Lipid and Protein Metabolism: Catabolism and Anabolism

Lecture 27: Lipid and Protein Metabolism

Overview

This lecture covers the metabolic pathways involved in the breakdown (catabolism) and synthesis (anabolism) of lipids and proteins. Understanding these processes is essential for comprehending how the body derives energy from nutrients and how it builds and maintains tissues.

Fatty Acid Catabolism

Triglycerides and Lipolysis

• Triglycerides are the main form of fat storage in the body, consisting of three fatty acids attached to a glycerol backbone.

• Lipolysis is the process by which triglycerides are broken down into free fatty acids and glycerol.

• Glycerol can be converted to glyceraldehyde-3-phosphate, an intermediate in glycolysis, allowing it to enter the pathway for ATP production.

Beta-Oxidation

• Approximately 95% of the energy stored in triglycerides comes from the oxidation of fatty acids via beta-oxidation.

• Beta-oxidation occurs in the mitochondrial matrix, especially in heart and skeletal muscle cells.

• This process sequentially removes two-carbon units from fatty acids, producing acetyl CoA, NADH, and FADH2.

• Acetyl CoA enters the citric acid cycle (Krebs cycle) for further energy extraction.

Equation for Beta-Oxidation:

$\text{Fatty Acid} + \text{CoA} + \text{FAD} + \text{NAD}^+ + \text{H}_2\text{O} \rightarrow \text{Acetyl-CoA} + \text{FADH}_2 + \text{NADH} + \text{H}^+$

Ketogenesis

• Ketogenesis is the process by which the liver converts excess acetyl CoA into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone).

• This occurs in the mitochondrial matrix of liver cells, especially during prolonged fasting or carbohydrate restriction.

• Excessive ketone body production can lower blood pH, leading to ketoacidosis.

Fatty Acid Anabolism

Lipogenesis

• Lipogenesis is the synthesis of fatty acids, essentially the reverse of beta-oxidation.

• This process occurs in the cytosol and is catalyzed by the enzyme fatty acid synthase.

• Fatty acid synthase elongates the fatty acid chain by adding two-carbon units (from malonyl-CoA) at a time.

• Newly synthesized fatty acids are attached to glycerol in the endoplasmic reticulum and stored as triglycerides in adipocytes (fat cells).

• Fatty acid synthesis often begins with glucose (via acetyl CoA) or amino acids as precursors.

• Excess glucose is converted to glycerol, and excess amino acids can be converted to triglycerides for storage.

Amino Acid Catabolism

Sources and Importance

• Proteins used for catabolism are either obtained from the diet or are already present in the cytosol of cells.

• It is important to maintain blood protein levels within normal limits to ensure proper physiological function.

Transamination and Deamination

• Transamination is the process by which the amino group is transferred from an amino acid to a keto acid, usually forming glutamate and a new amino acid.

• The amino group is eventually removed as ammonia (NH3) in the liver, which is converted to urea for excretion (deamination).

• The remaining carbon skeleton can be used for energy production (via the citric acid cycle) or for gluconeogenesis.

Amino Acid Anabolism

Protein Synthesis

• Protein synthesis involves joining amino acids by peptide bonds according to the genetic code in DNA.

• The body can synthesize 11 non-essential amino acids by adding an amino group to a carbon skeleton (transamination).

• The remaining 9 essential amino acids must be supplied by the diet.

• Unlike carbohydrates and lipids, proteins are not stored for future use; excess amino acids are converted to glucose or fatty acids for storage.

Big Picture of Nutrient Catabolism and Anabolism

Integration of Metabolic Pathways

• Carbohydrates, lipids, and proteins are interconnected through metabolic pathways.

• Glycerol and amino acids can enter glycolysis or gluconeogenesis.

• Fatty acids undergo beta-oxidation to form acetyl CoA, which enters the citric acid cycle.

• Amino acids can be deaminated and their carbon skeletons used for energy or converted to glucose or fatty acids.

Summary Table: Major Pathways of Lipid and Protein Metabolism

Process	Location	Main Substrates	Main Products	Purpose
Lipolysis	Adipose tissue, cytosol	Triglycerides	Fatty acids, glycerol	Release stored energy
Beta-oxidation	Mitochondrial matrix	Fatty acids	Acetyl CoA, NADH, FADH2	ATP production
Ketogenesis	Liver mitochondria	Acetyl CoA	Ketone bodies	Alternative energy source
Lipogenesis	Cytosol (mainly liver, adipose)	Acetyl CoA, NADPH	Fatty acids	Energy storage
Protein synthesis	Ribosomes (cytosol, RER)	Amino acids	Proteins	Growth, repair, function
Transamination/Deamination	Liver (mainly)	Amino acids	Keto acids, ammonia (urea)	Energy production, nitrogen excretion

Example: Fasting State Metabolism

• During fasting, triglycerides are broken down to provide fatty acids for beta-oxidation, and amino acids are used for gluconeogenesis.

• Ketone bodies become an important energy source for the brain and muscles when glucose is scarce.

Additional info: The diagrams referenced in the slides illustrate the integration of carbohydrate, lipid, and protein metabolism, showing how different nutrients can be converted into each other or used for energy depending on the body's needs.