chapter 6: metabolism - energy and...

BIOLS102 Dr. Tariq Alalwan

Chapter 6: Metabolism - Energy & Enzymes 1

Biology, 10e

Sylvia S. Mader

Lectures by Tariq Alalwan, Ph.D.

Learning Objectives

Define energy, emphasizing how it is related to work and to heat

State and apply two energy laws to energy t f ti transformations.

Give reasons why ATP is called the energy currency in cells.

Give examples to show how ATP hydrolysis is coupled to energy‐requiring reactions.

Describe a metabolic pathway and how they function.

Learning Objectives (cont.)

Explain how enzymes lower the energy of activation and speed chemical reactions.

List conditions that affect enzyme speed.

Explain how a cell can use enzyme inhibition to regulate metabolism.

Describe the difference between oxidation and reduction reactions.



Forms of Energy

Energy – the capacity to do work; cells continually use energy to develop, grow, repair, reproduce, etc.

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 (cont.)

Potential – stored energy or energy that is "waiting to happen"

Chemical ‐ energy stored in the bonds of atoms and l l (i f d )molecules (i.e. foods)

Nuclear – energy stored in the nucleus of an atom (the energy that holds the nucleus together)

Stored mechanical energy (e.g. compressed springs & muscle movement)

Laws of Thermodynamics

Thermodynamics ‐ the study of energy and its transformations governs all activities of the universe

First law (conservation of energy):

Energy cannot be created or destroyed

Energy CAN be transferred or converted from one form to another



Laws of Thermodynamics (cont.)

Second law:

Energy cannot be changed from one form to another without a loss of usable energy

Heat is a form of energy that dissipates into the environment; heat can never be converted back to another form of energy

Carbohydrate Synthesis

Carbohydrate Metabolism

Cells and Entropy

Entropy:

A measure of the disorder, or randomness of energy

Organized, usable energy has a low entropyg gy py

Disorganized energy, such as heat, has a high entropy

No energy conversion is ever 100% efficient, because much energy is dispersed as heat, increasing entropy

The total entropy of the universe always increases over time

Examples – car rust, dead trees decay, buildings collapse

Cells and Entropy



Energy Flow in Living Things

Metabolic Reactions andEnergy Transformations

Metabolism:

Sum of all the biochemical reactions in a cell

Reactants participate in a reaction and products form as result of a reaction

Two main types of metabolism:

Anabolism – complex molecules are synthesized from simpler substances (requires energy)

Catabolism – larger molecules are broken down into smaller ones (releases energy)

Exergonic Reactions

Free energy (G)– the amount of energy that is free to do work after a chemical reaction

Change in free energy is noted as (∆G); a negative ∆G Change in free energy is noted as (∆G); a negative ∆G means that products have less free energy than reactants; the reaction occurs spontaneously from higher to lower free energy

Exergonic reactions have a negative ∆G and energy is released



Endergonic Reactions

Endergonic reaction – a reaction in which there is a gain of free energy

ΔG has a positive value (the free energy of the products ΔG has a positive value (the free energy of the products is greater than the free energy of the reactants)

Requires an input of energy from the environment

ATP: Energy for Cells

Adenosine triphosphate (ATP)

High energy compound used to drive metabolic reactions

Constantly being generated ADP + Pi + energy → ATP

Nucleotide consisting of three parts: adenine (nitrogen‐containing organic base); ribose (five‐carbon sugar); and three phosphate groups

Hydrolysis of ATP, an exergonic reaction, yields ADP and inorganic phosphate

ATP

Biological advantages to the use of ATP

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

Wh ATP b ADP P th t f When ATP becomes ADP + Pi the amount of energy released is sufficient for biological function with little waste of energy (~ 7.3 kcal per mole)

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



The ATP Cycle

ATP and Coupled Reactions Coupled reactions

Occur in the same place, at the same time

Energy released by an exergonic reaction is captured in ATPATP

That ATP is used to drive an endergonic reaction

ATP breakdown is often coupled to cellular reactions that require energy

ATP supply is maintained by breakdown of glucose during cellular respiration

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 Pi 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



