Biological Chemistry II

⚗️Biological Chemistry II Unit 1 – Metabolism and Bioenergetics

Metabolism and bioenergetics form the foundation of life processes. These concepts explain how organisms obtain, transform, and use energy to maintain cellular functions and drive biochemical reactions. Understanding these principles is crucial for grasping the complexities of biological systems. This unit covers key topics like metabolic pathways, thermodynamics, enzyme regulation, and energy production. It explores carbohydrate, lipid, and amino acid metabolism, as well as the integration of these pathways. The knowledge gained here is essential for comprehending various physiological processes and metabolic disorders.

Key Concepts and Definitions

  • Metabolism encompasses all chemical reactions involved in maintaining the living state of cells and organisms
  • Bioenergetics studies energy transformations and energy exchanges within living systems
  • Catabolism breaks down complex molecules into simpler ones, releasing energy in the process
  • Anabolism constructs complex molecules from simpler ones, requiring an input of energy
  • Metabolic pathways are series of enzymatic reactions that transform initial reactants into final products
  • Enzymes are biological catalysts that speed up chemical reactions without being consumed in the process
  • Coenzymes are small, organic, non-protein molecules that carry chemical groups between enzymes (NAD+, FAD, Coenzyme A)
  • Adenosine triphosphate (ATP) is the primary energy currency of the cell, storing and releasing energy through its phosphate bonds

Thermodynamics in Biochemistry

  • Thermodynamics is the study of energy transformations and the direction of spontaneous processes
  • The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another
  • The second law of thermodynamics states that entropy (disorder) tends to increase in a closed system over time
  • Gibbs free energy (ΔG) predicts the spontaneity of a reaction at constant temperature and pressure
    • Reactions with negative ΔG are spontaneous and release energy
    • Reactions with positive ΔG are non-spontaneous and require an input of energy
  • Exergonic reactions release energy and have a negative ΔG (ATP hydrolysis, glucose oxidation)
  • Endergonic reactions absorb energy and have a positive ΔG (ATP synthesis, glucose synthesis)
  • Coupling of exergonic and endergonic reactions allows for energy transfer and drives metabolic processes

Metabolic Pathways Overview

  • Glycolysis is a cytosolic pathway that breaks down glucose into pyruvate, generating ATP and NADH
  • The citric acid cycle (Krebs cycle) is a mitochondrial pathway that oxidizes acetyl-CoA, producing CO2, NADH, FADH2, and ATP
  • Oxidative phosphorylation is the process by which electrons from NADH and FADH2 are transferred through the electron transport chain to generate a proton gradient, which is used to synthesize ATP
  • Fatty acid oxidation breaks down fatty acids into acetyl-CoA units, which can enter the citric acid cycle
  • Amino acid metabolism involves the breakdown of amino acids into carbon skeletons that can be used in the citric acid cycle or for gluconeogenesis
  • Gluconeogenesis is the synthesis of glucose from non-carbohydrate precursors (amino acids, lactate, glycerol)
  • The pentose phosphate pathway generates NADPH for reductive biosynthesis and ribose-5-phosphate for nucleotide synthesis
  • Metabolic integration coordinates the activities of various pathways to meet the energy and biosynthetic needs of the cell

Enzymes and Metabolic Regulation

  • Enzymes lower the activation energy of reactions, increasing reaction rates by factors of 106 to 1012
  • Enzyme activity is regulated by various mechanisms to control metabolic flux
  • Allosteric regulation involves the binding of effectors at sites other than the active site, modulating enzyme activity
    • Allosteric activators increase enzyme activity (AMP activating glycogen phosphorylase)
    • Allosteric inhibitors decrease enzyme activity (ATP inhibiting phosphofructokinase)
  • Covalent modification, such as phosphorylation or dephosphorylation, can alter enzyme activity (glycogen synthase, pyruvate dehydrogenase)
  • Feedback inhibition occurs when the end product of a pathway inhibits the activity of an earlier enzyme in the pathway (CTP inhibiting aspartate transcarbamoylase)
  • Compartmentalization of enzymes and substrates in different organelles allows for spatial regulation of metabolic processes
  • Hormonal regulation controls metabolic pathways at the organismal level (insulin promoting glucose uptake and storage)

Energy Production and ATP

  • ATP is composed of adenosine (adenine + ribose) and three phosphate groups
  • The hydrolysis of ATP to ADP + Pi releases ~7.3 kcal/mol of energy under standard conditions
  • ATP is synthesized by substrate-level phosphorylation and oxidative phosphorylation
    • Substrate-level phosphorylation directly transfers a phosphate group from a high-energy intermediate to ADP (phosphoenolpyruvate to ATP in glycolysis)
    • Oxidative phosphorylation uses the proton gradient generated by the electron transport chain to drive ATP synthesis via ATP synthase
  • The ATP/ADP cycle allows for the storage and release of energy in cells
  • The phosphorylation potential (ΔGp\Delta G_{p}) represents the free energy of ATP hydrolysis under intracellular conditions
  • The ATP/AMP ratio is a key indicator of cellular energy status and regulates metabolic pathways through allosteric enzymes (AMP-activated protein kinase)

