Introduction to Metabolism

Chapter 6

Metabolic Pathways

O Specific molecules are catalyzed each step

along the way by enzymes.

First Law of Thermodynamics

O The energy of the universe is constant.

O Energy can be transferred or transformed.

O Energy can not be created or destroyed

O Ex. Animals convert chemical energy in

food to kinetic energy of movement.

Second Law of Thermodynamics

O Every energy transfer or

transformation increases the entropy

of the universe.

O Ex-If an animal moves, disorder

around it increases by the release of

heat and small molecules that are the

result of metabolism.

What is Entropy?

OA measure of the disorder or

randomness which makes the

universe more disordered.

Free Energy

O The portion of a system’s energy

that can perform work when

temperature and pressure are

uniform throughout the system.

∆G = G(Final State) – G(Initial State)

Exergonic Reactions

O Proceeds with a net release of free energy.

O ∆G will be negative (less than 0)

O Will occur spontaneously like in cellular

respiration

O ∆G = - 686 kcal/mole

Catabolic PathwaysO This is a breakdown pathway

O Example is when glucose breaks down during cellular respiration.

O The energy stored in the bonds becomes available…

O Will have a –∆G, which means the energy at the beginning is higher than the energy at the end.

O Said to be spontaneous.

Spontaneous with a –∆G

Endergonic Reactions

O Absorbs free energy.

O ∆G will be positive (greater than 0)

O Will NOT occur spontaneously like photosynthesis. Energy is needed from the sun

O ∆G = + 686 kcal/mole

Anabolic PathwaysO This is a biosynthetic pathway

O Example is when glucose is formed during photosynthesis.

O The energy received gets stored in the bonds

O Will have a +∆G, which means the energy at the beginning is lower than the energy at the end.

O Said to be not spontaneous.

Not Spontaneous with a +∆G

What If You Need an Endergonic Reaction But Don’t

Have The Energy?

OGood News…..ATP

O This is called energy coupling.

O Use of an exergonic process to

drive an endergonic one.

ATPO Contains a sugar ribose, the nitrogenous

base adenine and three phosphate groups.

ATP = Energy

O When the terminal phosphate group is hydrolyzed

with water, the phosphate group is released as well

as energy.

O Sometimes called a high energy phosphate bond

ATP + water � ADP + P

∆G = -7.3 kcal/mole

How Do You Get ATP Back?

O ADP + P will turn into ATP and water with some help from the cell.

∆G = + 7.3 kcal/mole

O In animals, this will happen during cellular respiration.

O 10 million ATP are consumed and regenerated each second.

Activation Energy

O The energy needed to contort the reactant

molecules so the bonds will break.

O EA

O Think of it as the energy needed to push the

reactants up so the “downhill” portion can

begin.

O Energy often comes from thermal energy

(heat) absorbed from the surroundings.

How Can I Lower The EA?

O In essence, speed up the reaction.

O We use catalysts.

O The initial and final energy levels

remain the same, just the energy

needed is lowered.

Enzyme Substrate Complex

O When the substrate fits into the

active site of the enzyme.

O Sometimes this causes an even

better fit, called the induced fit.

O Caused by weak bonds forming

around the substrate.

O Enzyme is not used up.

Example

Enzyme Enzyme Enzyme

+ Substrate +

Substrate Complex Products

Sucrase

Sucrase Sucrase Sucrase

+ Sucrose +

Sucrose Complex Glucose

Fructose

Affects of Temperature

O Temperature has a huge effect on an

enzymes ability to function.

O If the temperature gets too high, hydrogen

and ionic bonds get disrupted and the

protein will loose it shape.

O It is said to be denatured.

Optimal Temperature

O In humans it’s 37 Celsius. Other organisms

may prefer hotter or colder conditions.

Optimal pH

OFor most reactions in humans,

an approximate pH of 7 works

very well.

OExceptions like the stomach

prefer a pH of 2 (acidic) and

the intestines prefer a higher

pH (slightly basic).

Altering the Active Site

O Many things may interfere with an enzymes

ability to function.

O A substance may move into the active site

and block the substrate, sometimes

irreversibly like in poisons or toxins. This is

called competitive inhibition.

O A substance may attach somewhere else but

affect the active site. This is called non-

competitive inhibition.

Allosteric RegulationO An activator may bind to an enzyme, not in

the active site, but activate the active site

because the change in shape is favorable.

Allosteric RegulationO An inhibitor may bind to an enzyme, not in

the active site, but inactivate the active site

because the change in shape is unfavorable.

Allosteric RegulationO Cooperativity, a substrate moves into the

active site and locks all of the active sites

into the active form.