Alexis Omar López

6 Energy and Metabolism

Lecture Outline

I. Biological work requires energyA. Energy is the capacity to do workB. Energy is expressed in units of work (kilojoules) or heat energy

(kilocalories)C. Organisms carry out conversions between potential energy and

kinetic energy1. Potential energy is stored energy; kinetic energy is energy of

motion

II. Two laws of thermodynamics govern energy transformationsA. The total energy in the universe does not change

1. First law of thermodynamics = law of conservation of energya) The total energy of a closed system remains constantb) Biological systems are open systemsc) Organisms can only convert energy to other forms

B. The entropy of the universe is increasing1. In most energy conversions, energy is lost as heat2. Second law of thermodynamics

a) As energy is converted from one form to another, the total amount of entropy (S) increases in the universe

b) Entropy may be defined as an increase in disorder or randomness

III. Metabolic reactions involve energy transformationsA. Metabolism of an organism includes anabolic and catabolic

pathways1. Anabolism includes synthetic pathways2. Catabolism includes reactions in which molecules are degraded

B. Enthalpy is the total potential energy of a system1. Bond energy is the amount of energy required to break a

molecular bond2. Enthalpy (H) is the total bond energy

C. Free energy is energy that is available to do cellular work, and hence is of biological interest

1. Free energy is the total energy available to do work and is expressed in kilojoules or kilocalories as G

2. G is inversely related to entropy3. H = G + TS

a) H is enthalpy; T is the absolute temperature (°K); S is entropy

b) If T is constant, E = free energy + entropy (unusable energy)c) As the temperature increases, entropy increases

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D. Chemical reactions involve changes in free energy1. The Greek letter delta () is used to refer to changes between

initial and final states2. G = H – TS

E. Free energy decreases during an exergonic reaction1. The total free energy of a system in the final state is less than the

total free energy in the original state2. Exergonic reactions have a –G3. Exergonic reactions release energy and are spontaneous

reactions4. These reactions still require activation energies

F. Free energy increases during an endergonic reaction1. An endergonic reaction is one in which there is a gain of free

energy2. Diffusion is an exergonic process

G. Free energy changes depend on the concentrations of reactants and products

1. Reactions proceed to a state of dynamic equilibrium2. Cellular reactions are typically never at equilibrium

H. Cells drive endergonic reactions by coupling them to exergonic reactions

1. Coupled reactions may drive thermodynamically unfavorable endergonic reactions

2. In energy coupling, one must look at the total G of both reactions

3. Endergonic reactions are coupled with exergonic reactionsa) The breakdown of ATP is a good example

IV. Adenosine triphosphate (ATP) is the energy currency of the cellA. The ATP molecule has three main parts

1. Nitrogen-containing base (adenine)2. Ribose, a pentose3. 3 phosphate groups in a series

B. ATP donates energy through the transfer of a phosphate group1. The bonds linking the 3 phosphate groups may be broken by

hydrolysisa) These reactions have a large –G (–7.6 kcal/mole)b) ATP is hydrolyzed to form ADP + Pi

c) This reaction may be coupled with an endergonic reactiond) Phosphorylation reactions occur when the phosphate group

is transferred to another moleculeC. ATP links exergonic and endergonic reactions

1. Phosphorylation is coupled to endergonic processesD. The cell maintains a very high ratio of ATP to ADP

1. The actual free energy of ATP under cellular conditions is -10 to -12 kcal/mole

2. The ratio of ATP to ADP is about 10:13. ATP cannot be stockpiled

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4. A resting human uses about 450 kg (100 lbs.) per day of ATP, but the amount present at any given time in the entire body is less than 1 gram (0.03 oz.)

