⚗️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.
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
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) 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
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)