Chapter 6 - MetabolismEnergy and Enzymes

Energy is the ability to do work.

Cells use acquired energy to:

- Maintain their organization - Carry out reactions that allow cells to develop, grow, and reproduce

Living things need to acquire energy; this is a characteristic of life.

There are two basic forms of energy.Kinetic energy is the energy of motion.Potential energy is stored energy.

Food eaten has potential energy because it can be converted into kinetic energy.

Potential energy in foods is chemical energy.

Organisms can convert chemical energy into a form of kinetic energy called mechanical energy for motion.

Forms of Energy

Two Laws of Thermodynamics

The flow of energy in ecosystems occurs in one direction; energy does not cycle.

The two laws of thermodynamics explain this phenomenon.

First Law: Energy cannot be created or destroyed, but it can be changed from one form to another.

Second Law: Energy cannot be changed from one form to another without loss of usable energy.

Flow of energy

When energy transformations occur, energy is neither created nor destroyed but there is always loss of usable energy, usually as heat.

For this reason, living things depend on an outside source of energy.

The ultimate source of energy for life is the sun, and this energy is passed from plants to animals.

Metabolic Reactions and Energy Transformations

Metabolism is the sum of all the chemical reactions that occur in a cell.

Reactants are substances that participate in a reaction; products are substances that form as a result of a reaction.

Biologists use the term “free energy” instead of entropy for cells.

Free energy, G, is the amount of energy to do work after a reaction has occurred.

ATP (adenosine triphosphate) is the energy currency of cells.

ATP is constantly regenerated from ADP (adenosine diphosphate) after energy is expended by the cell.

1) ATP is used in many types of reactions.

2) When ATP is converted to ADP + P, the energy released is sufficient for cellular needs and little energy is wasted.

The ATP cycle

In coupled reactions, energy released by hydrolyzing ATP drives the conversion of substrates to products.

Coupled Reactions

Summary of ATP functions

ATP is used for:

Chemical work – ATP supplies energy to synthesize macromolecules, and in turn, build the organism

Transport work – ATP supplies energy needed to pump substances across the plasma membrane

Mechanical work – ATP supplies energy for cellular movements

Metabolic Pathways and Enzymes

Cellular reactions are usually part of a metabolic pathway, a series of linked reactions, illustrated as follows:

E1 E2 E3 E4 E5 E6 A → B → C → D → E → F → G

The letters A-G represent substrates and products

E1-E6 represent enzymes.

An enzyme is a protein molecule that functions as an organic catalyst to speed a chemical reaction.

An enzyme brings together particular molecules and causes them to react.

The addition of an enzyme does not change the free energy of the reaction, rather an enzyme lowers the energy of activation.

The reactants in an enzymatic reaction are called the substrates for that enzyme.

Every reaction in a cell requires a specific enzyme.

Enzymes are named for their substrates:

Substrate EnzymeLipid LipaseUrea UreaseMaltose MaltaseRibonucleic acid Ribonuclease

Enzymes can be synthetic (anabolic) and degradative (catabolic)

Enzyme – Substrate Complex 

An enzyme is a protein

that catalyzes

biochemical reactions.

2. Enzymes are very specific and catalyze only one type of reaction.

3. Enzymes work best over very specific pH and temperature ranges

4. Enzymes are very efficient at their task, which is reaction rate enhancement.

5. Nomenclaturea) Originally enzymes were named by adding –ase to the reactants.b) A more systematic system has arisen, but we are not covering it in

any detail. A full explanation is given in your text.

6. Enzymes are proteins and as such have a definite three-dimensional shape. This shape is the key to enzyme catalysis.

a) The reactant is called the substrate.

b) The active site is where the actual reaction will occur.

c) An oversimplified explanation is that the enzyme and substrate have certain shapes that must be complimentary. The specificity of an enzyme can be explained in this manner. This is referred to as the lock and key theory.

LOCK KEY

d. Some enzymes can work with different substrates and will change its shape to accommodate the substrate. This is referred to as the induced-fit model.

Temperature and pH

As the temperature rises, enzyme activity increases because more collisions occur between enzyme and substrate.

