Chapter 6: Metabolism – Energy and Enzymes (Outline)

Forms of Energy Two Laws of Thermodynamics

Cells and Entropy

Metabolic Reactions ATP – Energy for Cells

Metabolic Pathways and Enzymes Energy of Activation

Enzyme-Substrate Complex

Redox reactions

Forms of Energy

The capability to do work or produce an effect and is divided into two types

Kinetic- the energy of motion, such as waves, electrons, atoms, molecules, substances and objects Mechanical energy of motion

Electrical (e.g. lightning)

Thermal (i.e. geothermal energy)

Radiant (e.g. visible light, x-rays, gamma rays and radio waves)

Forms of Energy

Potential: Stored energy or energy that is "waiting to

happen" Chemical - energy stored in the bonds of

atoms and molecules Nuclear – energy stored in the nucleus of an

atom (the energy that holds the nucleus together)

Stored mechanical energy (e.g. compressed springs)

Laws of Thermodynamics

First law: Law of conservation of energy

Energy cannot be created or destroyed, but

Energy CAN be changed from one form to another

Laws of Thermodynamics

Second law: Energy cannot be changed from one form to

another without a loss of usable energy

Therefore, no process requiring a conversion of energy is ever 100% efficient

Carbohydrate Synthesis

Carbohydrate Metabolism

Cells and Entropy

Law of entropy: The spontaneous movement from order to

disorder (randomness)

Waste energy goes to increase disorder

Entropy – a measure of the degree of a system’s disorder which increases over time

Cars rust, dead trees decay, buildings collapse; all these are examples of entropy in action

Cells and Entropy

Energy Flow in Living Things

Metabolic Reactions andEnergy Transformations

Metabolism:

Sum of cellular chemical reactions in a cell

Reactants participate in a reaction

Products form as result of a reaction

Free energy – the amount of energy available to perform work

Concept of free energy was developed by Gibbs

For a reaction to occur spontaneously free energy must decrease - the products must have less free energy than the reactants

Metabolic Reactions &Energy Transformations Energy Changes in Metabolic Reactions

Exergonic Reactions - products have less free energy than reactants (i.e. spontaneous and releases energy) Example: ATP breakdown

Endergonic Reactions - products have more free energy than reactants (i.e. not spontaneous and requires energy) Example:

Muscle contraction

ATP: Energy for Cells Adenosine triphosphate (ATP)

High energy compound used to drive metabolic reactions Constantly being generated from adenosine diphosphate

(ADP) and a molecule of inorganic phosphate Composed of

Adenine, ribose (C5 sugar), and 3 phosphate groups Biological advantages to the use of ATP

It provides a common energy currency used in many types of reactions

When ATP becomes ADP + ℗ the amount of energy released is sufficient for biological function with little waste of energy (~ 7.3 kcal per molecule)

ATP breakdown can be coupled to endergonic reactions that prevents energy waste

The ATP Cycle

ATP and Coupled Reactions

Coupled reactions Energy released by an exergonic reaction is

captured in ATP That ATP is used to drive an endergonic reaction Occur in the same place, at the same time, why? Because the energy released by the hydrolysis of

ATP is higher than the energy consumed by the endergonic reaction

Coupled Reactions

A cell has two ways to couple ATP hydrolysis

ATP is used to energize a reactant

ATP is used change the shape of a reactant

Both can be achieved by transferring ℗ to the reactant so that the product is phosphorylated

Example: Ion movement across the plasma membrane of a

cell through carrier proteins

Attachment of amino acids to a growing polypeptide chain

Coupled Reactions – Muscle Contraction

Enzymes

Protein molecules that function as catalysts, however ribozymes are made of RNA not proteins

The reactants of an enzymatically accelerated reaction are called substrates

Each enzyme accelerates a specific reaction

Each reaction in a metabolic pathway requires a unique and specific enzyme

End product will not appear unless ALL enzymes are present and functional

E1 E2 E3 E4 E5 E6

A B C D E F G

Metabolic Pathways

Reactions usually occur in a sequence Products of an earlier reaction become reactants

of a later reaction

Such linked reactions form a metabolic pathway

Begins with a particular reactant,

Proceeds through several intermediates, and

Terminates with a particular end product

AB C D E FG“A” is InitialReactant

“G” is EndProductIntermediates

Energy of Activation Reactants are often “reluctant” to participate in

the reaction Energy must be added to at least one reactant to

initiate the reaction Energy of activation - minimum amount of energy

required to trigger a chemical reaction

Enzyme Operation: Enzymes operate by lowering the energy of

activation Accomplished by bringing the substrates into contact

with one another under mild conditions Does not get consumed by the reaction nor does it

alter the equilibrium of the reaction, thus it remains intact

Energy of activation (Ea)

