Lipid Metabolism and Cholesterol Biosynthesis

Lipid Metabolism

Fate of Newly Synthesized Fatty Acids

The partitioning of fatty acids between triacylglycerol (TAG) and phospholipids depends on physiological needs, such as:

  • Active growth

  • Food supply

Acetyl-CoA, derived from glucose, amino acids (AA), and diet, is a central precursor. It enters the mitochondria, where it can be converted to fatty acids (FA). These fatty acids are then used for the synthesis of TAG and phospholipids.

Triacylglycerol & Phospholipid Synthesis

TAG and phospholipids are synthesized via a common pathway that utilizes glycerol-3-phosphate. Most glycerol-3-phosphate is derived from dihydroxyacetone phosphate (DHAP), an intermediate of glycolysis. A minor pathway in the liver and kidney can also produce glycerol-3-phosphate directly from glycerol.

Steps in TAG and Phospholipid Synthesis

  1. Activation of Fatty Acids: Fatty acids (R-COO-) are activated by esterification to Co-A, forming acyl-CoA, a reaction catalyzed by acyl-CoA synthetase.

  2. Acyl Chain Transfer: Acyl chains (R1, R2) are transferred to the -OH groups of glycerol-3-phosphate, catalyzed by acyltransferases. This process involves GPAT (Glycerol-3-Phosphate AcylTransferase).

  3. Phosphatidic Acid Formation: Esterification of two acyl chains onto glycerol-3-phosphate produces phosphatidic acid, a precursor that can be used to synthesize either triacylglycerol or phospholipids.

  4. Branchpoint: Headgroup attachment generates phospholipids, OR phosphatidic acid phosphatase (PAP) removes the phosphate, generating diacylglycerol.

  5. Final Acylation: A third acyl chain is attached to diacylglycerol to generate triacylglycerol. These reactions occur on the cytosolic face of the endoplasmic reticulum and mitochondria.

TAG Storage in Lipid Droplets

TAG is stored in lipid droplets, which are present in all cells and form at the endoplasmic reticulum, budding off into the cytoplasm. The surface of these droplets is covered by proteins, including:

  • Perilipins: Forming a 'shell' around the droplet.

  • Lipase Enzymes: Involved in TAG breakdown.

  • Lipase Regulatory Proteins

  • Enzymes for TAG Synthesis: ACS, GPAT, AGPAT, DGAT.

Location of TAG Synthesis

  • Newborn Mammals and Hibernating Animals: Thermogenin forms channels in the mitochondrial membrane, preventing a proton gradient. Fatty acid oxidation doesn't produce ATP but generates heat. TAG breakdown leads to FA oxidation and heat production.

  • Lactating Mammary Gland: Milk production.

  • Liver: Lipoprotein synthesis.

  • Adipose Tissue: Storage.

  • Brown Adipose Tissue: Heat generation.

Regulation of TAG Synthesis by Hormones

  • Insulin: Stimulates TAG synthesis.

  • Glucagon and Adrenaline: Inhibit TAG synthesis.

TAG synthesis is a storage reaction that occurs at a maximum rate when the organism has a plentiful supply of nutrients.

Molecular Mechanisms (Not Well Understood)

  • Enzymes have multiple isoforms and are highly hydrophobic.

  • Several potential regulatory points: GPAT, PAP, DGAT.

Glycerol Phosphate Acyl Transferase (GPAT)

  • GPAT has the lowest specific activity in the pathway and is inhibited by phosphorylation via PKA (glucagon, adrenaline) and AMPK (energy status).

Glucagon/adrenaline and low energy status (low ATP:AMP) activate PKA and AMPK, leading to:

  • Decreased fatty acid synthesis

  • Decreased glycogen synthesis

  • Decreased TAG synthesis

Coordinated regulation of anabolic and catabolic pathways ensures that fatty acids are channeled away from storage and toward oxidation.

Phosphatidic Acid Phosphatase (PAP)

  • PAP moves between the ER membrane and the cytosol. Its association with the ER membrane is stimulated by fatty acids, resulting in feed-forward activation by substrates.

Key Points

  • TAG and phospholipids are synthesized from glycerol-3-phosphate and fatty acids.

  • TAG synthesis is regulated by hormones.

  • Key enzymes: GPAT (first step) and PAP (branchpoint).

Regulation of Cholesterol Biosynthesis

Suggested reading: Lehninger, Chapter 21.2 (pages 760-765), focusing on triacylglycerol biosynthesis and the triacylglycerol cycle.

Cholesterol

Cholesterol is a crucial molecule in biology, recognized by numerous Nobel Prizes. It is:

  • A component of biological membranes, influencing fluidity.

  • A precursor for steroid hormones.

  • Associated with cardiovascular disease.

  • Efficiently absorbed from the diet.

  • Synthesized by all animal cells, primarily in the liver.

Cholesterol Biosynthesis Overview

The synthesis starts with Acetyl-CoA, which, through a multistep process, is converted to HMG-CoA. HMG-CoA reductase then catalyzes the conversion of HMG-CoA to mevalonate, a key regulatory step. Mevalonate is further converted to cholesterol through multiple steps.

Acetate→Acetyl-CoA→HMG-CoA→Mevalonate→Cholesterol\text{Acetate} \rightarrow \text{Acetyl-CoA} \rightarrow \text{HMG-CoA} \rightarrow \text{Mevalonate} \rightarrow \text{Cholesterol}

Regulation of HMG-CoA Reductase

HMG-CoA reductase is regulated by multiple mechanisms:

  • Covalent Modification: Inhibited by phosphorylation.

  • Transcription: Inhibited by cholesterol (SREBP-2).

  • Degradation: Stimulated by cholesterol.

  • Translation: Inhibited by cholesterol.

The rate of cholesterol synthesis responds to cellular cholesterol levels, hormones, and ATP levels.

Short-Term Regulation by Phosphorylation

Low energy status (Low ATP:AMP) and hormones like glucagon and adrenaline activate protein kinase A (PKA) and AMP-dependent protein kinase, leading to phosphorylation and inactivation of HMG-CoA reductase. Insulin activates protein phosphatase, which dephosphorylates and activates HMG-CoA reductase.

Transcriptional Control by SREBP (Sterol Regulatory Element-Binding Proteins)

  • High Cholesterol/Sterols: SREBP retained in the ER and remains inactive.

  • Low Cholesterol: SREBP is cleaved and moves to the nucleus to activate HMG-CoA reductase transcription.

Control by Proteolysis

  • Accumulation of cholesterol triggers proteolysis (protein degradation) of HMG-CoA reductase.

  • In the presence of sterols, insig triggers degradation of HMG-CoA reductase by inducing binding to ubiquitin ligase (gp78).

  • gp78 ubiquitinates HMG-CoA reductase, leading to its extraction from the membrane and degradation.

Regulation Summary

  • Short-Term Regulation (Fine Control):

    • Hormones

    • Energy levels

    • Phosphorylation

  • Longer-Term Regulation (Coarse Control):

    • Cholesterol levels

    • Transcription

    • Degradation

    • Cholesterol uptake

Summary

Cholesterol is synthesized from acetyl-CoA, and the key enzyme, HMG-CoA reductase, is regulated by multiple mechanisms.