Essential Metabolic Pathways and Biochemical Principles
Glycolysis
Glycolysis is the process of breaking down glucose into pyruvate in the cytoplasm of cells to produce energy. It is the first and most important pathway of carbohydrate metabolism and occurs in both aerobic and anaerobic conditions. In this pathway, one molecule of glucose containing six carbon atoms is converted into two molecules of pyruvate containing three carbon atoms each through a series of enzymatic reactions. During glycolysis, energy is released in the form of ATP and NADH. The pathway consists of two phases: the energy investment phase, where ATP is used, and the energy generation phase, where ATP is produced. The net gain from one molecule of glucose is 2 ATP and 2 NADH molecules. In the absence of oxygen, pyruvate is converted into lactic acid, while in the presence of oxygen, it enters the Krebs cycle for further energy production. Glycolysis is important because it provides quick energy to cells, especially muscles during exercise, and supplies intermediates for other metabolic pathways. Disorders or defects in glycolysis can lead to reduced energy production and metabolic diseases.
Citric Acid Cycle (Krebs Cycle)
The Citric Acid Cycle, also called the Krebs cycle or Tricarboxylic Acid (TCA) cycle, is an important metabolic pathway that occurs in the mitochondrial matrix of cells. It is the final common pathway for the oxidation of carbohydrates, fats, and proteins. In this cycle, acetyl-CoA combines with oxaloacetic acid to form citric acid, which undergoes a series of enzymatic reactions to produce energy. During one turn of the cycle, carbon dioxide is released and high-energy molecules such as NADH, FADH₂, and GTP (ATP) are formed. These molecules later participate in the electron transport chain to generate large amounts of ATP. The cycle begins with the formation of citrate and ends with the regeneration of oxaloacetic acid, allowing the cycle to continue repeatedly. The Citric Acid Cycle is important because it plays a central role in cellular respiration, provides energy for the body, and supplies intermediates for the synthesis of amino acids and other biomolecules. Any defect in this cycle can affect energy production and lead to metabolic disorders.
Urea Cycle
The Urea Cycle is a metabolic pathway that occurs mainly in the liver and is responsible for converting toxic ammonia, produced during protein metabolism, into urea, which is less toxic and easily excreted through urine by the kidneys. The cycle takes place partly in the mitochondria and partly in the cytoplasm of liver cells. In this process, ammonia combines with carbon dioxide to form carbamoyl phosphate, which then enters a series of reactions involving ornithine, citrulline, argininosuccinate, and arginine. Finally, urea is formed and ornithine is regenerated to continue the cycle. The Urea Cycle is very important because the accumulation of ammonia in the body is harmful, especially to the brain. It helps maintain nitrogen balance and removes excess nitrogen from the body. Defects in the enzymes of the urea cycle can lead to hyperammonemia and serious metabolic disorders.
Glycogenesis and Glycogenolysis
Glycogenesis is the process of formation of glycogen from glucose for storage in the body, mainly in the liver and muscles. It occurs when excess glucose is available, especially after meals. In this process, glucose is first converted into glucose-6-phosphate and then into UDP-glucose, which is finally polymerized to form glycogen with the help of the enzyme glycogen synthase. Glycogenesis is stimulated by the hormone insulin and helps maintain normal blood glucose levels by storing excess glucose for future use.
Glycogenolysis is the process of breakdown of stored glycogen into glucose when the body requires energy, such as during fasting or exercise. In this pathway, glycogen is converted into glucose-1-phosphate by the enzyme glycogen phosphorylase, which is later converted into glucose-6-phosphate. In the liver, glucose-6-phosphate can be converted into free glucose and released into the blood to maintain blood sugar levels. Glycogenolysis is stimulated by glucagon and adrenaline. Both glycogenesis and glycogenolysis are important processes for the regulation of blood glucose and energy supply in the body.
Enzyme Kinetics and Inhibition
Enzyme kinetics is the study of the rate of enzyme-catalyzed reactions and the factors affecting these reactions. The rate of an enzyme reaction depends on enzyme concentration, substrate concentration, temperature, pH, and the presence of inhibitors or activators. As the substrate concentration increases, the reaction velocity also increases until all active sites of the enzyme become saturated, after which the reaction reaches a maximum velocity (Vmax). The substrate concentration at which the reaction rate becomes half of Vmax is called the Michaelis constant (Km), which indicates the affinity of the enzyme for its substrate.
Enzyme inhibition is the process by which the activity of an enzyme is decreased or stopped by certain substances called inhibitors:
- Competitive inhibition: The inhibitor resembles the substrate and competes for the active site; this can be overcome by increasing substrate concentration.
- Non-competitive inhibition: The inhibitor binds to a site other than the active site and changes the enzyme structure; increasing substrate concentration does not reverse this effect.
Classification and Properties of Enzymes
Enzymes are biological catalysts made mainly of proteins that increase the rate of biochemical reactions without being consumed. According to the International Union of Biochemistry (IUB), enzymes are classified into six major classes:
- Oxidoreductases: Catalyze oxidation-reduction reactions.
- Transferases: Transfer functional groups from one molecule to another.
- Hydrolases: Catalyze hydrolysis reactions.
- Lyases: Remove or add groups without hydrolysis.
- Isomerases: Catalyze rearrangement within molecules.
- Ligases: Join two molecules together using energy from ATP.
Enzymes are highly specific, efficient, and function best at an optimum temperature and pH. They lower the activation energy of reactions, and many require non-protein components called cofactors or coenzymes for proper functioning.
Disorders of Carbohydrate Metabolism
These conditions occur when the digestion, absorption, storage, or utilization of carbohydrates is disturbed due to enzyme defects, hormonal imbalance, or genetic abnormalities. Common disorders include:
- Diabetes Mellitus: Deficiency or ineffective action of insulin, resulting in high blood glucose.
- Glycogen storage disease: Defects in enzymes leading to abnormal glycogen accumulation.
- Galactosemia: Hereditary inability to metabolize galactose.
- Hereditary fructose intolerance: Inability to metabolize fructose.
- Lactose intolerance: Deficiency of the enzyme lactase.
Structure and Functions of DNA
Deoxyribonucleic Acid (DNA) is the genetic material present in the nucleus of most living cells. It is made up of repeating units called nucleotides, consisting of a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine, thymine, guanine, and cytosine). DNA has a double helical structure where two polynucleotide strands are held together by hydrogen bonds between complementary base pairs (A-T and G-C).
DNA stores genetic information, controls protein synthesis, and is responsible for the transmission of hereditary characters. During cell division, DNA replicates itself to pass identical genetic information to daughter cells.
Structure and Functions of RNA
Ribonucleic Acid (RNA) is a nucleic acid mainly involved in protein synthesis. It is usually single-stranded and consists of ribose sugar, a phosphate group, and nitrogen bases (adenine, uracil, guanine, and cytosine). There are three main types:
- mRNA: Carries genetic information from DNA to ribosomes.
- tRNA: Transports amino acids to ribosomes.
- rRNA: Forms the structural part of ribosomes.
Carbohydrates and Their Classification
Carbohydrates are organic compounds made of carbon, hydrogen, and oxygen. They are the main source of energy for the body. They are classified as follows:
- Monosaccharides: Simplest units (e.g., glucose, fructose).
- Oligosaccharides: 2–10 monosaccharide units (e.g., sucrose, lactose).
- Polysaccharides: Complex chains (e.g., starch, glycogen).
- Homopolysaccharides: One type of unit (e.g., cellulose).
- Heteropolysaccharides: Different types of units (e.g., heparin).