CBB 35: Fat Biosynthesis
• Explain the general process used to generate acetyl-CoA for Fatty acid synthesis
• Explain the process of the citrate shuttle and how it is regulated
• Describe the process of malonyl-CoA synthesis and any coenzymes needed
• Explain the mechanisms of regulation of acetyl-CoA Carboxylase
• List the steps of the fatty acid synthesis spiral and list any coenzymes needed
• Describe briefly the processes of fatty acid elongation and desaturation
• Explain why essential fatty acids must be obtained from the diet and when
arachidonic acid becomes an essential fatty acid
• Describe the synthesis of phosphatidic acid, triglycerides and phospholipids
• Explain the basic functions of prostaglandins, thromboxane, and leukotrienes and
the enzymes used to form these from arachidonic acid
• Explain the function of leptin and how it relates to the amount of adipose tissue
General Process to Generate Acetyl-CoA for Fatty Acid Synthesis
• Source: Excess carbohydrates in the fed state (high insulin).
• Pathway:
1. Glycolysis → Glucose → Pyruvate (cytosol).
2. Pyruvate enters mitochondria.
3. Pyruvate Dehydrogenase (PDH) converts pyruvate → Acetyl-CoA.
• Problem: Acetyl-CoA is made in mitochondria but FA synthesis occurs in cytosol.
• Solution: Export via Citrate Shuttle (see below).
Citrate Shuttle & Regulation
Purpose: Move acetyl units from mitochondria → cytosol.
Steps:
1. In mitochondria:
o Pyruvate → OAA (Pyruvate carboxylase).
o Acetyl-CoA + OAA → Citrate (Citrate synthase).2. Citrate transporter moves citrate to cytosol.
3. ATP-citrate lyase (uses ATP) → Citrate → Acetyl-CoA + OAA.
4. OAA → Malate → Pyruvate (Malic enzyme, generates NADPH for FA synthesis).
Regulation:
• Substrate-level:
• Hormonal:
o High mitochondrial Acetyl-CoA → inhibits PDH, shunts pyruvate to OAA.
o High energy (ATP) → inhibits isocitrate dehydrogenase → citrate accumulates.
o Insulin ↑: activates PDH, citrate lyase, malic enzyme → ↑ shuttle activity.
o Glucagon/Epi ↓: reduces activity.
Malonyl-CoA Synthesis & Coenzymes
• Enzyme: Acetyl-CoA Carboxylase (ACC).
• Reaction: Acetyl-CoA + CO₂ + ATP → Malonyl-CoA.
• Coenzyme: Biotin (CO₂ carrier).
• Importance:
o Rate-limiting step of FA synthesis.
o Commits acetyl-CoA to FA synthesis.
o Malonyl-CoA inhibits CPT I → prevents simultaneous β-oxidation.
Regulation of Acetyl-CoA Carboxylase (ACC)
Allosteric:
• Activator: Citrate (signals abundant substrate).
• Inhibitor: Palmitate (end-product feedback).
Covalent (phosphorylation):
• Active: Dephosphorylated (via insulin → protein phosphatase).
• Inactive: Phosphorylated (via glucagon/epi → PKA; AMPK when low energy).
Integration:
• Fed state (high insulin): ACC active → ↑ FA synthesis.• Fasting/exercise (high glucagon/epi, low ATP): ACC inactive → ↓ FA synthesis.
Fatty Acid Synthesis Spiral & Coenzymes
Enzyme complex: Fatty Acid Synthase (FAS) — large, multifunctional, dimeric enzyme.
Key domains:
• ACP (acyl carrier protein) — contains pantothenic acid (Vit B5), sulfhydryl group.
• Cysteinyl-SH site — holds growing chain.
Cycle (adds 2 carbons per turn):
1. Loading:
o Acetyl-CoA → ACP → Cys-SH.
o Malonyl-CoA → ACP-SH.