Enzyme-Substrate Complex The “lock and key” model of enzyme activity

The active site is made up of amino acids and has a very specific shape

The enzyme and substrate slot together to form a complex, as a key slots into a lock

This complex reduces the activation energy for the reaction

Then the products no longer fit into the active site and are released allowing another substrate in

Enzyme-Substrate Complex

Induced fit model The active site complexes with the substrates by

weak interactions, such as hydrogen bonds, etc Causes active site to change shape Shape change forces substrates together, initiating

bond Some enzymes participate in the reaction (e.g.

Trypsin – active site contains 3 amino acids with R groups interacting with the peptide bond (to break the bond and introduce H2O

Synthesis vs. Degradation Synthesis:

Enzyme complexes with two substrate molecules

Substrates are joined together and released as single product molecule

Degradation:

Enzyme complexes with a single substrate molecule

Substrate is broken apart into two product molecules

Enzymatic action

Naming enzymes

Enzyme names usually end in ase Many enzyme names have 3 parts: (substrate

name) (type of reaction) (ase)

Substrate Enzyme

Lipid Lipase

Urea Urease

Maltose Maltase

Cellulose Cellulase

Lactose Lactase

Sucrose Sucrase

Factors Affecting Enzyme Activity

Substrate concentration Enzyme activity increases with substrate

concentration More collisions between substrate molecules and

the enzyme When the active sites of the enzyme are filled,

with increasing substrate, the enzyme’s rate of activity cannot increase any more

Amount of active enzyme can also increase or limit the rate of an enzymatic reaction

Factors Affecting Enzyme Activity

Temperature Enzyme activity increases with temperature Warmer temperatures cause more effective

collisions between enzyme and substrate However, hot temperatures destroy enzymes,

how? Denaturation – enzyme’s shape changes and it

can no longer bind its substrate efficiently However, there are exceptions such as

Some prokaryotes living in hot springs

Coat color pattern in Siamese cats

Factors Affecting Enzyme Activity

Optimal pH Most enzymes are optimized for a particular pH

Changes in pH can make and break intra- and intermolecular bonds, and/or hydrogen bonds thus changing the globular shape of the enzyme

pH change can alter the ionization of R side chains

Under extreme conditions of pH, the enzyme becomes inactive (due to altered shape)

Example: Pepsin (stomach) and trypsin (small intestine)

Effect of Temperature and pH

Enzyme Cofactors

Many enzymes require additional help in catalyzing their reaction from a coenzyme or cofactor

Cofactor is used to refer to inorganic metallic ions such as zinc, copper and iron, which is required by an enzyme

Coenzyme is a non-protein organic molecule Many of the coenzymes are derived from vitamins Enzyme activity decreases if vitamin is not available

(i.e. vitamin-deficient disorder) Niacin deficiency results in skin disease (pellagra),

while riboflavin deficiency results in cracks at the corner of mouth

Enzyme Inhibition

Competitive inhibition Substrate and the inhibitor are both able to

bind to the active site

If a similar molecule is present, it will compete with the real substrate for the active sites

Product will form only when the substrate, not the inhibitor, is at the active site

This will regulate the amount of product

Enzyme Inhibition

Noncompetitive inhibition Noncompetitive inhibitors are considered to be

substances which when added to the enzyme change the enzyme in a way that it cannot accept the substrate

The inhibitor binds to another location (allosteric site) on the enzyme and inactivates the enzyme molecule

Both competitive and noncompetitive inhibition are examples of feedback inhibition

Competitive and Noncompetitive Inhibitors

Feedback Inhibition

The end product of a pathway inhibits the pathway’s first enzyme

The metabolic pathway is shut down when the end product of the pathway is bound to an allosteric site on the first enzyme of the pathway

Normally, enzyme inhibition is reversible and the enzyme is not damaged

Oxidation and Reduction

Oxidation

The loss of one or more electrons

Reduction

The gain of one or more electrons

Reduction-oxidation (redox) reactions

Simultaneous reaction in which one molecule is oxidized and another is reduced

Redox reactions occur during photosynthesis and cellular respiration

Electron Transfer in Redox Reactions