M E TA BO L I S M A N D E N Z Y M E S

ENERGY IN LIVING SYSTEMS

METABOLISM

• The totality of an organism’s chemical reactions is called metabolism.• A cell’s metabolism is an elaborate road map of

the chemical reactions in that cell.• Metabolic pathways alter molecules in a series of

steps.

METABOLISM

• Enzymes selectively accelerate each step.• The activity of enzymes is regulated to maintain an

appropriate balance of supply and demand.

• Catabolic pathways release energy by breaking down complex molecules to simpler compounds.• This energy is stored in organic molecules until need to

do work in the cell.

• Anabolic pathways consume energy to build complicated molecules from simpler compounds.• The energy released by catabolic pathways is

used to drive anabolic pathways.

ENERGY

• Order in biological systems is maintained by constant input of free energy.

• Energy is the capacity to do work - to move matter against opposing forces.• Energy is also used to rearrange matter.

• Kinetic energy is the energy of motion.• Objects in motion, photons, and heat are examples.

• Potential energy is the energy that matter possesses because of its location or structure.• Chemical energy is a form of potential energy in molecules

because of the arrangement of atoms.

• Energy can be converted from one form to another. • We eat food, which is potential energy, and then we use the

chemical potential energy in that food to move.

ENERGYLAW OF THERMODYNAMICS

• Thermodynamics is the fundamental laws of energy that govern the universe.

1. Energy can not be created or destroyed, but can be transferred from one form to another. Energy in an isolated system is constant

2. Disorder (entropy) increases over time. 1. The greater the entropy the greater the disorder. 2. Every energy transfer or transformation increases entropy.3. Energy spontaneously tends to flow only from being concentrated in

one place to becoming diffused or dispersed and spread out. 4. The second law is blocked by the strength of chemical bonds. 5. Our bodies use second-law energy flow from the oxidation of food for

the synthesis of essential compounds and for all activity, from biochemical to muscular to mental.

6. In most energy transformations, ordered forms of energy are converted at least partly to heat.

7. Organisms take in ordered energy (light) and replace them with less ordered forms (heat.

ENTROPY

ENERGY

• Living organisms follow these laws by using energy in the form of light (in plants), or food (in animals)to drive the chemical reactions that maintain order in our bodies. • Without an input of energy, entropy is increased,

order is quickly lost, and organisms die.• The disorder in their constituent parts then

continue to increase until it reached equilibrium with the environment. • Combining the two laws, the quantity of energy is

constant, but the quality is not.

ORDER AND THE CELL

• Cells maintain order by using energy. • ATP is used to drive a reaction or create a

gradient. • Example: Sucrose into and out of the cell. Needs

a co-transporter (protons). ATP is used to pump protons out of the cell, decreasing proton entropy, and producing a gradient. Sucrose is coupled to the protons and flow back down the gradient, increasing proton entropy.

AN INTERESTING SIDE NOTE…

• The Arrow of Time• Entropy explains why time moves forward and why we

age. The universe is an isolated system and its entropy can never decrease-keeps increasing.

• Thus time is a result of the entropy of the universe moving towards its maximum state of disorder.

• Aging can be viewed as an increase in the body’s entropy.

• Death is the highest state of entropy and therefore most thermodynamically favored level of entropy.

• So much so, that no amount of energy input can prevent it.

FREE ENERGY

• Free Energy (G-”Gibbs free energy”) is the energy in a system that is able to perform work. • Systems that are high in free energy - compressed springs,

separated charges - are unstable and tend to move toward a more stable state - one with less free energy.

• Systems that tend to change spontaneously are those that have high energy, low entropy, or both.

• For our purpose free energy is the energy available to break and make bonds.

• What you need to care about is ΔG which is the difference in bond energies between reactants and products minus the degree of disorder.

• Any reaction whose products contains less G than the reactants (-ΔG) tends to occur spontaneously. Water runs down hill.

A PICTURE IS WORTH…

FREE ENERGY

• A system at equilibrium is at maximum stability.• In a chemical reaction at equilibrium, the rate of forward

and backward reactions are equal and there is no change in the concentration of products or reactants.

• At equilibrium delta G = 0 and the system can do no work.

• Movements away from equilibrium are nonspontaneous and require the addition of energy from an outside energy source (the surroundings).

TYPES OF REACTIONS

• Chemical reactions can be classified as either exergonic or endergonic based on free energy.• An exergonic reaction proceeds with a net

release of free energy and delta G is negative

EXERGONIC

• Energy is RELEASED!• The magnitude of delta G for an exergonic

reaction is the maximum amount of work the reaction can perform.• For the overall reaction of cellular respiration:

• C6H12O6 + 6O2 -> 6CO2 + 6H2O

• delta G = -686 kcal/mol• Through this reaction 686 kcal have been made available

to do work in the cell.• The products have 686 kcal less energy than the

reactants.

ENDERGONIC

• An endergonic reaction is one that absorbs free energy from its surroundings.• Endergonic reactions

store energy,• delta G is positive, and• reaction are

nonspontaneous.

ENDERGONIC

• ABSORBES energy!• If cellular respiration releases 686 kcal, then

photosynthesis, the reverse reaction, must require an equivalent investment of energy.• Delta G = + 686 kcal / mol.

• Photosynthesis is steeply endergonic, powered by the absorption of light energy.

ENERGY IN A SYSTEM

• Reactions in closed systems eventually reach equilibrium and can do no work.• A cell that has reached metabolic equilibrium has

a delta G = 0 and is dead!• Metabolic disequilibrium is one of the defining

features of life. • Cells maintain disequilibrium because they are

open with a constant flow of material in and out of the cell. • A cell continues to do work throughout its life.

