Biochemistry: Metabolic Pathways Quiz

Introduction to Metabolism

• Metabolism is the sum of all chemical reactions occurring in cells.
• It's a highly coordinated cellular activity with a specific purpose and direction, involving numerous enzyme systems.
• It involves the exchange of matter and energy with the environment.
• Specific functions include:
  • Obtaining chemical energy from sunlight or food.
  • Converting nutrients into cellular components.
  • Assembling these components into the cell's own macromolecules.
  • Forming and breaking down molecules for specialized cellular functions.

Digestion

• Digestion breaks down carbohydrates, lipids, and proteins into absorbable compounds (glucose, fatty acids, and amino acids, respectively).

Absorption

• Absorption involves the passage of digested products, along with vitamins, minerals, water, etc., through the digestive system into the organism.

Phases of Metabolism

• Absorption: The phase where chemicals and energy from the environment enter the protoplasm.
• Transformation: This involves all actions where the protoplasm changes the absorbed species and energy, including secretion, digestion, assimilation, and dissimilation.
• Excretion: Removal of chemicals not incorporated into the protoplasm.

Stages of Metabolism

• First stage: Breakdown of large molecules into monomers (e.g., polysaccharides to glucose, lipids to glycerol and fatty acids, proteins to amino acids). No usable energy is released in this stage.
• Second stage: Further breakdown of the numerous small molecules from the first stage into a smaller set of central metabolic molecules, tending toward acetyl CoA. A small amount of ATP is generated in this stage.
• Third stage: Oxidation of acetyl CoA into water and carbon dioxide. Most of the energy from food is produced as ATP in this stage.

Division of Metabolism: Anabolism and Catabolism

• Anabolism: A constructive metabolic process where large molecules are built from smaller ones, consuming energy (ATP). It's used to build proteins from amino acids.
• Catabolism: A degradative metabolic process where large molecules from food or the organism's own reserves are broken down into smaller ones. This process releases energy, some of which is used directly and the rest stored in special molecules for later use.
• Amphibolic pathways: Mixed routes for interconversion of metabolic intermediates between the beginning of anabolic pathways and the end of catabolic pathways.

Energy

• Energy is the ability to do work or cause change in matter. It can take the form of heat, light, electricity, and motion.

Metabolic Pathways

• A series of ordered reactions where the product of one reaction becomes the substrate for the next.
• Examples include glycolysis.
• A metabolic pathway is a sequence of chemical reactions transforming an initial substrate into one or more final products through intermediary metabolites.

Types of Metabolic Pathways

• Catabolic pathways: Oxidative routes releasing energy and reducing power, and synthesizing ATP. Examples include glycolysis and beta-oxidation.
• Anabolic pathways: Reductive routes consuming energy (ATP) and reducing power. Examples include gluconeogenesis and the Calvin cycle.
• Amphibolic pathways: Mixed catabolic and anabolic pathways like the Krebs cycle, which produces energy and reducing power, and precursors for biosynthesis.

Glycolysis

• Glycolysis (or the Embden-Meyerhof pathway) is the metabolic pathway that oxidizes glucose to produce energy for the cell.
• It comprises 10 enzymatic reactions that convert glucose into two pyruvate molecules which can participate in subsequent metabolic pathways to further produce energy.
• Functions: produce ATP and NADH, form energy source molecules for aerobic respiration and fermentation.
• Features:
  • Takes place in the cytosol.
  • Does not require oxygen.
  • Starts with one glucose molecule (6C).
  • Ends with two pyruvate molecules (3C each).
  • Considered an amphibolic pathway.

Pentose Phosphate Pathway

• An alternative glucose catabolic pathway also known as the phosphogluconate pathway.
• Glucose is oxidized but no ATP is produced.
• Primarily produces NADPH (nicotinamide adenine dinucleotide phosphate reduced form) in eukaryotic cells.
• Occurs in the cytoplasm.
• Consists of irreversible oxidation and reversible interconversions. -More complex than glycolysis.
• Not present in skeletal muscle.
• Functions include:
  • Provide reducing power in the cytoplasm (NADPH), essential for anabolic reactions, and a potent antioxidant in some cells.
  • Provide pentoses (ribose), necessary for nucleotide and many cofactor enzyme synthesis.
• Divided into two phases:
  • Oxidative phase: Produces NADPH + H+.
  • Non-oxidative phase: Produces various monosaccharides; pentose(ribose) is one important product.

Krebs Cycle

• Part of cellular respiration.
• Involves reactions leading from glycolysis to the respiratory chain.
• Occurs in mitochondria in three phases:
  • Oxidation of pyruvate to acetyl CoA.
  • Krebs cycle.
  • Oxidative phosphorylation.

Cori Cycle

• Cyclic circulation of glucose and lactate between muscle and liver.
• Muscles use glucose from glycogen reserves or the bloodstream.
• Intense muscle activity leads to substantial lactate production.
• Lactate diffuses into the blood and travels to the liver.
• In the liver, lactate is converted back to glucose by gluconeogenesis.
• Glucose returns to muscles.
• Objectives: provides quick energy for muscle in low oxygen conditions, prevents lactic acidosis in muscles.

Gluconeogenesis

• Anabolic pathway creating glucose from non-carbohydrate precursors like amino acids, lactate, pyruvate, glycerol, and intermediates of the citric acid cycle.
• Most important in liver and kidneys.
• Enables organisms to produce glucose when it's not available from food.
• Essential for maintaining blood glucose levels during fasting states.

Transamination

• Transfer of an amino group from an amino acid to a keto acid.
• Amino acid converts to a keto acid.
• Keto acid accepts amino group to form corresponding amino acid.
• Reversible reaction catalyzed by transaminases.
• Uses pyridoxal phosphate (vitamin B6).

Oxidative Deamination

• Removal of an amino group from an amino acid.
• Catalyzed by glutamate dehydrogenase.
• Uses NAD or NADP coenzymes.
• An important reaction for ammonia (NH3) removal.
• Occurs in the mitochondrial matrix.
• Allosteric enzyme activated by ADP.

Urea Cycle

• Metabolic process for processing protein derivatives and producing urea as the final product.
• A hepatic process, classifying animals by nitrogen excretion methods:
  • Ammonotelic: Excrete ammonia (aquatic animals, fish & amphibians).
  • Ureotelic: Convert ammonia to urea (humans, mammals, adult amphibians & sharks).
  • Uricotelic: Convert ammonia to uric acid (birds & reptiles).
• Five reactions, two in mitochondria and three in the cytoplasm.
  • Key steps outlined, including enzymes and substrates.

Glucogenic and Ketogenic Amino Acids

• Glucogenic: Produce intermediates for gluconeogenesis (e.g., pyruvate, oxaloacetate).
• Ketogenic: Produce ketone bodies (e.g., leucine, lysine).
• Some amino acids are both glucogenic and ketogenic (e.g., phenylalanine, tyrosine, tryptophan, isoleucine).