21-Hydroxylase

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Identifiers
EC number	1.14.99.10
CAS number	9029-68-9
Alt. names
IntEnz	IntEnz view
BRENDA	BRENDA entry
ExPASy	NiceZyme view
KEGG	KEGG entry
MetaCyc	metabolic pathway

21-Hydroxylase

Steroid 21-hydroxylase is an essential cytochrome P450 enzyme intricately involved in the biosynthesis of critical steroid hormones, namely aldosterone and cortisol. In humans, the genetic information for the synthesis of this enzyme is provided by the CYP21A2 gene.

Names and Classification

• Common Names: Steroid 21-monooxygenase, 21α-Hydroxylase.
• Alternate Name: 21β-Hydroxylase.

Function

Steroid 21-Hydroxylase belongs to the larger family of cytochrome P450 enzymes, a superfamily renowned for its role as monooxygenases. These enzymes participate in a variety of metabolic processes, including drug metabolism and the synthesis of vital molecules like cholesterol, steroids, and other lipids. Specifically, 21-Hydroxylase is found in the endoplasmic reticulum, where it hydroxylates steroids at the 21st carbon position. This enzymatic action is pivotal for the biosynthesis of steroid hormones, including cortisol and aldosterone.

Reaction Mechanism

21-Hydroxylase functions as a catalyst, facilitating the hydroxylation of the 21st carbon atom in steroid molecules, which translates to the addition of an "–OH" group. This modification is fundamental for the appropriate synthesis and function of certain hormones.

• 

  Progesterone

• 

  11-deoxycorticosterone

• 

  Steroid numbering - #21 is highlighted near center top.

Biosynthetic Pathway

The transformation catalyzed by 21-Hydroxylase is integral to several metabolic pathways, particularly those associated with steroid hormone synthesis.

Human steroidogenesis, illustrating the two key reactions facilitated by 21-Hydroxylase, located centrally at the top.

A depiction of corticosteroid biosynthesis in the rat, showcasing the vital role of 21-Hydroxylase.

Clinical Relevance

Deficiencies or anomalies in the CYP21A2 gene can lead to compromised 21-Hydroxylase synthesis, which in turn results in congenital adrenal hyperplasia due to 21-hydroxylase deficiency. This condition can have significant clinical repercussions. Additionally, a pseudogene closely related to the CYP21A2 gene exists. gene conversion events between this pseudogene and the functional gene are considered the root cause for many instances of 21-hydroxylase deficiency.

External Resources

• GeneReviews/NCBI/NIH/UW article on 21-Hydroxylase-Deficient Congenital Adrenal Hyperplasia
• OMIM database entry on 21-Hydroxylase-Deficient Congenital Adrenal Hyperplasia
• Graphical Representation of Desoxycorticosterone Synthesis from Progesterone via 21-Hydroxylase
• Steroid+21-Hydroxylase at the US National Library of Medicine Medical Subject Headings (MeSH)

21-Hydroxylase Resources
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This concise article on 21-Hydroxylase incorporates public domain text from the US National Library of Medicine.