Biosynthesis of Cholesterol

Overview: Five Major Stages

Stage 1: Acetyl-CoA → Mevalonate

• Two molecules of acetyl-CoA condense via cytosolic thiolase to form acetoacetyl-CoA (4C)
• A third acetyl-CoA is added by HMG-CoA synthase (cytosolic isoform) to produce 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) (6C)

Fig. 18.3 - Lippincott's Biochemistry: Synthesis of HMG-CoA

Note: There are two HMG-CoA synthase isoenzymes - the cytosolic one feeds cholesterol synthesis; the mitochondrial one feeds ketogenesis. The two pathways are distinct compartmentally.

• HMG-CoA is reduced to mevalonate (6C) by HMG-CoA reductase (HMGCR)
• Reaction: HMG-CoA + 2 NADPH → Mevalonate + CoA
• This is irreversible, uses 2 NADPH, and is the rate-limiting, regulated step of the entire pathway

Harper's Biochemistry Fig. 26-1: HMG-CoA reductase is the key regulatory enzyme

Stage 2: Mevalonate → Isopentenyl Pyrophosphate (IPP)

• Mevalonate undergoes two sequential phosphorylations by ATP kinases → 5-pyrophosphomevalonate
• Decarboxylation (requires ATP) produces isopentenyl pyrophosphate (IPP), the fundamental 5-carbon isoprene unit
• IPP is isomerized to dimethylallyl pyrophosphate (DMAPP / DPP) by IPP isomerase

IPP is the precursor of the entire isoprenoid family - cholesterol is a sterol isoprenoid; non-sterol isoprenoids produced here include dolichol, ubiquinone (CoQ10), and prenyl groups on proteins like Ras.

Stage 3: IPP → Squalene (30C)

1. IPP + DMAPP → geranyl pyrophosphate (GPP, 10C)
2. GPP + IPP → farnesyl pyrophosphate (FPP, 15C)
3. Two FPP molecules combine tail-to-tail (NADPH required) → squalene (30C)

FPP is also used in protein prenylation (farnesylation of Ras oncoproteins) - a key reason statins may have anti-tumor effects.

Stage 4: Squalene → Lanosterol (first sterol ring)

• Squalene epoxidase (SER) uses O₂ and NADPH to form squalene-2,3-epoxide
• The epoxide undergoes cyclization catalyzed by oxidosqualene cyclase (lanosterol cyclase), forming the tetracyclic steroid ring structure: lanosterol (30C)

Stage 5: Lanosterol → Cholesterol

• Three methyl groups are removed (losing 3C as CO₂)
• The side chain double bond is reduced
• The ring double bond migrates from C-8 to C-5
• Final step: 7-dehydrocholesterol reductase (DHCR7) reduces the Δ7 double bond to yield cholesterol

Smith-Lemli-Opitz syndrome is caused by a partial deficiency of DHCR7, leading to accumulation of 7-dehydrocholesterol, low cholesterol levels, and multiple developmental abnormalities.

Regulation of Cholesterol Synthesis

Lippincott's Fig. 18.7 - Regulation of HMG-CoA reductase

1. Transcriptional Regulation (SREBP-2 Pathway)

• When intracellular cholesterol is low, SCAP (SREBP cleavage-activating protein) escorts SREBP-2 from the ER to the Golgi
• Proteolytic cleavage releases the active transcription factor, which moves to the nucleus and binds SRE (sterol regulatory element)
• This upregulates transcription of HMG-CoA reductase and the LDL receptor
• When cholesterol is high, INSIG proteins retain the SCAP-SREBP complex in the ER - SREBP-2 is not released, transcription stops

2. Accelerated Enzyme Degradation

• When ER sterol levels are high, the reductase binds INSIG proteins, is ubiquitinated, and undergoes proteasomal degradation

3. Phosphorylation/Dephosphorylation

• AMP-activated protein kinase (AMPK) phosphorylates and inactivates HMG-CoA reductase
• Phosphoprotein phosphatase dephosphorylates and activates it
• When AMP (low energy) is high, AMPK is activated → reductase is inactivated → cholesterol synthesis decreases (similar to fatty acid synthesis control)

