Energy, Enzymes, and Biological ReactionsChapter 4
Energy
Definition: The Capacity to do work
Types of Energy: Potential: Stored energy, measured as a
capacity to do work. example: stretched spring Kinetic: Energy of motion, released potential
energy. example: releasing of a stretched spring Thermal: Energy released as heat Chemical: Potential energy stored in molecules.
Measured as Kilocalories (Kcal) aka Calories (C)(1 calorie (c) = heat req’d to raise 1g of H2O 1C)
Why do cells need energy?
Chemical work, build, rearrange, tear apart compounds
Mechanical work, move cilia, flex a muscle
Electrochemical work, nerve impulses
Where does energy come from?
The universe contains a huge, but finite amount of energy
The original source of energy for most life on earth is from the sun
Energy is governed by the Laws of Thermodynamics
First Law of Thermodynamics
The total amount of energy in the universe remains constant
Energy can be converted from one form to another, but it is never destroyed
Second Law of Thermodynamics
Entropy tends to increase in a closed system
(No energy conversion is 100% efficient) Overall energy flows in one direction from
useable (lots of potential energy) to nonuseable (little potential energy) forms
So how can life exist?
Energy flows from the sun to plants, which lose energy directly or indirectly to other organisms
Overall energy flows in one direction and entropy increases as at each step energy is lost
Producers builds complex molecules from simpler building blocks using the energy of the sun
i.e. – the sun is constantly supplying us with new energy
Energy and chemical reactionsReactant(s) → Product(s)
Energy is stored in chemical bonds – all molecules contain energy
Endergonic reactions: reactions in which the products contain more energy than reactants
Exergonic reactions: reactions in which the products contain less energy than the reactants
Endergonic Reactions
Requires energy input
Energy in
energy-poorreactants
glucose - a product with more energy
+ 6O2
Endergonic Reaction: Photosynthesis Original source of energy for
most life on earth Overall reaction:
6CO2 + 6H2O C6H12O6 + 6O2
Very endergonic – where does the plant get the energy?
→ SUN
Exergonic Reactions
Releases energy
Energy out
glucose - energy-rich starting
substance
+ 6O2
products with less energy
6 6
Exergonic Reaction – Cellular Respiration
Breakdown of glucose; very exergonic The source of ATP energy in cells Overall reaction:
C6H12O6 + 6O2 6CO2 + 6H2O -686Kcal
ATP is the cell’s energy currency nearly all energy in a cell is stored within the ATP molecule
Energy releasing rxns→ ATP→ Energy requiring rxns Cells cleave ATP into ADP & Pi releasing energy This energy can be used to do work such as
synthesize other molecules or move muscles
Adenosine Triphosphate (ATP)
How is ATP synthesized? ATP are renewable and are recycled by cells:
How is the energy from ATP utilized?
Reaction coupling: thermodynamically unfavorable reactions (endergonic) are coupled to the favorable reactions of ATP cleavage (exergonic)
ATP → ADP + Pi = –7.3Kcal X → → → → Y = +5Kcal Net energy = -2.3Kcal Total reaction still increases entropy and
conforms to the 2nd Law of Thermodynamics
Chemical Reactions (Rxn)
The conversion, accumulation, & disposal of substances by a cell is done through energy-driven reactions
Parts of a Reaction (Rxn) Reactants: substances that enter into a reaction Intermediates: substances formed in the middle
of a reaction Products: end results of a reaction
How are cellular reactions defined?
Catabolism: breaking down of complex molecules
Anabolism: the building up of complex molecules
Metabolism: the sum of all these reactions
Anabolic and Catabolic Reactions
ATP
BIOSYNTHETIC PATHWAYS(ANABOLIC)
ENERGY INPUT
DEGRADATIVE PATHWAYS
(CATABOLIC)
energy-poor products
large energy-rich molecules
simple organic compounds
ADP + Pi
Types of Reaction Sequences
BRANCHING PATHWAY
LINEAR PATHWAY CYCLIC PATHWAY
A B C D EF
K J I G
N M L H
Activation Energy Exergonic reactions are spontaneous -
Why don’t exergonic reactions happen all the time?
