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Unformatted text preview: ENZYMES,METABOLISM,AND CELLULAR RESPIRATION
CHAPTER 7 Metabolism Sum total of all chemical reactions that occur within an organism Also refers to specific chemical reactions at the cellular level 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 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
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 to another increases entropy or degree of disorder of a system Change in free energy determines direction Energy transformations involve an increase in entropy Entropy - a measure of the disorder that cannot be harnessed to do work H = G + 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? Occur without input of additional energy Not necessarily fast Key factor is the free energy change G = H - T S Exergonic G < 0 or negative free energy change Spontaneous Endergonic G > 0 or positive free energy change Requires addition of free energy Not spontaneous ATP Hydrolysis of ATP G = -7.3 kcal/mole Reaction favors formation of products Energy liberated can drive a variety of cellular processes Cells use ATP hydrolysis An endergonic reaction can be coupled to an exergonic reaction Endergonic reaction will be spontaneous if net free energy change for both processes is negative Glucose + phosphate glucose-phosphate + H2O G = +3.3 Kcal/mole endergonic ATP + H2O ADP + Pi G = -7.3 Kcal/mole exergonic Coupled reaction
Glucose + ATP glucose-phosphate + ADP G = -4.0 Kcal/mole exergonic Enzymes A spontaneous reaction is not necessarily a fast reaction Catalyst- agent that speeds up the rate of a chemical reaction without being consumed during the reaction Enzymes- protein catalysts in living cells 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 2 common ways Large amounts of heat Using enzymes to lower activation energy Small amount of heat can now push reactants to transition state 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 Active site- location where reaction takes place Substrate- reactants that bind to active site Enzyme-substrate complex formed when enzyme and substrate bind Substrate binding Enzymes have a high affinity or high degree of specificity for a 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 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 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 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 Electrons removed by oxidation are used to create energy intermediates like NADH NAD+ Nicotinamide adenine dinucleotide NADH... Oxidized to make ATP Can donate electrons during synthesis reactions Anabolic reactions Biosynthetic reactions Make large macromolecules or smaller molecules not available from food Many proteins use ATP as a source of energy 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 sites Enormous importance of ATP as energy source Regulation of metabolic pathways
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. Cellular respiration Process by which living cells obtain energy from organic molecules Primary aim to make ATP and NADH Aerobic respiration uses oxygen O2 consumed and CO2 released Organic molecules + O2 CO2 + H2O + Energy Glucose metabolism 1. 2. 3. 4. 4 metabolic pathways Glycolysis Breakdown of pyruvate to an acetyl group Citric acid cycle Oxidative phosphorylation Stage 1: Glycolysis
Glycolysis can occur with or without oxygen Steps in glycolysis nearly identical in all living species 10 steps in 3 phases 1. 2. 3. Energy investment Cleavage Energy liberation 3 phases of glycolysis
1. 2. 3. Energy investment Steps 1-3 2 ATP hydrolyzed to create fructose-1,6 bisphosphate Cleavage Steps 4-5 6 carbon molecule broken into two 3 carbon molecules of glyceraldehyde-3-phosphate Energy liberation Steps 6-10 Two glyceraldehyde-3-phosphate molecules broken down into two pyruvate molecules producing 2 NADH and 4 ATP Net yield in ATP of 2 Stage 2: Breakdown of pyruvate to an acetyl group In eukaryotes, pyruvate in transported to the mitochondrial matrix Broken down by pyruvate dehydrogenase Molecule of CO2 removed from each pyruvate Remaining acetyl group attached to CoA to make acetyl CoA 1 NADH is made for each pyruvate Stage 3: Citric acid cycle Metabolic cycle Particular molecules enter while other leave, involving a series of organic molecules regenerated with each cycle Acetyl is removed from Acetyl CoA and attached to oxaloacetate to form citrate or citric acid Series of steps releases 2CO2, 1ATP, 3NADH, and 1 FADH2 Oxaloacetate is regenerated to start the cycle again Stage 4: Oxidative phosphorylation High energy electrons removed from NADH and FADH2 to make ATP Typically requires oxygen Oxidative process involves electron transport chain Phosphorylation occurs by ATP synthase Group of protein complexes and small organic molecules embedded in the inner mitochondrial membrane Can accept and donate electrons in a linear manner in a series of redox reactions Movement of electrons generates H+ electrochemical gradient/ proton-motive force Excess of positive charge outside of matrix Electron transport chain Free energy change Movement from NADH to O2 is a very negative free energy change Spontaneous in forward direction Highly exergonic Some energy used to pump H+ across inner mitochondrial membrane and create H+ electrochemical gradient ATP synthase Enzyme harnesses free energy as H+ flow through membrane embedded region Energy conversion- H+ electrochemical gradient or proton motive force converted to chemical bond energy in ATP Racker and Stoeckenius confirmed ATP uses an H+ electrochemical gradient Rotary machine that makes ATP as it spins Yoshida and Kinosita demonstrated that the subunit of the ATP synthase spins Masasuke Yoshida, Kazuhiko Kinosita, and colleagues set out to experimentally visualize the rotary nature of the ATP synthase Released membrane embedded portion and adhered it to a slide Visualize g subunit using fluorescence Added ATP to make reaction run backward Rotated counterclockwise to hydrolyze ATP Rotate clockwise to synthesize ATP Other organic molecules Other carbohydrates, proteins and fats also used for energy Enter into glycolysis or citric acid cycle at different points Utilizing the same pathways for breakdown increases efficiency Metabolism can also be used to make other molecules (anabolism) Anaerobic metabolism For environments that lack oxygen or during oxygen deficits 2 strategies Use substance other than O2 as final electron acceptor in electron transport chain If confined to using O2, carry out glycolysis only Pyruvate converted to lactate or lactic acid in muscles or ethanol is yeast Fermentation produces far less ATP Secondary Metabolism Primary metabolism- essential for cell structure and function Secondary metabolism- synthesis of secondary metabolites that are not necessary for cell structure and growth Secondary metabolites unique to a species or group Roles in defense, attraction, protection, competition 4 categories Phenolics Antioxidants with intense flavors and smells Alkaloids Bitter-tasting molecules for defense Terpenoids Intense smells and colors Polyketides Chemical weapons ...
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- Spring '08