5. Each second, approximately 10 million molecules of ATP are made and recycled in every cell

V. Cells transfer energy by redox reactions A. Oxidation is the loss of electrons; reduction is the gain of electrons

1. Oxidation involves the loss of energy; reduction is the gain of energy

2. These processes occur simultaneously, called redox reactionsB. Most electron carriers carry hydrogen atoms

1. Electron carriers transfer energy to an acceptor2. Electrons lose energy as they are transferred between acceptors3. Nicotinamide adenine dinucleotide (NAD+) is a common

hydrogen acceptor in respiratory and photosynthetic pathways4. Nicotine adenine dinucleotide phosphate (NADP+) is involved in

photosynthesis5. Flavin adenine dinucleotide (FAD) is involved in cellular

respiration6. Cytochromes are proteins containing iron and are also electron

carriers

VI. Enzymes are chemical regulatorsA. Nearly all enzymes are protein catalysts that speed the rate of

chemical reactions1. Not all organic catalysts are enzymes; some nucleotide-based

molecules function as enzymes as well2. Enzymes that catalyze specific chemical steps allow for cellular

controlB. All reactions have a required energy of activation (EA)C. An enzyme lowers the activation energy needed to initiate a

chemical reaction1. Enzymatic action has no effect on the overall free energy

D. An enzyme works by forming an enzyme-substrate (ES) complex1. Enzymes have at least one 3-dimensional area, known as the

active site or active sites2. The current model of enzymatic action is the induced-fit model

a) The active site is not rigid, but binding of the substrate to the active site involves conformational changes in both the enzyme and (typically) the substrate

b) The enzyme and substrate form an ES or enzyme-substrate complex

c) After binding to the substrate, the product is released, and the enzyme can be reused

E. Most enzyme names end in -ase1. Sucrase is an enzyme that reacts with sucrose2. Other enzymes have older names ending in –zyme3. Enzyme names such as pepsin and trypsin give no clues as to

their functionF. Enzymes are specific

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1. Because of the binding at the active site, the substrate is very specific to the enzyme

2. Not all enzymes are specific (e.g. lipases react with a variety of fats)

G. Many enzymes require cofactors1. Many enzymes are composed of a protein (an apoenzyme) and a

cofactor2. Inorganic cofactors include elements such as Mg, Ca, Fe, Cu, Zn,

and Mn3. Organic non-protein cofactors bind with the enzyme, forming a

coenzymea) Coenzymes are typically transfer agentsb) NADH, NADPH, and FADH2 are coenzymesc) ATP is a coenzymed) Coenzyme A is important in the transfer of groups derived

from organic acids4. Most vitamins are coenzymes or are parts of coenzymes

H. Enzymes are most effective at optimal conditions1. Each enzyme has an optimal temperature

a) In the human body, the optimal temperature for enzymes is near body temperature

b) High temperatures denature protein enzymes2. Each enzyme has an optimal pH

a) In the human body, the optimal pH for most enzymes is between pH 6 and 8

b) A suboptimal pH denatures protein enzymesI. Enzymes are organized into teams in metabolic pathways

1. Enzymes often work in sequencesJ. The cell regulates enzymatic activity

1. The amount of enzyme produced can control the rate of a reaction; this is typically accomplished by feedback mechanisms

2. Allosteric enzymes have a receptor site to which allosteric regulators binda) Some allosteric regulators are inhibitors of the enzyme;

others are activators of the enzymeK. Enzymes can be inhibited by certain chemical agents

1. Reversible inhibition may be competitive or noncompetitivea) Reversible competitive inhibition involves a molecule that is

structurally similar to the normal substrate(1) The inhibitor binds to the active site temporarily

b) Reversible noncompetitive inhibition involves binding at a site other than the active site temporarily (similar to allosteric inhibition)

2. Irreversible inhibitors permanently inactivate the enzymea) Examples: nerve gases (cyanide), heavy metals (Hg, Pb)b) Some drugs are enzyme inhibitors

(1) Examples include sulfa drugs and penicillin, which are used to combat bacterial infections

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Research and Discussion Topics

Insecticides such as Malathion and Parathion act by inhibiting biologically important enzymes. Investigate the effects of these chemicals on mammalian life.

Many household items use enzymes. Discuss the action of proteolytic enzymes in laundry detergents and stain removers, as well as meat tenderizers.

Most animals digest their food by secreting digestive enzymes into a digestive tube (intestine). Several animals digest, or at least predigest, their food outside of their bodies. Research and describe the feeding mechanism of spiders. Starfish feed on bivalves, like mussels, by secreting digestive enzymes into the mussel to kill and predigest it. Describe the mechanism.