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Proteins/UNIPROT:P18405
Catalyzes the irreversible stereospecific reduction of the delta 4,5 bond (double bond between carbons 4 and 5) of various 3-oxo steroids (3-keto steroids) producing their 5alpha dihydro-3-oxo forms. Converts testosterone into 5-alpha-dihydrotestosterone (DHT), a more potent androgen as it is the preferred ligand for androgen receptor (AR) transactivation, making this reaction a key step in male sexual differentiation during development. Besides testosterone, it can also act on other steroids, including progesterone, androstenedione, and corticoids, producing metabolites with diverse roles. Hence, it plays a central role in sexual differentiation and androgen physiology..
Function from UniProtKB (P18405), read 2026-09-23. CC BY 4.0.
Curated compounds routed: 1 · substrate of
1 compound
1 compound.
0 reported routes1 assembled route
71 statements
71 statements · 6 reference databases · Subject: SRD5A1 · Sources: Bgee, Hetionet, Human-GEM, Reactome, Rhea, WikiPathways
8 reactions
proton+NADPH(4-)+testosterone17beta-hydroxy-5alpha-androstan-3-one+NADP(3-)
Catalysed by SRD5A3 or SRD5A1 or SRD5A2
Reaction pageLeft to rightBalancedEC 1.3.1.22cytosol
Human-GEM MAR01976CC BY 4.0Reference data
proton+NADPH(4-)+testosterone17beta-hydroxy-5alpha-androstan-3-one
3 names
Resolves to: SRD5A1
Reaction pageLeft to rightBalancedEC 1.3.1.22endoplasmic reticulum
Human-GEM MAR01977CC BY 4.0Reference data
5alpha-pregnane-3,20-dione+NADP(3-)proton+NADPH(4-)+progesterone
Catalysed by SRD5A3 or SRD5A1 or SRD5A2
Reaction pageReversibleBalancedEC 1.3.1.22cytosol
Human-GEM MAR02002CC BY 4.0Reference data
androst-4-ene-3,17-dione+proton+NADPH(4-)5alpha-androstan-3,17-dione+NADP(3-)
Catalysed by SRD5A3 or SRD5A1 or SRD5A2
Reaction pageReversibleBalancedEC 1.3.1.22endoplasmic reticulum
Human-GEM MAR02015CC BY 4.0Reference data
5alpha-pregnane-3,20-dione+NADP(3-)progesterone+NADPH(4-)+hydron
Reaction pageRight to leftBalanced
Rhea RHEA:21952CC BY 4.0Reference data
androst-4-ene-3,17-dione+NADPH(4-)+hydron5alpha-androstan-3,17-dione+NADP(3-)
Reaction pageReversibleBalanced
Rhea RHEA:50816CC BY 4.0Reference data
17beta-hydroxy-5alpha-androstan-3-one+NADP(3-)testosterone+NADPH(4-)+hydron
Catalysed by SRD5A1 or SRD5A2 or SRD5A3
Reaction pageRight to leftBalanced
Rhea RHEA:50820CC BY 4.0Reference data
CHEBI:13601+NADP(3-)CHEBI:47909+NADPH(4-)+hydron
Catalysed by SRD5A1 or SRD5A2 or SRD5A3
Reaction pageRight to leftBalance undeterminable
Rhea RHEA:54384CC BY 4.0Reference data