Cholesterol, Steroid Hormones, and Calcitriol: Structure, Synthesis, and Regulation

Cholesterol, Steroid Hormones & Calcitriol

Overview

This section covers the structure and biosynthesis of cholesterol, the steroid nucleus, regulation of cholesterol synthesis, steroid hormone families, and the activation of vitamin D3 (calcitriol). Understanding these pathways is essential for grasping lipid metabolism and hormone regulation in biochemistry.

• Recognize and draw the steroid ring structure

• Describe cholesterol structure and synthesis

• Explain regulation of HMG-CoA reductase

• Understand steroid hormone synthesis

• Explain vitamin D3 to calcitriol activation

• Apply concepts to clinical and physiological contexts

The Steroid Nucleus

17-Carbon Steroid Ring System

The steroid nucleus is a core structure found in all steroids, consisting of four fused rings labeled A, B, C, and D. This planar, rigid structure defines the steroid family.

• 4 fused rings: Three six-membered rings (A, B, C) and one five-membered ring (D)

• 17 carbons: Numbered beginning at the A-ring

• All steroids share this nucleus

Cholesterol Structure

Key Features

• Steroid: Contains the four fused rings

• Sterol: Steroid with a hydroxyl group (–OH) at C3

• Amphipathic: Polar –OH at C3, nonpolar hydrocarbon tail and ring system

• Numbering starts at the A-ring, important for understanding modifications (e.g., hormones, bile acids)

Major Sites of Cholesterol Synthesis

Tissue and Cellular Localization

• High-capacity tissues: Liver (primary), intestine, adrenal cortex, gonads, skin

• Intracellular location: Cytosol and smooth endoplasmic reticulum (SER)

Overview of Cholesterol Synthesis

Pathway and Key Intermediates

Cholesterol synthesis is a multi-step process starting from acetyl-CoA and proceeding through several key intermediates:

1. Acetyl-CoA

2. HMG-CoA

3. Mevalonate

4. Isopentenyl pyrophosphate (IPP, isoprene donor)

5. Dimethylallyl pyrophosphate (DMAPP, isoprene donor)

6. Geranyl pyrophosphate (C10)

7. Farnesyl pyrophosphate (C15)

8. Squalene (C30)

9. Lanosterol

10. Cholesterol (C27)

HMG-CoA Reductase

Role and Regulation

• Catalyzes: HMG-CoA → Mevalonate

• Uses: 2 NADPH as reducing agents

• Significance: This is the rate-limiting, regulated step in cholesterol synthesis

$\text{HMG-CoA} + 2\ NADPH + 2\ H^+ \rightarrow \text{Mevalonate} + 2\ NADP^+ + \text{CoA}$

Mevalonate to Isoprene Donors

Formation of IPP and DMAPP

• Mevalonate is phosphorylated and decarboxylated to form:

• IPP: Isopentenyl pyrophosphate

• DMAPP: Dimethylallyl pyrophosphate

Isoprene to Squalene

Condensation Reactions

• IPP + DMAPP → Geranyl PP (C10)

• Geranyl PP + IPP → Farnesyl PP (C15)

• 2 Farnesyl PP → Squalene (C30)

• Enzymes: Prenyl transferases and squalene synthase

Cyclization of Squalene

Formation of Lanosterol

• Squalene monooxygenase: Adds O2 and NADPH, converts squalene to squalene epoxide

• Squalene epoxide is cyclized to lanosterol

• Monooxygenases: Insert one atom of O2 into substrate, reduce the other to H2O

Lanosterol to Cholesterol

Multistep Pathway

• 19 reactions convert lanosterol to cholesterol

• Key intermediate: 7-dehydrocholesterol (precursor to vitamin D)

• Final product: Cholesterol (C27)

Regulation of HMG-CoA Reductase

Mechanisms of Regulation

• Gene expression: SREBP/SRE pathway

• Degradation: Proteasomal degradation of the enzyme

• Phosphorylation state: Regulated by kinases and phosphatases

Effects of High Intracellular Cholesterol

• Decreased synthesis of HMG-CoA reductase

• Increased degradation of HMG-CoA reductase

• Decreased SREBP activation → reduced gene transcription

AMP, Insulin, and Glucagon Effects

• High AMP: Activates AMPK → phosphorylation → inactive HMG-CoA reductase

• Insulin: Stimulates phosphatase → dephosphorylation → active enzyme

• Glucagon: Activates PKA → phosphorylation → inactive enzyme

Steroid Hormone Overview

Families and Synthesis

All steroid hormones are derived from cholesterol. Major families include:

• Progestagens (C21)

• Corticosteroids (C21): Glucocorticoids and mineralocorticoids

• Androgens (C19)

• Estrogens (C18)

Cholesterol Side-Chain Cleavage

Formation of Pregnenolone

• Enzyme: Side Chain Cleavage Enzyme (CYP11A1)

• Reaction: Cholesterol (C27) → Pregnenolone (C21)

• Occurs in mitochondria of adrenal cortex and gonads

• Requires NADPH and O2

$\text{Cholesterol} + 3\ O_2 + 3\ NADPH \rightarrow \text{Pregnenolone} + 3\ NADP^+ + 4\ H_2O + \text{Isocaproic acid}$

Families of Steroid Hormones

Family	Carbons	Example	Where Made
Progestagens	C21	Progesterone	Ovary, placenta
Glucocorticoids	C21	Cortisol	Zona fasciculata
Mineralocorticoids	C21	Aldosterone	Zona glomerulosa
Androgens	C19	Testosterone	Leydig cells
Estrogens	C18	Estradiol	Ovary, adipose

ACTH and Cortisol

Regulation of Cortisol Synthesis

• ACTH (adrenocorticotropic hormone): Major stimulant of cortisol synthesis, released from anterior pituitary

• Mechanism: ACTH binds GPCR → increases cAMP → activates PKA

• Stimulates cholesterol transport into mitochondria and increases steroidogenesis enzymes

Mechanism of Steroid Hormone Action

Intracellular Receptors and Gene Regulation

• Steroid hormones cross the cell membrane

• Bind to intracellular receptors

• Receptor-hormone complex binds DNA and alters transcription

• Effects are slow in onset but long in duration

Vitamin D3 (Cholecalciferol) and Calcitriol

Synthesis and Activation

• 7-dehydrocholesterol (skin): UV light converts to cholecalciferol (vitamin D3)

• Liver: Cholecalciferol → 25-hydroxyvitamin D (25-OH-D) (measured for vitamin D status)

• Kidney: 25-OH-D → 1,25-dihydroxyvitamin D (calcitriol, active hormone) via 1α-hydroxylase

$\text{Cholecalciferol} \xrightarrow{\text{liver}} 25\text{-OH-D} \xrightarrow{\text{kidney}} 1,25\text{-diOH-D (Calcitriol)}$

• Calcitriol is the active hormone, essential for calcium homeostasis

Example: Clinical Application

• Statins inhibit HMG-CoA reductase, lowering cholesterol synthesis

• Vitamin D deficiency can lead to rickets or osteomalacia due to impaired calcitriol synthesis