Chapter - 6


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Unformatted text preview: AN INTRODUCTION TO ENERGY, ENZYMES, AND METABOLISM METABOLISM CHAPTER 6 Metabolism Metabolism Sum total of all chemical reactions that occur Sum within an organism within Also refers to specific chemical reactions at the Also cellular level cellular Chemical reactions 2 factors govern fate of a chemical reaction – Direction Many cells use ATP to drive reactions in 1 direction – Rate Catalysts called enzyme can speed the reaction rate aA + bB ↔ cC + dD Energy Energy Ability to promote change 2 forms – Kinetic- associated with movement – Potential- due to structure or location Chemical energy- energy in molecular bonds 2 Laws of thermodynamics Laws 1. 2. First law Law of conservation of energy Energy cannot be created or destroyed Second law Transfer or transformation of energy from one form Transfer to another increases entropy or degree of disorder of a system system Change in free energy determines direction Change Energy transformations involve an increase Energy in entropy in Entropy - a measure of the disorder that Entropy cannot be harnessed to do work cannot H = G + TS G = H - TS H= enthalpy or total energy G= free energy or amount of energy for work S= entropy or unusable energy T= absolute temperature in Kelvin (K) Spontaneous reactions? Spontaneous Occur without input of additional energy Not necessarily fast Key factor is the free energy change ΔG = Δ H - T Δ S Exergonic – ΔG < 0 or negative free energy change – Spontaneous Endergonic – ΔG > 0 or positive free energy change – Requires addition of free energy – Not spontaneous ATP Hydrolysis of ATP ΔG = -7.3 kcal/mole Reaction favors formation Reaction of products of Energy liberated can drive a Energy variety of cellular processes variety Cells use ATP hydrolysis An endergonic reaction can be coupled to an exergonic An reaction reaction Endergonic reaction will be spontaneous if net free energy Endergonic change for both processes is negative change Glucose + phosphate → glucose-phosphate + H2O Glucose ΔG = +3.3 Kcal/mole endergonic ATP + H2O → ADP + Pi ΔG = -7.3 Kcal/mole exergonic Coupled reaction Glucose + ATP → glucose-phosphate + ADP ΔG = -4.0 Kcal/mole exergonic Enzymes and Ribozymes Enzymes A spontaneous reaction is not necessarily a fast reaction Catalyst- agent that speeds up the rate of a chemical reaction Catalystwithout being consumed during the reaction without Enzymes- protein catalysts in living cells Ribozymes – RNA molecules also are biological catalysts Activation energy Initial input of energy to start reaction Allows molecules to get close enough to cause bond rearrangement Can now achieve transition state where bonds are stretched Overcoming activation energy Overcoming 2 common ways – Large amounts of heat – Using enzymes to lower activation energy Small amount of heat can now push reactants to Small transition state transition Lowering activation energy Straining bonds in reactants to make it easier to achieve transition state Positioning reactants together to facilitate bonding Changing local environment – Direct participation through very temporary bonding Other enzyme features Other Active site- location where reaction takes Active place place Substrate- reactants that bind to active site Enzyme-substrate complex formed when Enzyme-substrate enzyme and substrate bind enzyme Substrate binding Substrate Enzymes have a high affinity or high degree of specificity for a Enzymes substrate substrate Used the example of a lock and key for substrate and enzyme binding Induced fit - interaction also involves conformational changes Other requirements for enzymes Prosthetic groups- small molecules permanently attached to the enzyme Cofactor- usually inorganic ion that temporarily binds to enzyme Coenzyme- organic molecule that participates in reaction but left unchanged afterward Enzymes are affected by environment Most enzymes function maximally in a narrow range of temperature and pH Outside of this narrow range, enzyme function decreases Overview of metabolism Overview Chemical reactions occur in metabolic pathways Each step is coordinated by a specific enzyme Catabolic pathways – Result in breakdown and are exergonic Anabolic pathways – Promote synthesis and are endergonic – Must be coupled to exergonic reaction Catabolic reactions Catabolic Breakdown of reactants Used for recycling Used to obtain energy for endergonic reactions – Energy stored in energy intermediates ATP, NADH 2 ways to make ATP 1. 2. Substrate-level phosphorylation – Enzyme directly transfers phosphate from one molecule to another molecule Chemiosmosis – Energy stored in an electrochemical gradient is used to make ATP from ADP and Pi Redox Redox Oxidation – Removal of electrons Reduction – Addition of electrons Redox – Electron removed from one molecule is added to another Ae- + B → A + Be A – Has been oxidized – Electron removed B – Has been reduced – Electron added Energy intermediates Energy Electrons removed by oxidation are Electrons used to create energy intermediates like NADH like NAD+ Nicotinamide adenine dinucleotide dinucleotide NADH… – Oxidized to make ATP – Can donate electrons during Can synthesis reactions synthesis Anabolic reactions Biosynthetic reactions Make large macromolecules or smaller molecules not available from food Many proteins use ATP as a source of energy Many Each ATP undergoes 10,000 cycles of hydrolysis and resynthesis every day Particular amino acid sequences in proteins function as ATP-binding sites Can predict whether a newly discovered protein uses ATP or not On average, 20% of all proteins bind ATP Likely underestimated because there may be other types of ATP-binding Likely sites sites Enormous importance of ATP as energy source Regulation of metabolic pathways Regulation 1. 2. Gene regulation – Turn on or off genes Cellular regulation – Cell-signaling pathways like hormones Biochemical regulation – Competitive inhibitors- compete for access to active site – Noncompetitive inhibitors- bind outside the active site Allosteric site- binding causes conformational change in enzyme active site inhibiting enzyme function Feedback inhibition- product of pathway inhibits early steps to prevent over accumulation of product 1. Recycling Recycling Most large molecules exist for a relatively short period of time Half-life: time it takes for 50% of the molecules to be broken Half-life: down and recycled down Important - to efficiently use and recycle organic molecules - prevent waste of energy prevent Expression of genome allows cells to respond to changes in their environment – RNA and proteins made when needed – Broken down when they are not mRNA degradation important – Conserve energy by degrading mRNAs for proteins no longer required – Remove faulty copies of mRNA mRNA degradation mRNA Exonucleases – Enzyme cleaves off nucleotides from end Exosome – Multiprotein complex uses exonucleases Role played by proteasome Role Recognition by proteases Protein complex – proteasome Ubiquitin – covalently binds to unwanted proteins Ubiquitin and directs them to the proteasome and Autophagy Autophagy Lysosomes contain hydrolases to break down proteins, Lysosomes carbohydrates, nucleic acids, and lipids carbohydrates, – Digest substances taken up by endocytosis – Autophagy- recycling worn out organelles Autophagosome ...
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This note was uploaded on 10/10/2010 for the course CELL 101 taught by Professor Burdsal during the Spring '08 term at Tulane.

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