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