Carbohydrate Metabolism

  • Glycolysis is a 10-step pathway that converts glucose into pyruvate
    • Preparatory phase (steps 1-5) consumes 2 ATP to convert glucose into fructose-1,6-bisphosphate
    • Payoff phase (steps 6-10) yields 4 ATP and 2 NADH, resulting in a net gain of 2 ATP and 2 NADH per glucose molecule
  • Pyruvate can be further oxidized in the citric acid cycle or converted to lactate under anaerobic conditions
  • The pentose phosphate pathway has an oxidative and a non-oxidative branch
    • Oxidative branch generates NADPH and ribulose-5-phosphate
    • Non-oxidative branch interconverts pentose phosphates and glycolytic intermediates
  • Glycogen is a storage polysaccharide in animals, synthesized by glycogen synthase and degraded by glycogen phosphorylase
  • Gluconeogenesis synthesizes glucose from non-carbohydrate precursors, sharing several enzymes with glycolysis but using unique, irreversible steps (pyruvate carboxylase, phosphoenolpyruvate carboxykinase, fructose-1,6-bisphosphatase, glucose-6-phosphatase)

Lipid Metabolism

  • Fatty acids are activated to acyl-CoA thioesters before being catabolized or synthesized
  • Fatty acid oxidation (β-oxidation) occurs in the mitochondrial matrix and breaks down fatty acids into acetyl-CoA units
    • Each cycle of β-oxidation shortens the fatty acid by two carbons and generates NADH, FADH2, and acetyl-CoA
    • Odd-chain fatty acids yield propionyl-CoA in the final step, which is converted to succinyl-CoA and enters the citric acid cycle
  • Ketone bodies (acetoacetate, β-hydroxybutyrate) are synthesized from acetyl-CoA in the liver during prolonged fasting or low-carbohydrate states
  • Fatty acid synthesis occurs in the cytosol and uses acetyl-CoA as a substrate
    • Acetyl-CoA carboxylase catalyzes the rate-limiting step, converting acetyl-CoA to malonyl-CoA
    • Fatty acid synthase is a multienzyme complex that condenses acetyl-CoA and malonyl-CoA to form palmitate
  • Triacylglycerols are stored in adipose tissue and hydrolyzed by lipases to release fatty acids during energy demand
  • Cholesterol is synthesized from acetyl-CoA through the mevalonate pathway, with HMG-CoA reductase as the rate-limiting enzyme

Amino Acid Metabolism

  • Amino acids are classified as glucogenic (can be converted to glucose), ketogenic (can be converted to ketone bodies or fatty acids), or both
  • Transamination reactions transfer the amino group from an amino acid to an α-ketoacid, forming a new amino acid and α-ketoacid
  • Deamination removes the amino group from an amino acid, releasing ammonia (NH3) or ammonium (NH4+)
  • The urea cycle converts ammonia into urea in the liver, preventing ammonia toxicity
    • Carbamoyl phosphate synthetase I, ornithine transcarbamoylase, argininosuccinate synthetase, argininosuccinate lyase, and arginase are the key enzymes in the urea cycle
  • Amino acid carbon skeletons can enter the citric acid cycle or be used for gluconeogenesis
  • Specific amino acids have unique metabolic fates (glycine in heme synthesis, phenylalanine and tyrosine in neurotransmitter synthesis)

Integration of Metabolic Pathways

  • Metabolic pathways are interconnected and regulated to maintain homeostasis
  • The fed state is characterized by high blood glucose, insulin secretion, and anabolic processes (glycogen synthesis, fatty acid synthesis, protein synthesis)
  • The fasting state is characterized by low blood glucose, glucagon secretion, and catabolic processes (glycogenolysis, gluconeogenesis, fatty acid oxidation, ketogenesis)
  • The Cori cycle shuttles lactate from anaerobic tissues (muscles) to the liver for gluconeogenesis
  • The glucose-alanine cycle transports amino groups from muscle to the liver in the form of alanine, which is then used for gluconeogenesis
  • The Cahill cycle (glucose-fatty acid cycle) describes the reciprocal regulation of glucose and fatty acid metabolism
    • Fatty acid oxidation inhibits glucose utilization in tissues (glucose-fatty acid cycle)
    • Elevated acetyl-CoA levels from fatty acid oxidation inhibit pyruvate dehydrogenase and activate pyruvate carboxylase, promoting gluconeogenesis
  • Metabolic flexibility allows organisms to adapt to changes in nutrient availability and energy demands

Clinical and Real-World Applications

  • Diabetes mellitus is characterized by impaired glucose homeostasis due to insulin deficiency (type 1) or insulin resistance (type 2)
    • Complications include hyperglycemia, ketoacidosis, and long-term tissue damage (retinopathy, neuropathy, nephropathy)
  • Inborn errors of metabolism are genetic disorders that affect specific metabolic pathways
    • Phenylketonuria (PKU) is caused by a deficiency in phenylalanine hydroxylase, leading to elevated phenylalanine levels and neurological damage if untreated
    • Maple syrup urine disease (MSUD) is caused by a defect in branched-chain α-ketoacid dehydrogenase, resulting in the accumulation of branched-chain amino acids and their toxic metabolites
  • Metabolic syndrome is a cluster of conditions (obesity, insulin resistance, hypertension, dyslipidemia) that increase the risk of cardiovascular disease and type 2 diabetes
  • Atherosclerosis is the buildup of plaque in arteries, often driven by dyslipidemia (high LDL cholesterol, low HDL cholesterol) and chronic inflammation
  • Cancer cells exhibit altered metabolism, such as increased aerobic glycolysis (Warburg effect) and glutamine addiction, to support rapid proliferation
  • Athletic performance can be enhanced by optimizing nutrient intake and training to improve metabolic efficiency and maximize energy production
  • Fasting and calorie restriction have been shown to induce metabolic adaptations (ketogenesis, autophagy) that may have health benefits, such as improved insulin sensitivity and longevity
  • Biotechnology applications leverage metabolic pathways for the production of biofuels, pharmaceuticals, and other valuable compounds (engineered microorganisms, cell-free systems)


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AP® and SAT® are trademarks registered by the College Board, which is not affiliated with, and does not endorse this website.
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