2. Condensation: Acetyl + Malonyl → β-ketoacyl-ACP + CO₂ (Ketoacyl-ACP synthase).
3. Reduction: β-keto → β-hydroxyacyl (NADPH).
4. Dehydration: β-hydroxyacyl → enoyl.
5. Reduction: Enoyl → saturated acyl (NADPH).
6. Translocation: Acyl moved to Cys-SH; ACP accepts new malonyl-CoA.
Repeat: 7 cycles → Palmitate (16:0).
Coenzymes:
• NADPH (from PPP & malic enzyme).
• Biotin (for ACC step).
• Pantothenic acid (ACP).
Elongation & Desaturation
• Elongation:
o Location: Smooth ER.
o Adds 2C units (malonyl-CoA) to palmitate → stearate, etc.
• Desaturation:
o Enzyme: Fatty acyl-CoA desaturase.
o Requires O₂, NADH, cytochrome b₅.o Humans can only introduce double bonds up to C9 (Δ9 desaturase).
Essential Fatty Acids & Arachidonic Acid
• Essential FAs: Linoleic acid (ω-6), α-linolenic acid (ω-3).
• Reason: Humans lack desaturases beyond C9 → cannot synthesize ω-3, ω-6.
• Arachidonic acid:
o Normally made from linoleic acid.
o Becomes essential if dietary linoleic acid is deficient.
Synthesis of Phosphatidic Acid, Triglycerides, Phospholipids
Phosphatidic Acid:
• Glycerol-3-phosphate + 2 fatty acyl-CoA.
• G3P sources:
o Liver: glycerol kinase (glycerol → G3P).
o Liver/adipose: DHAP → G3P (glycerol-3-P dehydrogenase).
Triglycerides:
• Phosphatidic acid → dephosphorylated to DAG → + fatty acyl-CoA → TAG.
• Stored in adipose or exported from liver via VLDL.
Phospholipids:
• From phosphatidic acid via DAG intermediate.
• Two activation strategies (CTP-dependent):
1. Activate head group (CDP-head group) → add to DAG.
2. Activate DAG (CDP-DAG) → add head group.
Eicosanoids: Prostaglandins, Thromboxane, Leukotrienes
Source: Arachidonic acid (from membrane phospholipids via phospholipase A₂/C).
Pathways:
• Cyclooxygenase (COX):o AA → PGG₂ → PGH₂ →
▪ Prostaglandins (PGs): vasodilation, pain, fever, inflammation.
▪ Thromboxane A₂ (TXA₂): platelet aggregation, vasoconstriction (TXA
synthase in platelets).
• Lipoxygenase:
o AA → Leukotrienes (inflammation, bronchoconstriction).
Clinical Pearl: Aspirin/NSAIDs inhibit COX → ↓ PGs & TXA₂.
Leptin & Adipose Tissue
• Source: Adipocytes.
• Secretion: Proportional to adipose mass.
• Action: Hypothalamus → ↓ appetite, ↑ energy expenditure.
• Pathways: Activates catabolic, inhibits anabolic.
• Clinical:
o Leptin mutation/receptor defect: rare, severe obesity, hyperphagia.
o Leptin resistance: common in obesity; satiety signaling impaired.
Board-Style Integration Table
Learning Objective Key Enzyme(s) Coenzymes Regulation Clinical Tie-In
Acetyl-CoA generation PDH, citrate
lyase —
PDH inhibited by
high Ac-CoA Fed vs fasting state
Citrate shuttle Citrate lyase,
malic enzyme
NADPH (malic
enzyme)
Insulin ↑, high
ATP ↑
PPP & malic enzyme
NADPH supply
Malonyl-CoA synthesis ACC Biotin, ATP, CO₂
Citrate ↑,
Palmitate ↓,
Insulin ↑
CPT I inhibition
ACC regulation ACC —
Allosteric +
covalent AMPK in exercise
FA synthesis spiral FAS NADPH, Vit B5
Substrate
availability
Palmitate end
productLearning Objective Key Enzyme(s) Coenzymes Regulation Clinical Tie-In
Elongation/desaturation Elongase,
desaturase
NADH,
cytochrome b₅
— ω-3/ω-6 essentiality
Essential FAs — — —
Arachidonic acid
essential in
deficiency
Phosphatid