• A catabolic process in a cell releases free energy in a series of reactions, not in a single step.• Some reversible reactions of respiration are

constantly “pulled” in one direction as the product of one reaction does not accumulate, but becomes the reactant in the next step.

SO…

• Sunlight provides a daily source of free energy for the photosynthetic organisms in the environment.• Nonphotosynthetic organisms depend on a

transfer of free energy from photosynthetic organisms in the form of organic molecules.

REVIEW-ENERGY INPUT AND OUTPUT

• Producers are autotrophs• Producers use free energy from the sun to create

food.• Photoautotrophs• Chemoautotrophs use chemical energy to fix carbon.

• Consumers are heterotrophs.• They feed on autotrophs.• Each tropic level has less energy. 100% goes in,

5-20% come out.

ENZYMES

• Enzymes are biological catalysts. They lower the energy barrier and therefore speed up metabolic reactions. • Catalysts are substances that can change the rate of a

reaction without being altered in the process

• Chemical reactions between molecules involve both bond breaking and bond forming. Must have an unstable transition state.• To hydrolyze sucrose, the bond between glucose and

fructose must be broken and then new bonds formed with a hydrogen ion and hydroxyl group from water.

ACTIVATION ENERGY

• The amount of energy required to make the reaction proceed is called the activation energy.

• Even in an exergonic reaction, the reactants must absorb energy from their surroundings, the free energy of activation or activation energy (EA), to break the bonds.• This energy makes the reactants unstable, increases the speed

of the reactant molecules, and creates more powerful collisions.

• In exergonic reactions, not only is the activation energy released back to the surroundings, but even more energy is released with the formation of new bonds.

ACTIVATION ENERGY

• Activation energy is the amount of energy necessary to push the reactants over an energy barrier.• At the summit the

molecules are at an unstable point, the transition state.

• The difference between free energy of the products and the free energy of the reactants is the delta G.

ACTIVATION ENERGY

• For some processes, the barrier is not high and the thermal energy provided by room temperature is sufficient to reach the transition state.• In most cases, EA is higher and a significant input

of energy is required.• A spark plug provides the energy to energize gasoline.• Without activation energy, the hydrocarbons of gasoline

are too stable to react with oxygen.

BACK TO THERMODYNAMICS

• The laws of thermodynamics would seem to favor the breakdown of proteins, DNA, and other complex molecules.• However, in the temperatures typical of the cell there is

not enough energy for a vast majority of molecules to make it over the hump of activation energy.

• Yet, a cell must be metabolically active.• Heat would speed reactions, but it would also denature

proteins and kill cells.

LOWER ACTIVATION ENERGY

• Enzyme speed reactions by lowering EA.• The transition state can then be reached even at

moderate temperatures.

• Enzymes do not change delta G.• It hastens reactions that would occur eventually.• Because enzymes

are so selective, they determine which chemical processes will occur at any time

SUBSTRATES

• A substrate is a reactant which binds to an enzyme.• When a substrate or substrates binds to an enzyme,

the enzyme catalyzes the conversion of the substrate to the product.• Sucrase is an enzyme that binds to sucrose and breaks the

disaccharide into fructose and glucose.

• They do this by orientating the substrate, or by adding charges, or otherwise inducing strain in the substrate so that bonds are destabilized and the substrate is more reactive.

LOCK AND KEY

• Older model-suggested the substrate fit perfectly and was drawn into the molecule. It has since been modified to recognize the flexibility of enzymes to the induced fit model.

INDUCED FIT MODEL

• The active site of an enzymes is typically a pocket/groove/cleft on the surface of the protein into which the substrate fits.

• The specificity of an enzyme is due to the fit between the active site and that of the substrate.

• As the substrate binds, the enzyme changes shape leading to a tighter induced fit, bringing chemical groups in position to catalyze the reaction.

ACTIVE SITE

• In most cases substrates are held in the active site by weak interactions, such as hydrogen bonds and ionic bonds. This is the enzyme-substrate complex• R groups of a few amino acids on the active site catalyze

the conversion of substrate to product.

REACTIONS

Catabolic

1. Substrate bind to the active site of the enzyme.

2. The substrate is subjected to stress which will facilitate the breaking of the bonds.

3. The substrate is cleaved and the two products are released.

Anabolic

1. The two substrates bind to the active site.

2. The substrate molecules are subjected to stress which aid in the formation of the bonds.

3. The two substrate molecules form a single product which is released to allow the enzyme to work again.

THEY RECYCLE

• A single enzyme molecule can catalyze thousands or more reactions a second.• Enzymes are unaffected by the reaction and are

reusable.• Most metabolic enzymes can catalyze a reaction

in both the forward and reverse direction.• The actual direction depends on the relative

concentrations of products and reactants.• Enzymes catalyze reactions in the direction of

equilibrium.

RATE OF REACTIONS

• The rate that a specific number of enzymes converts substrates to products depends in part on substrate concentrations.• At low substrate concentrations, an increase in

substrate speeds binding to available active sites.• However, there is a limit to how fast a reaction

can occur.• At some substrate concentrations, the active sites

on all enzymes are engaged, called enzyme saturation.• The only way to increase productivity at this point

is to add more enzyme molecules.

ENZYMES ARE SENSITIVE

• Enzymes can be affected by several factors such as temperature and pH. • In most plants and animals there is little activity

at low temperature. As the temperature increases, enzyme activity increases. • Until it reaches a temperature that is high enough

to damage the structure of the enzyme and denatures it. • Extreme pH will do this as well. • Poisons