If the temperature is too high, enzyme activity levels out and then declines rapidly because the enzyme is denatured.

Each enzyme has an optimal pH at which the rate of reaction is highest.

Rate of an enzymatic reaction as a function of temperature and pH

pHEnzymes usually work in a

narrow pH range.

pH optimum: pH at which each enzyme exhibits peak activity.

this reflects the pH of the body fluid in which the enzyme is found.

8. Other Factors Affecting Enzyme Action

• Cofactors – (inorganic) can be a metal ion such as Cu, Zn, Fe

• Coenzymes - organic plugin to an enzyme. The plugin is mostly made of vitamins

9. Substrate concentration – when all the enzymes are involved with substrates, the rate of reaction reaches a plateau.

Temperature can affect the reaction rate by providing too much or too little energy.

The pH must be stable in order to maintain the tertiary and quaternary structures.

Cofactors and Coenzymes

Needed for the activity of certain enzymes.

Cofactor:- Ions (Ca++, Mg++, etc.)- often transform and turn

on the active site.- help bind the substrate

Coenzyme:- derived from vitamins- participate in the rxn

ALLOSTETIC SITE

Vitamin Deficiencies cause Diseases

Vitamin C - Scurvy

Niacin (B3)- Pellagra (Skin Disease)

Vitamin D - Rickets

Vitamin A - Night Blindness

Vitamin K - Blood clotting problems

Substrate Concentration

- Maximum rate occurs when enzyme is saturated.

- Additional substrate does not increase reaction rate.

Inhibiting Enzymes

A) Competitive Inhibition

occurs when an active enzyme is prevented from combining with its substrate because a competitive substrate latches on. Some poisons work this way. Cyanide poisons human enzymes, Penicillin poisons bacterial enzymes. Antifreeze (wood alcohol) poisons enzymes of the liver, etc.

Inhibiting Enzymes

B) Non-competitive Inhibition

inhibitor does not resemble the substrate

does not compete for active site binding but attaches to the enzyme and changes the shape of the active site

no competition with the substrate occurs

addition of more substrate cannot overcome this type of inhibitor

Ex ---> Lead Poisoning // Mercury Poisoning

A cell regulates which enzymes are present or active at any one time.

Genes must be turned on or off to regulate the quantity of enzyme present.

Another way to control enzyme activity is to activate or deactivate the enzyme.

- feedback regulation- covalent modification- phosphorylation

Control of enzymes

Oxidation-Reduction Reactions

Oxidation is the loss of electrons.Reduction is the gain of electrons.

Because oxidation and reduction occur simultaneously in a reaction, such a reaction is called a redox reaction.

Oxidation also refers to the loss of hydrogen atoms, and reduction refers to the gain of hydrogen atoms in covalent reactions in cells.

Consider NaCl

Oxidized – loses electrons Reduced – gains electrons

Oxidation-reduction reactions are exemplified by the overall reactions of photosynthesis and cellular respiration.

The pathways of photosynthesis and cellular respiration permit the flow of energy from the sun though all living things.

Photosynthesis (in Chloroplast)

6CO2 + 6H2O + energy → C6H12O6 + 6O2

During photosynthesis, hydrogen atoms are transferred from water to carbon dioxide, and glucose is formed.

water has been oxidized (ripped apart for electrons

carbon dioxide has been reduced. The electrons were used to put glucose together

Energy to form glucose comes from the sun.

Cellular respiration (Mitochondria)

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy

It is opposite that of photosynthesis

In this case,- glucose is oxidized (torn apart) (to CO2)- oxygen is reduced (gets electrons to make) (to water).

The complete oxidation of a mol of glucose releases 686 kcal of energy that is used to synthesize ATP.

For photosynthesis, chloroplasts capture solar energy and use it to convert water and carbon dioxide into carbohydrates that provide food for other living things.

Cellular respiration, the breakdown of glucose into carbon dioxide and water, occurs in mitochondria.

It is the cycling of molecules between chloroplasts and mitochondria that allows a flow of energy from the sun through all living things.

Relationship of chloroplasts to mitochondria