acne, also known as ''acne vulgaris'', is a long-term skin condition that occurs when dead skin cells and oil from the skin clog hair follicles. Typical features of the condition include blackheads or whiteheads, pimples, oily skin, and ...
,
hirsutism
Hirsutism is excessive body hair on parts of the body where hair is normally absent or minimal. The word is from early 17th century: from Latin ''hirsutus'' meaning "hairy". It usually refers to a "male" pattern of hair growth in a female that ...
, male-pattern baldness, irregular menstruation, and infertility.
Fewer studies have been published about males with NCCAH comparing to those about females, because males are generally asymptomatic.
Males, however, may present with acne
and early balding.
While symptoms are usually diagnosed after puberty, children may present with premature
adrenarche
Adrenarche is an early stage in sexual maturation that happens in some higher primates and in humans, typically peaks at around 20 years of age, and is involved in the development of pubic hair, body odor, skin oiliness, axillary hair, sexual ...
.
See also
*
Steroidogenic enzyme __NOTOC__
Steroidogenic enzymes are enzymes that are involved in steroidogenesis and steroid biosynthesis. They are responsible for the biosynthesis of the steroid hormones, including sex steroids (androgens, estrogens, and progestogens) and cortic ...
*
Cytochrome P450 oxidoreductase deficiency
Cytochrome P450 oxidoreductase deficiency (PORD) is a rare disease and inborn error of metabolism caused by deficiency of cytochrome P450 oxidoreductase (POR). POR is a 2- flavin protein that is responsible for the transfer of electrons from NADP ...
References
External links
GeneReviews/NCBI/NIH/UW entry on 21-Hydroxylase-Deficient Congenital Adrenal Hyperplasia OMIM entry on 21-Hydroxylase-Deficient Congenital Adrenal HyperplasiaSynthesis of Desoxycorticosterone from Progesterone through 21-Hydroxylase (Image)
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{{Enzymes
Enzymes
EC 1.14.99
21
Metabolism
Human proteins