Bioenergetics and Metabolism

Transcription

1 Bioenergetics and Metabolism Lecture 24 Key Concepts Energy Conversion in Biological Systems Review of thermodynamic principles Sunlight is the ultimate source of energy on planet Earth Energy conversion by coupled oxidation and reduction reactions The adenylate system is used for short term energy storage Overview of Metabolic Pathways Metabolic pathways consist of linked enzymatic reactions The six major groups of primary metabolic pathways in nature 1

2 Energy Conversion in Biological Systems Concentration of biomolecules and ions inside of a cell must be very different than that of the environment Gradients Compartmentalization An organism at equilibrium with the environment is no longer alive 2

3 Energy conversion in biological systems is used to perform work chemical work in the form of macromolecular biosynthesis of organic molecules osmotic work to maintain a concentration of intracellular salts and organic molecules that is different than the extracellular milieu mechanical work in the form of flagellar rotation or muscle contraction Cycling of resources and waste between environment and a living cell provides materials for energy conversion 3

4 Thermodynamic principles govern energy conversion in metabolic processes 1. First Law of Thermodynamics 2. Second Law of Thermodynamics 3. Gibbs Free Energy First Law of Thermodynamics Energy cannot be created or destroyed it can only be converted between forms Antoine Laurent Lavoisier and the guinea pig,

5 Using a "bomb" calorimeter to measure heat transfer as a result of combustion in pure oxygen Example: glucose Temperature increase of the surrounding water is a measurement of the amount of energy stored in the glucose C 6 H 12 O O 2 6 CO H 2 O + heat Where heat = q = E = 3.75 ºC/kilogram of water Calorie Calorie (kcal) was originally defined by the amount of heat energy required to raise 1 kilogram of water from 14.5 ºC to 15.5 ºC. This can also be expressed in the international unit of measurement the Joule (J) in which 1 Calorie = 1 kcal = kj 5

6 Total energy potential of 1 gram of glucose is the same regardless of the metabolic path taken Second Law of Thermodynamics All natural processes in the Universe tend towards disorder Entropy (S) Energy is required to restrain the natural tendency toward disorder An increase in entropy is the natural process that happens when a cell dies. 6

7 Think cool thoughts G ( delta G naught ) Gibbs Free Energy Change in free energy ( G) under standard conditions (what are these again?) spontaneity of a reaction A B G = - RT ln(k eq ) (remember lecture 2?) 7

8 Concepts to review Gº = 0 Gº < 0 Gº > 0 Equilibrium Exergonic Endergonic G G is important in Metabolism Use of shared intermediates The free energy released from ATP hydrolysis is used drive unfavorable reactions The actual change in free energy ( G) of a reaction can be determined 8

9 Use of shared intermediates Product of reaction 1 is the substrate for reaction 2 The Gº of a coupled reaction is the sum of the Gº values for each individual reaction A B B C A C Gº = +4 kj/mol Gº = -10 kj/mol Gº = -6 kj/mol Free energy released from ATP hydrolysis is used drive unfavorable reactions ATP hydrolysis: Gº = kj/mol The first step in glycolysis is catalyzed by the enzyme hexokinase and utilizes ATP hydrolysis to drive the unfavorable reaction of glucose phosphorylation in a coupled reaction Glucose + Pi glucose 6-phosphate + H 2 O ATP + H 2 O ADP + Pi Glucose + ATP glucose 6-phosphate + ADP Gº = kj/mol Gº = kj/mol Gº = kj/mol 9

10 The actual change in free energy ( G) of the reaction A B is the sum of the change in standard free energy G and the term: RT ln[b] actual /[A] actual G = Gº + RT ln[b] actual / [A] actual mass action ratio = [B] actual /[A] actual This is NOT K eq : K eq = [B] equilibrium /[A] equilibrium If the system is a equilibrium, it is DEAD! Sunlight is the ultimate source of energy 10

11 Energy conversion by coupled oxidation and reduction reactions redox reactions involve the transfer of electrons from one compound to another the oxidation of one compound results in the reduction of another Metabolic redox reactions Catalyzed by enzymes called dehydrogenases and oxidases Involve coenzymes such as nicotinamide adenine dinucleotide (NADH and NAD + ) and flavin adenine dinucleotide (FADH 2 and FAD) that serve as electron donors and acceptors 11

12 Example: Oxidation of Glucose leads to reduced forms of cofactors NADH or FADH 2 The adenylate system is used for short term energy storage ATP synthesis reaction captures redox energy in the form of phosphoanhydride bond energy Highest Energy! 12

13 Catabolic processes produce ATP Anabolic processes require ATP Catabolic processes: break down molecules Anabolic processes: build molecules The Adenylate System Interconversion of low and high energy forms of adenylate ATP, ADP, and AMP a 70 kg person requires ~100 moles of ATP every day molecular weight of ATP is 507 g/mol, which means we hydrolyze as much as 50 kg of ATP every day How do we do it? 13

14 Recycle adenylate forms by reforming ATP from ADP + Pi (adenylate kinase) AMP + ATP 2 ADP (phosphorylating systems) oxidative phosphorylation and photophosphorylation 2 ADP + 2 Pi ATP + ATP Net AMP + 2 Pi ATP Energy Charge (EC) ATP is the high energy form of the adenylate system 14

15 Normal Range of EC for Cells [ATP] is higher than [ADP] or [AMP] Overview of Metabolism 15

16 A metabolic pathway is a set of linked reactions metabolism of shared intermediates first reaction step and regulation often linked Positive vs. Negative regulation Feedback inhibition Different types of metabolic pathways Substrate concentration and enzyme activity levels affect flux through metabolic pathways 16

17 Example: Glucose metabolism in the liver before and after breakfast Glucagon signaling in liver cells activates both a catabolic pathway (glycogen degradation) and an anabolic pathway (gluconeogenesis), while at the same time inhibiting the catabolism of glucose by the glycolytic pathway. Example: Glucose metabolism in the liver before and after breakfast Within an hour of eating a bowl of cereal and drinking a cup of fruit juice, your insulin levels increase due to elevated blood glucose causing activation of the insulin signaling pathway and stimulation of glucose uptake, glycogen synthesis, and an increase in glucose catabolism by the glycolytic pathway. 17

18 6 major groups of metabolic pathways in nature Metabolic pathways are highly interdependent and exquisitely controlled by substrate availability and enzyme activity levels Key to understanding metabolic integration in terms of nutrition, exercise, and disease (e.g., diabetes and obesity) is learning how metabolic flux between pathways is regulated and controlled Should you memorize this chart? 18

19 Interdependence of six major groups of pathways Hierarchical Nature of Metabolism Four classes of macromolecules (proteins, nucleic acids, carbohydrates, and lipids) Six primary metabolite groups (amino acids, nucleotides, fatty acids, glucose, pyruvate, acetyl CoA) Six small biomolecules (NH4+, CO2, NADH, O2, ATP, H2O) 19

20 We will start off the discussion of each new pathway by answering the following four questions about the pathway: 1. What does the pathway accomplish for the cell? 2. What is the overall net reaction of the pathway? 3. What are the key regulated enzymes in the pathway? 4. What are examples of this pathway in real life? 20