CELLS @ WORK

CELLS @ WORKCELLS @ WORK

Cell Metabolism and ATPCell Metabolism and ATP• ATP is the energy molecule

of the cell.• Billions are used and

reassembled every second!!• Endergonic Rxn – Requires

energy (ie. Photosynthesis)

• Exgergonic Rxn – releases energy

(ie.Cellular Respiration)• Glucose = 1 dollar• ATP = 1 penny

EnzymesEnzymesOur “Key” to Biochemical

Reactions

ENZYMESENZYMES

• Proteins that accelerate chemical reactions

• Almost all processes in the cell need enzymes in order to occur – Cellular Respiration, Photosynthesis, food digestion etc.

• Are extremely selective – very specific to certain reactions

ENZYMESENZYMES

• For Example:For Example:

Lysozyme digests bacterial cell walls, and is found in human tears, egg-white, etc

ENZYMESENZYMES

• Enzymes are known to catalyze about 4,000 reactions in the human body

• Named according to the reaction they catalyze … “ase” is added to the name of the substrate

• Ex: Lactase breaks down lactose

HOW ENZYMES WORK:HOW ENZYMES WORK:

• By providing a lower activation energy for a reaction and dramatically accelerating its rate

• For example… (Do not copy)– the reaction catalysed by orotidine-

phosphate decarboxylase will consume half of its substrate in 78 million years if no enzyme is present. However, when the decarboxylase is added, the same process takes just 25 milliseconds

HOW ENZYMES WORK:HOW ENZYMES WORK:

EnzymeEnzymeSUBSTRATE PRODUCT(S)

Eg:Eg:

SucraseSucraseSucrose + Water Glucose + Fructose

Enzymes help a reaction to occur … without being directly involved!!!

Check these out!Check these out!

• http://www.yellowtang.org/animations/enzymes.swf

• http://www.yellowtang.org/animations/enyme_action_final.swf

Enzymes lower the activation Enzymes lower the activation energy of a reactionenergy of a reaction

Final energy state of products

Initial energy stateof substrates

Activation energyof uncatalysed reactionsActivation energy

of enzyme catalysedreaction

Progress of reaction (time)

Ene

rgy

leve

ls o

f m

olec

ules

Enzymes lower activation energy by Enzymes lower activation energy by forming an enzyme/substrate complexforming an enzyme/substrate complex

Substrate + Enzyme

Enzyme/substrate complex

Enzyme/product complex

Product + Enzyme

In anabolic reactions enzymes bring the substrate molecules together.

In catabolic reactions the enzyme active site affects the bonds in substrates so they are easier to break

HOW ENZYMES WORK:HOW ENZYMES WORK:

• ““Lock and Key” Model:Lock and Key” Model:

HOW ENZYMES WORK:HOW ENZYMES WORK:

Lock-and-key hypothesis assumes the Lock-and-key hypothesis assumes the active site of an enzyme is rigid in its active site of an enzyme is rigid in its

shapeshape

How ever crystallographic studies indicate proteins are flexible.

The Induced-fit hypothesis suggests the The Induced-fit hypothesis suggests the active site is flexible and only assumes its active site is flexible and only assumes its catalytic conformation after the substrate catalytic conformation after the substrate molecules bind to the site.molecules bind to the site.

When the product leaves the enzyme the active site reverts to its inactive state.

Enzyme Reaction RatesEnzyme Reaction Rates

Rates of enzymesRates of enzymes

• Rate of enzyme action is dependent on number of substrate molecules present

Vmax = maximum rate of reaction

Vmax approached as all active sites become filled

Some active sites free at lower substrate concentrations

Substrate concentration

Rat

e of

Rea

ctio

n (M

)

Temperature -Enzymes denature at Temperature -Enzymes denature at 6060ooCC

Temperature

Rat

e of

rea

ctio

n

Rate doubles every 10oC

Enzyme denaturing and losing catalytic abilities

Optimum temperature

Some thermophilic bacteria have enzymes with optimum temperatures of 85oC

pH - affects the formation of hydrogen pH - affects the formation of hydrogen bonds and sulphur bridges in proteins bonds and sulphur bridges in proteins and so affects shape.and so affects shape.

pepsintrypsin cholinesterase

2 4 8 106pH

Rat

e of

Rea

ctio

n (M

)

In Summary...In Summary...• Work at optimal concentrations, temp.

& pH

• If it is too hot, an enzyme can become DENATURED… and will no longer function properly

Define the following terms:Define the following terms:1. Anabolic reactions:

2. Catabolic reactions:

3. Metabolism:

4. Catalyst:

5. Metabolic pathway:

6. Specificity:

7. Substrate:

8. Product:

Reactions that build up molecules

Reactions that break down molecules

Combination of anabolic and catabolic reactions

Sequence of enzyme controlled reactions

Only able to catalyse specific reactions

The molecule(s) the enzyme works on

Molecule(s) produced by enzymes

A substance that speeds up reactions without changing the produced substances

Competitive InhibitorsCompetitive Inhibitors

• A molecule similar in shape to the substrate bonds with the enzyme’s active site and inhibit its function.

• Can be reversible or irreversible

• Poisons: cyanide and arsenic bind to key enzymes in this manner…death may result!

Non-Competitive Non-Competitive InhibitorsInhibitors

• Attach to a binding site on the enzyme other than the active site...causing the shape to change.

• The enzyme loses affinity for substrate

Allosteric RegulationAllosteric Regulation

• Inhibits or stimulates enzyme activity

• Enzyme can be turned on or off...therefore cellular reactions are controlled through feedback.

The switch: Allosteric The switch: Allosteric inhibitioninhibition

Allosteric means “other site”

E

Active site

Allosteric site

Switching offSwitching off

• These enzymes have two receptor sites

• One site fits the substrate like other enzymes

• The other site fits an inhibitor molecule

Inhibitor fits into allosteric site

Substratecannot fit into the active site

Inhibitor molecule

The allosteric site the The allosteric site the enzyme “on-off” switchenzyme “on-off” switch

E

Active site

Allosteric site

emptySubstratefits into the active site

The inhibitor

molecule is absent

Conformational change

Inhibitor fits into

allosteric site

Substratecannot fit into the active site

Inhibitor molecule

is present

E

Negative Feedback Negative Feedback InhibitionInhibition

This example demonstrates how an end product can inhibit the first step in its production.

1.Isoleucine binds to the allosteric site of threonine deaminase and prevents threonine from binding to the active site because the shape of the active site is altered. 2.When the level of isoleucine drops in the cell’s cytoplasm, the isoleucine is removed from the allosteric site on the enzyme, the active site resumes the activated shape and the pathway is “cut back on” and isoleucine begins to be produced.

Example!Example!

• http://www.yellowtang.org/animations/feedback_inhibition.swf

The End!!!The End!!!