Metabolism Overview

Catabolism and Anabolism

Metabolism encompasses all chemical reactions in the body, divided into catabolism and anabolism. Catabolism breaks down large molecules, releasing energy, while anabolism builds molecules, requiring energy input.

• Catabolism: Breakdown of complex molecules into simpler ones; releases energy (exothermic).

• Anabolism: Synthesis of complex molecules from simpler ones; requires energy (endothermic).

• ATP: Adenosine triphosphate, the main energy currency of the cell.

Three Parts of Catabolism

Catabolic pathways for glucose involve three main stages:

1. Glycolysis: Occurs in the cytoplasm; anaerobic (no oxygen required). Glucose (6C) is split into two pyruvate (3C) molecules, yielding 2 ATP.

2. Citric Acid Cycle (Krebs Cycle): Occurs in mitochondria; pyruvate is converted to Acetyl CoA and enters the cycle. Electron carriers FAD and NAD+ are reduced to FADH2 and NADH + H+, storing energy.

3. Oxidative Phosphorylation (Aerobic Respiration): FADH2 and NADH donate electrons to the electron transport chain. Oxygen is the final electron acceptor, forming water. ATP Synthase uses the proton gradient to generate ATP, producing up to 34 ATP.

Example: Aerobic respiration is analogous to a dam, where water flows to turn a turbine and generate electricity.

Carbohydrate Metabolism

Digestion and Absorption

Carbohydrates are digested by enzymes and absorbed as monosaccharides, primarily glucose.

• Amylase: Breaks down starch and glycogen into smaller sugars.

• Disaccharidases: Maltase, sucrase, and lactase complete digestion to monosaccharides.

• Glucose: Main monosaccharide used for energy.

Glucose Oxidation

Complete oxidation of glucose yields:

• Glycolysis: 2 ATP

• Citric Acid Cycle + Oxidative Phosphorylation: 34 ATP

• Total: 36 ATP per glucose molecule

• Byproducts: Carbon dioxide and water

Lipid Metabolism

Lipid Breakdown and Digestion

Lipid digestion begins in the mouth and continues in the duodenum, aided by bile and lipase enzymes.

• Lingual Lipase: Initiates fat digestion in the mouth.

• Pancreatic Lipase: Continues digestion in the duodenum.

• Bile: Emulsifies fats, increasing surface area for lipase action.

• Triglycerides: Split into glycerol and three fatty acids.

Lipoproteins

Absorbed triglycerides are packaged into lipoproteins for transport:

• Chylomicrons: Formed in the gut, transported to the liver.

• LDL (Low-Density Lipoprotein): Delivers fats and cholesterol to tissues; "bad cholesterol".

• HDL (High-Density Lipoprotein): Removes fats and cholesterol from tissues; "good cholesterol".

Fatty Acid Catabolism

Fatty acids are broken down via beta-oxidation to Acetyl CoA, which enters aerobic respiration for ATP production.

• Beta-oxidation: Sequential removal of two-carbon units from fatty acids.

• Fatty acids are also precursors for cholesterol, bile, steroid hormones, phospholipids, glycolipids, prostaglandins, and triglycerides.

Protein and Amino Acid Metabolism

Protein Digestion

Proteins are digested into amino acids by stomach and pancreatic enzymes.

• Pepsin and HCl: Initiate protein breakdown in the stomach (pH 2).

• Proteases: Trypsin, chymotrypsin, and carboxypeptidase in the duodenum complete digestion.

Amino Acid Processing

Amino acids are processed in the liver for protein synthesis, conversion, or energy production.

• Deamination: Removal of amino group, producing ammonium ion (toxic).

• Urea Cycle: Converts ammonium and CO2 to urea (nontoxic), excreted in urine.

• Essential Amino Acids: 10 must be obtained from diet; others are synthesized by the body.

• Amination: Addition of amino group to organic acid.

• Transamination: Transfer of amino group between molecules.

• Deamination: Removal of amino group from amino acid.

Absorptive and Post-Absorptive States

Absorptive State

Regulated by insulin, this state promotes storage and synthesis of nutrients after eating.

• Increased glucose and amino acid uptake

• Increased triglyceride, protein, and fat synthesis

• Increased glycogen storage

Post-Absorptive State

Regulated by glucagon, glucocorticoids, and epinephrine, this state mobilizes stored nutrients for energy between meals.

• Glucocorticoids: Break down proteins and lipids

• Glucagon: Increases glycogen breakdown and glucose formation

• Epinephrine: Increases glycogen breakdown in muscle

Energy Content of Nutrients

The energy released per gram of nutrient is as follows:

Fat is the most efficient energy storage molecule.

Phenylketonuria (PKU)

Phenylketonuria is a genetic disorder where the amino acid phenylalanine cannot be metabolized due to a missing enzyme. Phenylalanine accumulates and cannot be converted to tyrosine.

• Aspartame (in diet sodas) contains phenylalanine; must be avoided by individuals with PKU.

Appetite Regulation

Feeding and Satiety Centers

Appetite is regulated by neural and hormonal signals.

• Feeding Center: Stimulated by neuropeptide Y and ghrelin; increases appetite.

• Satiety Center: Stimulated by CCK, increased blood glucose, digestive tract stretch, and leptin; decreases appetite.

• Leptin: Provides long-term regulation of appetite.

Key Equations

ATP Yield from Glucose

The total ATP yield from complete aerobic oxidation of glucose:

Beta-Oxidation of Fatty Acids

General reaction for beta-oxidation:

Urea Cycle

Conversion of ammonium ion to urea:

Additional info: The urea cycle is essential for detoxifying ammonia produced during amino acid catabolism.