Essential Concepts of Metabolism Chapter 5 Microbiology 130

Metabolism: An Overview Anabolism Catabolism- - electron transfer Oxydation- Reduction-

How do microbes obtain energy? Autotrophs- self feeders, use CO2 to sysnthesis organic compounds - Photoautotrophs- use sunlight for energy - Chemoautotrophs- use inorganics such as sulfides and nitrites for energy Heterotrophs- other feeders, use organic molecules, - Photoheterotrophs-obtain chemical energy from light - Chemoheterotrophs- obtain energy from ready made organic compounds

What Is Energy? Capacity to do work Forms of energy Potential energy, Kinetic energy Chemical energy What Can Cells Do With Energy? Cells use energy for: Chemical work Mechanical work Electrochemical work

One-Way Flow of Energy The sun is life’s primary energy source Producers trap energy from the sun and convert it into chemical bond energy All organisms use the energy stored in the bonds of organic compounds to do work

Endergonic Reactions Energy input required Product has more energy than starting substances glucose, a high energy product + 6O2 ENERGY IN 6 6 low energy starting substances 6 6

Exergonic Reactions Energy is released Products have less energy than starting substance glucose, a high energy starting substance + 6O2 ENERGY OUT low energy products 6 6

three phosphate groups The Role of ATP Cells “earn” ATP in exergonic reactions Cells “spend” ATP in endergonic reactions base three phosphate groups sugar

ATP/ADP Cycle When adenosine triphosphate (ATP) gives up a phosphate group, adenosine diphosphate (ADP) forms ATP can re-form when ADP binds to inorganic phosphate or to a phosphate group that was split from a different molecule Regenerating ATP by this ATP/ADP cycle helps drive most metabolic reactions

Participants in Metabolic Reactions Energy carriers Enzymes Cofactors Transport proteins Reactants Intermediates Products

Chemical Equilibrium At equilibrium, the energy in the reactants equals that in the products Product and reactant molecules usually differ in energy content Therefore, at equilibrium, the amount of reactant almost never equals the amount of product

Chemical Equilibrium

Redox Reactions Cells release energy efficiently by electron transfers, or oxidation-reduction reactions (“redox” reactions) One molecule gives up electrons (is oxidized) and another gains them (is reduced) Hydrogen atoms are commonly released at the same time, thus becoming H+

Electron Transfer Chains Arrangement of enzymes, coenzymes, at cell membrane As one molecule is oxidized, next is reduced Function in aerobic respiration and photosynthesis

Uncontrolled vs. Controlled Energy Release H2 1/2 O2 Explosive release of energy as heat that cannot be harnessed for cellular work H2O

Metabolic Pathways Defined as enzyme-mediated sequences of reactions in cells Biosynthetic (anabolic) – ex: photosynthesis Degradative (catabolic) – ex: aerobic respiration

Enzyme Structure and Function Enzymes are catalytic molecules They speed the rate at which reactions approach equilibrium

Four Features of Enzymes 1) Enzymes do not make anything happen that could not happen on its own. They just make it happen much faster. 2) Reactions do not alter or use up enzyme molecules.

Four Features of Enzymes 3) The same enzyme usually works for both the forward and reverse reactions. 4) Each type of enzyme recognizes and binds to only certain substrates.

Activation Energy For a reaction to occur, an energy barrier must be surmounted Enzymes make the energy barrier smaller activation energy without enzyme starting substance activation energy with enzyme energy released by the reaction products

How Catalase Works

Induced-Fit Model Substrate molecules are brought together Substrates are oriented in ways that favor reaction Active sites may promote acid-base reactions Active sites may shut out water

Factors Influencing Enzyme Activity Temperature pH Salt concentration Allosteric regulators Coenzymes and cofactors

Enzyme Helpers Cofactors Coenzymes Metal ions NAD+, NADP+, FAD Accept electrons and hydrogen ions; transfer them within cell Derived from vitamins Metal ions Ferrous iron in cytochromes

Allosteric Activation allosteric activator enzyme active site vacant allosteric binding site active site cannot bind substrate active site altered, can bind substrate

Allosteric Inhibition allosteric inhibitor allosteric binding site vacant; active site can bind substrate active site altered, can’t bind substrate

END PRODUCT (tryptophan) Feedback Inhibition enzyme 2 enzyme 3 enzyme 4 enzyme 5 A cellular change, caused by a specific activity, shuts down the activity that brought it about enzyme 1 END PRODUCT (tryptophan) SUBSTRATE

Effect of Temperature Small increase in temperature increases molecular collisions, reaction rates High temperatures disrupt bonds and destroy the shape of active site