ATP Hydrolysis and Muscle Contraction

Metabolic Pathways and Enzymes

Enzymes – protein catalysts that speed chemical reactions without the enzyme being affected by the reaction

Ribozymes – RNA molecules that function as biological Ribozymes RNA molecules that function as biological catalysts; involved in RNA and protein synthesis

The reactants of an enzymatically accelerated reaction are called substrates

Each enzyme accelerates a specific reaction

Each reaction requires a unique and specific enzyme

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

Metabolic Pathways

Reactions usually occur in an orderly sequence

Products of an earlier reaction become reactants of a later reaction

Such linked reactions form a metabolic pathway Such linked reactions form a metabolic pathway

Begins with a particular reactant,

Proceeds through several intermediates, and

Terminates with a particular end product

“A” is InitialReactant

“G” is EndProductIntermediates

E1 E2 E3 E4 E5 E6

A→ B → C → D → E → F → G



Energy of Activation

Molecules often do not react with each other unless activated in some way

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

Energy of activation (Ea) ‐ the energy that must be added to cause molecules to react

Enzyme Operation:

Enzymes operate by lowering the energy of activation

Brings the substrates into contact with one another and even participates in the reaction

Does not get consumed by the reaction nor does it alter the equilibrium of the reaction, thus it remains intact

Energy of Activation (cont.)

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 (ES) 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 (cont.)

Induced fit model

The active site complexes with the substrates by weak interactions, such as hydrogen bonds, etc hydrogen bonds, etc

Causes active site to change shape

Shape change forces substrates together, initiating bond

Active site returns to its original state after the product(s) is released

Some enzymes (e.g., trypsin) actually participate in the reaction

Synthesis vs. Degradation

Synthesis (Anabolism):

Enzyme complexes with two substrate molecules

Substrates are joined together and released as single product molecule

Degradation (Catabolism):

Enzyme complexes with a single substrate molecule

Substrate is broken apart into two product molecules

Only a small amount of enzyme is needed in a cell because enzymes are not consumed during catalysis

Enzymatic action



Regulation of Metabolism

A particular reactant(s) may produce more than one type of product(s)

Presence or absence of enzyme determines which ti t k lreaction takes place

If reactants can form more than one product, the enzymes present determine which product is formed

Enzymes are sometimes named for their substrates, and usually end in ase

Examples – lipase (lipid), cellulase (cellulose), lactase (lactose), etc.

Factors Affecting Enzyme Activity

Substrate concentration

Enzyme activity increases with substrate concentration

More collisions between substrate 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 (cont.)

Temperature

Enzyme activity increases with temperature

Warmer temperatures cause more effective collisions between enzyme and substratey

Hot temperatures destroy enzymes, how?

Denaturation – enzyme’s shape changes and it can no longer bind to its substrate efficiently

However, there are exceptions such as

Some prokaryotes living in hot springs

Coat color pattern in Siamese cats




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 → alters intermolecular bonds, and/or hydrogen bonds → alters 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 pH on Enzymatic Reactions


Enzyme Cofactors

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

Cofactor – an inorganic ion (e.g., zinc, copper and iron)Cofactor an inorganic ion (e.g., zinc, copper and iron)

Coenzyme – a nonprotein organic molecule

Many of the coenzymes are derived from vitamins

Vitamins – small organic molecules required in trace amounts for synthesis of coenzymes

Enzyme activity decreases if vitamin is not available (i.e. vitamin‐deficient disorder)

Examples : Niacin (pellagra) & riboflavin deficiency



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 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 (cont.) Noncompetitive inhibition

Noncompetitive inhibitors are substances which when added to the enzyme changes 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



Enzyme Inhibition (cont.)

The end product of a pathway can inhibit the pathway’s first enzyme

The metabolic pathway is shut 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

Redox Reactions

Energy is transferred through the transfer of electrons from one substance to another (redox reactions)

One substance loses electrons (oxidation)

One substance gains electrons (reduction)

Redox reactions often occur in a series of electron transfers

Important in cellular respiration, photosynthesis, and many other chemical processes

Electron Transfer in Redox Reactions

chapter 6: metabolism - energy and...

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