Chapter 6: Metabolism and Energy
Key Terms: catabolic vs. anabolic, potential vs. kinetic energy, first and second law of thermodynamics,
free energy, exergonic vs. endergonic reactions, catalyst, activation energy, substrate, product,
competitive inhibition, alosteric regulation, feedback inhibition, cooperativity.
INTRODUCTION
Bioenergetics: How Energy Behaves in Living Systems
Metabolism: The Sum of All Chemical Reactions Carried Out by an Organism
Anabolism: expend energy to make or transform bonds
Catabolism: harvest energy when bonds broken
WHAT IS ENERGY?
Energy: The Ability to Do Work
Exists in two states
Kinetic energy: energy of motion
Potential energy: stored energy that has the capacity of moving
Living organisms transform potential energy into kinetic energy
Thermodynamics: The Study of Energy
Energy is readily measured by its conversion into heat
Unit of heat: 1000 calories = 1 kilocalorie (kcal)
THE LAWS OF THERMODYNAMICS DESCRIBE HOW ENERGY CHANGES
First Law of Thermodynamics
Energy cannot be created or destroyed but it can be changed into another form
Total amount of energy in the universe remains constant
Animals transfer food potential energy into their own chemical bonds
Energy is not lost but may be changed into other forms
Converted to kinetic energy, light, electricity
Also dissipated as heat
Second Law of Thermodynamics
All objects tend to become less ordered, disorder is increasing
Entropy : measure of disorder of a system = S
FREE ENERGY
Bonds Between Atoms Hold Molecules Together
Free energy: energy available to break and form chemical bonds = G
Enthalpy: energy within a cell that is available to do work = H
Temperature = T
Free Energy = Ordering Influences - Disordering Influences
G = H - TS
Change in free energy: ΔG = ΔH - TΔS
Negative ΔG: Exergonic reactions
Products contain less free energy or more disorder than reactants
Reactions occur spontaneously, release excess usable free energy
Positive ΔG: Endergonic Reactions
Products contain more free energy than the reactants
Reactions do not occur spontaneously, requires input of energy
ACTIVATION ENERGY: PREPARING MOLECULES FOR REACTION
Reactions Require an Input of Energy to Get Started
Must break chemical bonds before new bonds can be created
Activation energy: required to destabilize existing chemical bonds
Catalysis: Stressing Chemical Bonds Making Them Easier to Break
Catalyst: substance that carries out catalysis
Cannot violate basic laws of thermodynamics
Accelerates reaction in both forward and reverse directions
Direction of reaction dependent on free energy
ENZYMES: BIOLOGICAL CATALYSTS
Enzymes: Agents That Carry Out Catalysis in Living Organisms
Are generally proteins with specialized shapes
Permit temporary associations with the molecules that are reacting
Lower activation energy required for new bonds to form
Bring two substrates together in the correct orientation
Stress particular bonds of a substrate
Example: formation of carbonic acid from carbon dioxide and water
Reaction proceeds in either direction
Reaction is slow because of a great activation energy
Carbonic anhydrase: enzyme that speeds the reaction
Enzymes given the name of their substrate with the ending -ase
Thousands of Different Enzymes Exist
Each enzyme catalyzes a different reaction
Different cells contain different complements of enzymes
How Enzymes Work
Globular protein enzymes possess surface clefts called active sites
Enzymes are specific in their choice of substrate
Amino acid side groups of enzyme react with substrate
The substrate must fit precisely into the active site
Induced fit: binding may induce shape adjustments in the protein
Substrate itself may act as activator
FACTORS AFFECTING ENZYME ACTIVITY
Temperature
Disrupts hydrogen bonds and hydrophobic interactions
Alters protein shape and peptide arms
Enzymes function best within narrow range, temperature optimum
Hot spring bacteria proteins have strong bonds in peptide arms
pH
Hydrogen ion concentration
Disrupts bonds between oppositely charged amino acids
With more H+ ions fewer negative, more positive charges occur
Most enzymes have a pH optimum
Enzymes that function in acids retain 3-D shape when many H+ present
Inhibitors and Activators
Activity dependent on presence of specific substances
Substances bind to enzyme and change its shape
When shape changes activity is altered
Inhibitors change shape and shut off activity
Competitive inhibitors bind at same site as substrate
Non-competitive inhibitors bind at different site
Feedback inhibition: end product inhibits reaction early in pathway
Allosteric site: region where non-competitive inhibitor binds
Allosteric inhibitor binds to allosteric site to reduce enzyme activity
Activators bind to allosteric sites
Keep enzymes in active configuration
Increase enzyme activity
Coenzymes and Other Cofactors
Cofactors
Additional components that aid enzyme action
Many metallic trace elements are cofactors
Coenzyme
Nonprotein organic molecule functions as cofactor , include vitamins
Serve as acceptors for electron pairs in redox reactions, shuttle energy
Example: nicotinamide adenine dinucleotide (NAD+)
Important biological hydrogen acceptor
NAD+ acquires an electron and hydrogen to become reduced NADH
NADH carries energy of electron and hydrogen around in cells
ATP: THE ENERGY CURRENCY OF LIFE
Adenosine Triphosphate (ATP) Is the Chief Energy Currency of All Cells
ATP Molecule Composed of Three Subunits
Five-carbon ribose sugar serves as the backbone
Adenine composed of two C-N rings attaches to the ribose
Nitrogen has unshared electrons
Weakly attracts hydrogen atoms
Called a nitrogenous base (one of four in DNA)
Triphosphate group attaches to the ribose
Covalent bonds linking phosphates are high-energy
Bonds are readily broken and energy transferred
ATP = ADP + Pi + 7.3 kcal/mole
Adenosine diphosphate = ADP
Pi is inorganic phosphate group
Cells Use ATP to Drive Endergonic Reactions
Products possess more energy than the reactants
Can power cell activities
Terminal high-energy bond is more exergonic than others
Activation energy is usually less than 7 kcal/mole
Cells contain a pool of ATP, ADP and phosphate
Cells do not stockpile ATP but create it as needed
ATP constantly recycled
BIOCHEMICAL PATHWAYS: THE ORGANIZATIONAL UNITS OF METABOLISM
Reactions in Biological Systems Occur in Sequence
Product of one reaction becomes substrate for another
Organized units of metabolism
Location of enzymes helps map out model of pathway
How Biochemical Pathways Evolved
First primitive biochemical processes
Energy-rich molecules scavenged from the environment
Molecules existed in the existing organic soup
Catalyzed reactions were simple one-step processes
As energy-rich molecules were depleted only those cells that could
synthesize energy-rich molecules could survive
Energy utilizing reaction became coupled to energy-producing reaction
Evolution of pathways works backwards
Occur one step at a time
Final reactions generally evolve first, initial reaction evolves last
How Biochemical Pathways Are Regulated
Output of pathways must be controlled
Primitive organisms evolved feedback mechanisms
Enzymes have secondary binding sites, bind nonsubstrate molecules
Binding alters shape of enzyme which changes its activity
Activity can be increased as well as decreased
Enzymes controlled in this manner are allosteric
Feedback inhibition
Second binding site binds with end product
Prevents enzyme activity
Lack of enzyme activity shuts down pathway
Stops production of product