4. Hormonal Regulation

• Insulin → promotes dephosphorylation → activates reductase (fed state, anabolism)
• Glucagon / Epinephrine → promotes phosphorylation → inactivates reductase (fasted/stress state)

Clinical Relevance: Statins

Summary Pathway

Acetyl-CoA (x3)
    ↓  [Thiolase + HMG-CoA synthase]
HMG-CoA (6C)
    ↓  [HMG-CoA reductase] ← RATE-LIMITING STEP (statin target)
Mevalonate (6C)
    ↓  [Kinases + decarboxylase, 3 ATP]
IPP / DMAPP (5C isoprene units)
    ↓  [Prenyltransferases]
Farnesyl-PP (15C) × 2
    ↓  [Squalene synthase, NADPH]
Squalene (30C)
    ↓  [Squalene epoxidase + Lanosterol cyclase, O₂ + NADPH]
Lanosterol (30C)
    ↓  [~19 SER reactions, NADPH]
Cholesterol (27C)

• Biochemistry, 8th ed. Lippincott Illustrated Reviews, Chapter 18 (Cholesterol Synthesis, pp. 617-627)
• Harper's Illustrated Biochemistry, 32nd Ed., Chapter 26 (Cholesterol Biosynthesis)

The Human Eye - Anatomy and Function

Overview

External View

Gross anatomy of the human eye - Neuroscience: Exploring the Brain, 5th Ed.

Cross-Sectional Anatomy (Internal Structures)

Cross-sectional anatomy of the eye showing the light path to the fovea - Neuroscience: Exploring the Brain, 5th Ed.

Schematic of eye anatomy - Ganong's Review of Medical Physiology, 26th Ed.

The Three Coats (Tunics) of the Eyeball

1. Fibrous Coat (Outer Layer)

2. Vascular Coat - Uvea (Middle Layer)

3. Neural Coat (Inner Layer)

Chambers and Fluids

Anterior Cavity

• Anterior chamber - between the cornea and iris; filled with aqueous humor
• Posterior chamber - narrow space between the iris, zonule fibers, and lens; also contains aqueous humor

Blockage of aqueous drainage → raised IOP → glaucoma (open-angle: reduced trabecular permeability; closed-angle: iris balloons forward to block the angle)

Posterior Cavity (Vitreous Chamber)

• Between the lens and retina
• Filled with vitreous humor - a clear, jellylike gel that keeps the eye spherical and maintains retinal apposition

The Lens and Accommodation

• Far vision (relaxed): Ciliary muscle relaxed → zonule fibers taut → lens is flat → less refractive power
• Near vision (accommodation): Ciliary muscle contracts → zonule fibers relax → lens bulges (becomes more convex) → greater refractive power

The Retina

Ophthalmoscopic view of the retina - Neuroscience: Exploring the Brain, 5th Ed.

Photoreceptors

The Light Path - From Entry to Perception

Light
  → Cornea (major refraction)
  → Aqueous humor (anterior chamber)
  → Pupil (regulates light quantity)
  → Lens (fine-tune focus / accommodation)
  → Vitreous humor
  → Retina (photoreceptors: rods and cones)
  → Bipolar cells
  → Ganglion cells → Optic nerve (CN II)
  → Optic chiasm
  → Lateral geniculate nucleus (thalamus)
  → Primary visual cortex (occipital lobe, V1)

Protection of the Eye

• Bony orbit - walls of the skull protect the eyeball from mechanical trauma
• Eyelids and blinking - physical protection; spread tears over the cornea
• Conjunctiva - clear mucous membrane lining the inner eyelids and covering the sclera; continuous with the corneal margin
• Lacrimal gland - in the upper outer orbit; produces tears that flow across the eye surface and drain into the nasolacrimal duct → nasal cavity. Tears keep the cornea moist, clear, and protected from infection

Clinical Correlations

• Ganong's Review of Medical Physiology, 26th Ed., Chapter 10 (Vision)
• Neuroscience: Exploring the Brain, 5th Ed., Chapter 9 (The Eye)