Chapter 6—An Introduction to

Metabolism

YOU are an energy transformer!

I. Energy, Metabolism, & Life

• Metabolism—

– All of an organism’s chemical reactions

• Emergent property arising from interactions

between molecules

• Manages the material and energy resources of

cells (supply and demand)

The Complexity of Metabolism

Catabolic vs. Anabolic

• Catabolic pathways— – Release energy by breaking down complex molecules

to simpler ones • Ex.—Cellular respiration

» C6H12O6 + 6O2 → 6H2O + 6CO2 + energy (ATP)

• Anabolic pathways— – Consume energy to build complex molecules from

simpler ones • Ex.—amino acids → protein

• Bioenergetics— – How organisms manage their energy resources

Let’s Review… Energy!!

• Energy = ability to rearrange matter

• Kinetic vs. Potential

• Chemical energy—potential or kinetic?

• All organisms are energy transformers

Yes, the Laws of Thermodynamics

Apply

• Thermodynamics—

– Energy transformations happening in a collection of matter (such as an organism)

– Organisms—closed or open systems?

• 1st Law of Thermodynamics— – Energy is transferred or transformed, but not created or

destroyed

» Examples? Cars? Plants?

• 2nd Law of Thermodynamics— – Energy transfers and transformations increase entropy

(disorder, randomness) of the universe

» Examples? Cars? People?

What is the ultimate fate of all

energy?

• HEAT—

– energy in its most random state, its lowest

grade

– All chemical energy eventually becomes heat

• The _________ of energy in the universe

is constant, but its _________ is not.

How can we reconcile the unstoppable

increase in entropy of the universe (2nd

Law)—with the orderliness of life?

• Organisms are islands of low entropy in an

increasingly random universe (because of

constant energy input!)

We owe it all to free energy…

• Free Energy—

– Portion of a system’s energy that can perform

work when temp. is uniform throughout the

system (i.e. a living cell)

• Organisms live at the EXPENSE of free energy

acquired from the surroundings

Relationship of Free Energy to Stability,

Work Capacity, & Spontaneous Change

Figure 6.5

Free Energy = Spontaneous

Change

• Free Energy (G)

– Measure of instability, the tendency to change

to a more stable state

– G = H – TS

• H = total energy of the system

• S = entropy of the system

• T = absolute temperature in Kelvin (K = °C + 273)

• Free energy < Total energy due to entropy

In any spontaneous process, the

free energy of a system decreases

• ∆G = ∆H – T∆S

– For a process to occur spontaneously, ∆G < 0

– At equilibrium, ∆G = 0 (no work can be done, death in living things)

• The greater the decrease in free energy, the more work can be performed

– Nature runs ―downhill‖

Exergonic vs. Endergonic

Photosynthesis = +686 kcal/mole

Respiration = -686 kcal/mole

ATP Powers Cellular Work

• Cellular Work = mechanical, transport,

chemical

• Energy coupling—

– ATP uses exergonic processes to drive

endergonic ones

ATP + H2O → ADP + Pi

∆G = -7.3 kcal/mol (standard conditions)

Exergonic

Loaded spring

How ATP Performs Work

ATP transfers its

phosphate group to

other molecules

making them unstable,

and more likely to react

chemically

Endergonic +

Exergonic = Overall

reaction is

spontaneous (∆G

The Regeneration of ATP

ATP couples the cell’s energy-yielding processes to the energy-

consuming ones.

ADP + Pi → ATP + H2O

∆G = +7.3 kcal/mol (endergonic)

ATP is recyclable

10 million/per

second/per cell

II. Enzymes speed up metabolic

reactions by lowering energy barriers

• Enzyme—

– Catalytic protein that changes the rate of a

reaction w/o being consumed by it

Enzyme for this reaction?

Energy profile of an exergonic

reaction

Activation Energy

(EA)—

Investment of

energy for starting

a reaction

(required to break

bonds in

reactants)

Usually comes

from heat in

surroundings

Why is EA important?

Enzymes lower the barrier of

activation energy

Enzymes speed up

reactions by

lowering EA

(transition

state can

occur at a

lower temp,

safe for cells)

ΔG stays the

same!

Enzyme + Substrate =

• Substrate— – The reactant an enzyme acts on

Enzyme

• Substrate(s) → Product(s)

Sucrase

• Sucrose + H2O → Glucose + Fructose

• Enzymes are substrate specific—each type of enzyme catalyzes a particular reaction – Specificity results from 3-D shape of enzyme

Catalytic Cycle of an Enzyme

The Active Site—‖where the magic

happens‖

• Substrate binds to active site to form enzyme-substrate complex (induced fit)

– Held in place by weak bonds (hydrogen/ionic)

• R groups/side chains catalyze the change from substrate to product

• Enzyme emerges unchanged, ready to be used again (and again and again…)

– FAST—1,000 reactions/second (typical enzyme)

How do they do that?

• Enzymes mechanisms:

– Induced fit

– Microenvironment

– Direct participation

Environmental factors affect enzyme activity

• i.e. pH, temp, chemicals

• Enzyme Helpers— – Cofactors

(inorganic) • Zn, Fe, Cu

– Coenzymes (organic)

• vitamins

Inhibition of Enzyme Activity

• Competive vs.

Noncompetive

Inhibitors

III. The Control of Metabolism

• Allosteric regulation

– Allosteric activators and inhibitors

Feedback Inhibition

• An end product

switches off a

metabolic pathway

• Allosterically inhibits

enzyme early in

pathway

Localizing enzymes within cells

orders metabolism