Lecture 6: Energy Flow

Covers 6.2-6.5

Some definitions and ground rules*

• Energy: the capacity to do work• Work: transfer of energy from one place to another• Chemical energy: energy contained in molecules and

released through chemical reactions. • Sugar, glycogen and fat are the molecules in our

bodies that can store chemical energy.• ATP is the molecule that can accept the energy and

transfer it from one chemical reaction to another.• This energy allows us to move and grow. EX: muscle

contraction results from the interaction of proteins that are fueled by ATP.

Definitions & ground rules*

• Chemical reaction: a process that forms or breaks chemical bonds.

• These reactions change molecules from one form to another:• Reactants (name for molecules BEFORE reaction)

go through a chemical reaction. At the end of the reaction, the reactants have become the products (name for molecules AFTER the chemical reaction.)

Reactants and End Products of Burning Glucose

Fig. 6-5

energy

C6H12O6

(glucose)6 O2

(oxygen)

6 CO2

(carbondioxide)

6 H2O(water)

Chemical Reactions can be EXERGONIC or ENDERGONIC*

• Exergonic: the result of the reaction is that reactants have been changed to products AND energy is RELEASED.

• Endergonic: the result of the chemical reaction is that reactants have been changed to products BUT ENERGY IS REQUIRED TO MAKE THE REACTION HAPPEN.

An Exergonic Reaction

Fig. 6-3

energy

reactants

products

+

+

An Endergonic Reaction

Fig. 6-4

reactants

products+

+

energy

HERE IS THE KEY*• Most organisms are powered by the breakdown of sugar into CO2

and H20. • The energy that is released from this reaction does all of the work

that a cell needs to do: make proteins, move materials around, muscle contraction, etc.

• But the energy from this reaction must be carried from the place the glucose is broken down to the place that the energy is needed.

• ATP IS THE CARRIER MOLECULE OF CHEMICAL ENERGY IN OUR BODIES. IT CANNOT STORE THE ENERGY FOR LONG PERIODS, ONLY ENOUGH TIME TO CATCH THE ENERGY FROM AN EXERGONIC REACTION AND MOVE IT TO A PLACE WHERE ENERGY IS NEEDED IN ANOTHER PART OF THE CELL. (LATER IT CAN PICK UP MORE ENERGY AND DELIVER IT TO OTHER AREAS OF THE CELL.)

• AND FINALLY, THE ENERGY FROM EXERGONIC REACTIONS OFTEN FUELS ENDERGONIC REACTIONS. (called coupled reactions)

ATP*

• Recall that ATP is a nucleotide composed of a nitrogen-containing base, a sugar, and three phosphate groups.

• ADP (the base, sugar and TWO phosphates) is a stable molecule. It gets unstable when energy from a reaction causes another Phosphate to be added to ADP and ATP is formed. ATP will then deliver the ENERGY CONTAINED IN THE LAST PHOSPHATE BOND to a place where it is needed.

• When it “drops off” the energy, ATP is converted back to ADP BECAUSE THE ENERGY IS IN THE PHOSPHATE BOND.

The Interconversion of ADP and ATP

Fig. 6-8

ADP

energy

(a) ATP synthesis: Energy is stored in ATP

(b) ATP breakdown: Energy is released

ATP

ATP

ADP phosphate

phosphate

energy

P P P

P P P

P P P

P P P

Other types of energy carriers

• ATP is not the only molecule that carries energy within its bonds:– Electron carriers: some energy in exergonic

reactions is transferred to electrons within special molecules called electron carriers. When the carriers reach their destination, the electrons (and the energy they carry) can be released. • NADH & FADH2 are electron carriers

Reactions may need enzymes to make them happen

• Some reactions (exergonic and endergonic) may never actually happen unless an catalyst is present.

• A catalyst is a molecule that speeds up a reaction without the catalyst itself being changed.

• ENZYMES are catalysts that make reactions in the body happen. They are proteins.

• Each enzyme can only catalyze ONE (or a small number of) reactions.

