Which enzyme catalyzes the conversion of pregnenolone to 17-hydroxypregnenolone in the adrenal cortex and gonads?

Explanation

## Steroid Hormone Synthesis Pathway **Key Point:** 17α-hydroxylase (CYP17A1) catalyzes the hydroxylation of pregnenolone at the 17-position to form 17-hydroxypregnenolone, an essential step in both cortisol and sex hormone synthesis. ### Enzymatic Steps in Steroidogenesis ```mermaid flowchart TD A[Cholesterol]:::outcome --> B[Pregnenolone]:::outcome B --> C[17-Hydroxypregnenolone]:::outcome C --> D[DHEA]:::outcome D --> E[Androstenediol]:::outcome E --> F[Testosterone]:::outcome F --> G[Estradiol]:::outcome B --> H[Progesterone]:::outcome H --> I[17-Hydroxyprogesterone]:::outcome I --> J[11-Deoxycortisol]:::outcome J --> K[Cortisol]:::outcome C -->|17,20-lyase activity| D style C fill:#fff4e6 style A fill:#e8f4f8 style K fill:#e8f4f8 style F fill:#e8f4f8 style G fill:#e8f4f8 ``` ### Enzyme Functions and Specificity | Enzyme | Substrate | Product | Tissue Location | Clinical Significance | |--------|-----------|---------|-----------------|----------------------| | **17α-Hydroxylase (CYP17A1)** | Pregnenolone or Progesterone | 17-OH-Pregnenolone or 17-OH-Progesterone | Adrenal, Gonads | Deficiency → hypertension, hypokalemia, ↓ sex hormones | | 3β-HSD | Pregnenolone or DHEA | Progesterone or Androstenedione | Adrenal, Gonads | Deficiency → salt wasting, ↑ DHEA | | 17,20-Lyase (CYP17A1) | 17-OH-Pregnenolone | DHEA | Adrenal, Gonads | Same enzyme as 17α-hydroxylase; catalyzes C17,20 bond cleavage | | Aromatase (CYP19A1) | Testosterone or Androstenedione | Estradiol or Estrone | Ovary, Adipose, Brain | Converts androgens to estrogens | **High-Yield:** CYP17A1 has **dual enzymatic activity**: both 17α-hydroxylase and 17,20-lyase functions. The 17α-hydroxylase activity converts pregnenolone → 17-OH-pregnenolone; the 17,20-lyase activity then cleaves the C17,20 bond to form DHEA. **Mnemonic:** **"17 Hydroxylase Happens First"** — 17α-hydroxylase acts before 17,20-lyase in the sequential pathway. The hydroxyl group is added before the side chain is cleaved. **Clinical Pearl:** 17α-hydroxylase deficiency (rare) presents with hypertension, hypokalemia, and absent secondary sexual characteristics because cortisol and sex hormone synthesis are blocked, but mineralocorticoid (aldosterone) synthesis is unopposed.