Because of Activation Energy (EA) – the energy required to get a reaction started
The EA of a reaction can prevent it from occurring or cause it to occur slowly
Activation Energy
Initial input of energy to start a reaction, even if it is spontaneous
Catalysts
Agents that speed up chemical reactions without getting used up
Biological Catalysts: Enzymes
Enzymes are protein catalysts (ribozymes are RNA catalysts)
They are required in small amounts They are not altered permanently by the reaction They do not change the thermodynamics of a
reaction They can only accelerate the rate at which a
favorable reaction proceeds
Role of Enzymes in Biological Reactions Enzymes accelerate reactions by reducing
activation energy
Enzymes combine with reactants and are released unchanged
Enzymes reduce activation energy by inducing the transition state
Enzymes and Activation Energy
Enzymes decrease activation energy required for a chemical reaction to proceed
Example:
A phosphatase enzyme can catalyze a rxn in 10 milliseconds
Without the enzyme the rxn would take…
1 trillion yrs. (1,000,000,000,000)
THE REACTION IS CONSIDERED SPONTANEOUS
Biological Catalysts
Enzyme Specificity Enzymes are usually very specific Substrates interact with enzyme’s active site
Enzyme Activity:Induced Fit Model
Transition State During catalysis, the substrate and active
site form an intermediate transition state
Fig. 4-12, p. 81
How do enzymes lower EA?
Catalytic mechanisms induce transition stateBringing substrates into close proximityOrienting substratesAltering environment around substrates
Factors That Affect Enzymes
Temperature: increasing temperature speeds up rxns
(both enzymatic and non-enzymatic) up to a point (WHY?)
High temperatures will destroy the enzymeEnzymes are proteinsProteins get denatured (unfolded) at high
temps
Factors That Affect Enzymes
Concentration of substrate and products: increasing substrate will increase reaction
up to a point increased product will slow reaction
(known as negative feedback)Concentration of enzyme Increasing concentration increases
enzyme activity up to a point
Factors That Affect Enzymes
pH: [H+] affects enzyme shape, so enzymes work
best at narrow ranges of pH Optimal pH – pH at which enzyme can
catalyze best For most enzymes, optimal pH is around neutral,
depending on the environment in which the enzymes work
E.g. Pepsin – digestive enzyme in stomach, optimal pH ~2
Controlling Enzyme Activity
Enzymes are very efficient at what they do Because of this they need to be carefully
controlled The cells needs to be able to regulate
when a reaction occurs The cell also has to be able to regulate
how much product is produced from a reaction
Enzyme inhibitors
Competitive inhibitors Bind to active site of enzyme Prevent substrate from binding
Non-competitive inhibitors Also called Allosteric inhibitors Bind to enzyme in a region other
than the active site called allosteric site
Change the shape of the active site to prevent substrates from binding
Enzyme Regulation
Enzyme activity is often regulated to meet the needs for reaction products
Allosteric regulation occurs with reversible combinations of regulatory molecules with an allosteric site on the enzyme High-affinity state (active form); enzyme binds
substrate strongly Low-affinity state (inactive form);enzyme binds
substrate weakly or not at all
Allosteric Regulation Allosteric activators and allosteric
inhibitors
Fig. 4-17, p. 84
Feedback inhibition If too much product is created the first enzyme may be shut off
by the product becoming an allosteric or competitive inhibitor:
Cofactors and Coenzymes
Some enzymes need assistance in the form of cofactors
Minerals – inorganic cofactors Examples: Potassium, Sodium, Calcium
Vitamins – organic cofactors or coenzymes Examples: The specialized nucleotides NAD+ and
FAD act as cofactors for enzymatic reactions; NAD+
contains the vitamin niacin and FAD contains the vitamin riboflavin
Ribozymes
RNA-based catalysts
Help remove surplus segments of RNA molecules with cutting and splicing reactions
In ribosomes, help join amino acids together when building proteins
Some coenzymes accept and hold onto electrons (e-) and protons (H+) during the breakdown glucose
Why are these coenzymes required?
Enzymes are not used up or modified during a reaction
If the enzyme accepted the e- or H+ it would be modified
Oxidation/Reduction (Redox) Reactions
One compound gains e- or H+ lost by another compound
The oxidized compound loses electrons or H+
The reduced compound gains electrons or H+
Reduction acts as a mechanism for storing energy
Redox Reactions