ATP SYNTHASE

• This protein is an enzyme that makes these reactions happen:

• ADP + Phosphate ATP • ATP ADP + Phosphate

Structure of an enzyme*• The function of an enzyme is determined by its structure. • Every enzyme has an active site (created as a byproduct of

it’s quartenary structure) • The molecule to be changed (the reactant, called the

substrate in the case of enzymes) in the reaction enters the active site

• Active site is altered once the substrate is attached (aa’s within the active site may bond with atoms of the substrate)

• The reaction occurs, and the products of the reaction are released from the active site along with the enzyme.

The Cycle of Enzyme-Substrate Interactions

Fig. 6-11

The substrates, bonded together, leave the enzyme; the enzyme is ready for a new set of substrates

The substrates andactive site change shape,promoting a reactionbetween the substrates

substrates

active siteof enzyme

Substrates enterthe active site in aspecific orientation

enzyme

1

3 2

Regulation of reactions

• In the human body, reactions are SOMETIMES linked in sequences called metabolic pathways: an initial reactant molecule is modified by one enzyme, that product is then modified by another enzyme and so on.

• EX: Glycolysis is the initial stage of the breakdown of glucose. Photosynthesis is a metabolic pathway that plants use to change H20 and CO2 into glucose.

Simplified Metabolic Pathways

Fig. 6-12

PATHWAY 1

Initial reactant Intermediates Final products

enzyme 1 enzyme 2 enzyme 3 enzyme 4

PATHWAY 2

enzyme 5 enzyme 6

A B D E

F

C

G

Speed of reactions depends on amount of substrate, enzymes

• The more substrate you have, the faster the given reaction will occur until all available enzymes are being used.

• HOWEVER, our bodies can regulate the speed of a reaction (it’s not just substrate in, reaction happens)…HOW? By changing the rate at which enzymes are produced. (if our body doesn’t make enzymes, all of the substrate in the world doesn’t matter. The enzymes are required for the reaction to occur.)

• WE MAKE ENZYMES (PROTEINS) VIA PROTEIN SYNTHESIS. MORE LATER.

• So, our cells can actually “turn on” the production of enzymes when there is a lot of substrate, and “turn off” the production of enzymes when there is less substrate around.

• Our bodies can also synthesize enzymes in an inactive form, so they will only “turn on” when they are activated.

• We can also inhibit enzymes from catalyzing reactions.

Inhibition/Regulation of enzymes• Competitive Inhibition: a substance that is not the normal

substrate for an enzyme can “sit” in the active site, thus inhibiting the normal substrate to enter

• Noncompetitive inhibition: a substance binds to the enzyme (BUT NOT IN THE ACTIVE SITE) and the presence of this substance will cause the enzyme to not bind its normal substrate

• Allosteric regulation: an enzyme can have 2 different configurations: an active and an inactive configuration depending on the presence of molecules that activate or inhibit the enzyme. These activators/inhibitors are often either the end product in the reaction OR intermediate products in a metabolic pathway.

(b) Competitive inhibition

A competitive inhibitor molecule occupies theactive site and blocksentry of the substrate

Fig. 6-13b

noncompetitiveinhibitor molecule

(c) Noncompetitive inhibition

A noncompetitiveinhibitor moleculecauses the active siteto change shape, so thesubstrate no longer fits

Fig. 6-13c

Allosteric Regulation of an Enzyme by Feedback Inhibition

Fig. 6-14

enzyme 1 enzyme 2 enzyme 3 enzyme 4 enzyme 5

isoleucine(end product)

threonine(initial reactant)

A B C D

As levels of isoleucine rise,it binds to the regulatory siteon enzyme 1, inhibiting it

isoleucine

enzyme 1

intermediates

Example of allosteric regulation/feedback inhibition

• Metabolic pathway that makes isoleucine from threonine. 5 enzymes needed for this pathway, with 4 intermediate products. As amount of isoleucine increases, it will actually inhibit the first enzyme in the pathway.

• EX #2: ATP can inhibit enzymes in the metabolic pathways that create it. (if too much ATP, no need to make more at the moment.)