Effect of pH

Producing the Universal Currency of Life All energy-releasing pathways require characteristic starting materials yield predictable products and by-products produce ATP

Main Types of Energy-Releasing Pathways Anaerobic pathways Evolved first Don’t require oxygen Start with glycolysis in cytoplasm Completed in cytoplasm Aerobic pathways Evolved later Require oxygen Completed in mitochondria

Energy-Releasing Pathways

Main Pathways Start with Glycolysis Glycolysis occurs in cytoplasm Reactions are catalyzed by enzymes Glucose 2 Pyruvate (six carbons) (three carbons)

The Role of Coenzymes NAD+ and FAD accept electrons and hydrogen from intermediates during the first two stages When reduced, they are NADH and FADH2 In the third stage, these coenzymes deliver the electrons and hydrogen to the transfer chain

Anaerobic Pathways Do not use oxygen Produce less ATP than aerobic pathways Two types of fermentation pathways Alcoholic fermentation Lactate fermentation

Fermentation Pathways Begin with glycolysis Do not break glucose down completely to carbon dioxide and water Yield only the 2 ATP from glycolysis Steps that follow glycolysis serve only to regenerate NAD+

Alcoholic Fermentation

Yeasts Single-celled fungi Carry out alcoholic fermentation Saccharomyces cerevisiae Baker’s yeast Carbon dioxide makes bread dough rise Saccharomyces ellipsoideus Used to make beer and wine

Lactate Fermentation Carried out by certain bacteria Electron transfer chain is in bacterial plasma membrane Final electron acceptor is compound from environment (such as nitrate), not oxygen ATP yield is low

Lactate Fermentation

Carbohydrate Breakdown and Storage Glucose is absorbed into blood Pancreas releases insulin Insulin stimulates glucose uptake by cells Cells convert glucose to glucose-6-phosphate This traps glucose in cytoplasm where it can be used for glycolysis

Glycolysis Occurs in Two Stages Energy-requiring steps ATP energy activates glucose and its six-carbon derivatives Energy-releasing steps The products of the first part are split into three-carbon pyruvate molecules ATP and NADH form

Energy-Requiring Steps

Energy-Releasing Steps

Net Energy Yield from Glycolysis Energy requiring steps: 2 ATP invested Energy releasing steps: 2 NADH formed 4 ATP formed Net yield is 2 ATP and 2 NADH

Overview of Aerobic Respiration C6H1206 + 6O2  6CO2 + 6H20 glucose oxygen carbon water dioxide

Overview of Aerobic Respiration

Second-Stage Reactions Occur in the mitochondria Pyruvate is broken down to carbon dioxide More ATP is formed More coenzymes are reduced

Two Parts of Second Stage Preparatory reactions Pyruvate is oxidized into two-carbon acetyl units and carbon dioxide NAD+ is reduced Krebs cycle The acetyl units are oxidized to carbon dioxide NAD+ and FAD are reduced

Preparatory Reactions pyruvate + coenzyme A + NAD+ acetyl-CoA + NADH + CO2 One of the carbons from pyruvate is released in CO2 Two carbons are attached to coenzyme A and continue on to the Krebs cycle

Using Glycogen When blood levels of glucose decline, pancreas releases glucagon Glucagon stimulates liver cells to convert glycogen back to glucose and to release it to the blood (Muscle cells do not release their stored glycogen)

Energy Reserves Glycogen makes up only about 1 percent of the body’s energy reserves Proteins make up 21 percent of energy reserves Fat makes up the bulk of reserves (78 percent)

Energy from Fats Most stored fats are triglycerides Triglycerides are broken down to glycerol and fatty acids Glycerol is converted to PGAL, an intermediate of glycolysis Fatty acids are broken down and converted to acetyl-CoA, which enters Krebs cycle

Energy from Proteins Proteins are broken down to amino acids Amino acids are broken apart Amino group is removed, ammonia forms, is converted to urea and excreted Carbon backbones can enter the Krebs cycle or its preparatory reactions

Reaction Sites

Evolution of Metabolic Pathways When life originated, atmosphere had little oxygen Earliest organisms used anaerobic pathways Later, noncyclic pathway of photosynthesis increased atmospheric oxygen Cells arose that used oxygen as final acceptor in electron transfer

Processes Are Linked Aerobic Respiration Reactants Sugar Oxygen Products Carbon dioxide Water Photosynthesis Processes Are Linked

Life Is System of Prolonging Order Powered by energy inputs from sun, life continues onward through reproduction Following instructions in DNA, energy and materials can be organized, generation after generation With death, molecules are released and may be cycled as raw material for next generation