metabolism 2015 enzymes pathways ss

HOW ENZYMES FUNCTION

Metabolism = all the chemical reactions in an organism

Cells need a mechanism for linking chemical reactions, by

1. Coupling energy between endergonic & exergonic reactions.

= ATP Cycle couples anabolism to catabolism

2. Reactions need to occur fast enough to pass their products onto the next reaction.

So – Run reactions in a series, one after another,in Metabolic pathways

- using protein catalysts called ENZYMES:

Enzymes speed up the cell’s chemical reactions by lowering energy barriers

• Although biological molecules possess much potential energy, it is not released spontaneously.

• An energy barrier must be overcome before a chemical reaction can begin.

• Weaken the reactants’ chemical bonds

• This energy is called the activation energy (because it “activates” the reactants).


• We can think of activation energy as the amount of energy needed for a reactant molecule to move “uphill” to a higher-energy but an unstable state so that the “downhill” part of the reaction can begin.

• One way to speed up a reaction is to add heat, which agitates atoms so that bonds break more easily and reactions can proceed, but too much heat will kill a cell.

© 2015 Pearson Education, Inc.

HOW ENZYMES FUNCTIONEnzymes speed up the cell’s chemical reactions by lowering energy barriers

Chemical ReactionAtoms break bonds and form new ones with different atoms- Change electron sharing “partners”

Three conditions necessary for a chemical reaction:1. Atoms or molecules carry enough energy

- to move with respect to each other- and also move electrons out of bonds

2. Reactants must physically contact3. Contact must occur in a specific orientation


• function as biological catalysts, • are NOT reactants themselves

• increase the rate of a reaction without being consumed by the reaction – “come out in the same form they came in,”

• are usually proteins (although some RNA molecules can function as enzymes),

• are specific for reactants – that is, the shape of an enzyme will only bind an exact type of chemical,

• Every reaction needs its own specific enzymeEnzymes speed up a reaction by lowering the activation energy needed for a reaction to begin.


– For a chemical reaction to begin• Reactants must absorb some energy, called the energy of

activation EA

EA barrier

Reactants

Products1 2E

nzym

e


A protein catalyst called an enzyme• Can decrease the energy of activation needed to begin a

reaction

Reactants

EA withoutenzyme

EA withenzyme

Net changein energy

Products

Ene

rgy

Progress of the reaction


A specific enzyme catalyzes each cellular reaction

Enzymes have unique three dimensional shapes that allows their determining which chemical reactions occur in a cell

http://en.wikipedia.org/wiki/Image:Phenylalanine_hydroxylase_brighter.jpg


Reactants

Progress of the reactionProducts

a

b

c

Reactant

Products

Reactant

Products

Ener

gy

Activationenergybarrier

Without enzyme

Enzyme

Ener

gy

Activationenergybarrierreduced byenzyme

With enzyme

Ener

gy


An enzyme• is very selective in the reaction it catalyzes and• has a shape that determines the enzyme’s

specificity.

• The specific reactant that an enzyme acts on is called the enzyme’s substrate.

• A substrate fits into a region of the enzyme called the active site.

• Enzymes are specific because only specific substrate molecules fit into their active site.

A specific enzyme catalyzes

each cellular reaction

• The following figure illustrates the catalytic cycle of an enzyme.

• The enzyme’s action depends on the substrate “fitting” exactly into the active site on the enzyme.

• The enzyme then changes shape to activate the substrate to form the product.


Glucose

Fructose

The productsare released The substrate

is convertedto products

H2O

Enzyme(sucrase)

Substratebinds toenzyme withinduced fit.

Substrate(sucrose)

Active site

The enzyme availablewith an empty active site

1

2

3

4


The cellular environment affects enzyme activity– Temperature, salt concentration, and pH can change the 3-D shape of proteins


• For every enzyme, there are optimal conditions under which it is most effective.

• Temperature affects molecular motion.• An enzyme’s optimal temperature produces the

highest rate of contact between the reactants and the enzyme’s active site.

• Most human enzymes work best at 35–40°C.• The optimal pH for most enzymes is near

neutrality.

Some enzymes require non-protein cofactors such as metal ions like Zinc, Copper, Iron, Magnesium, etc.

or

Organic molecules called coenzymes

- These are chemicals that help the enzyme transfer electrons &/or atoms during the reaction.

- The cofactor or coenzyme is regenerated in its original form after the reaction.

Two coenzymes used by enzymes in cellular respiration

1. NADH carries high-energy electrons extracted from food

Nicotinamide adenine dinucleotide

2. FADH2 also carries high-energy electrons from food

Flavin adenine dinucleotide

Enzyme inhibitors block enzyme action

• Inhibitors interfere with an enzyme’s activity to form products

• Many poisons, pesticides, and drugs are enzyme inhibitors

Like aspirin, ibuprofen, cyanide, arsenic, some anti-cancer drugs like methotrexate

Enzyme inhibition can regulate enzyme activity in a cell



A chemical that interferes with an enzyme’s activity is called an inhibitor.• Competitive inhibitors

• block substrates from entering the active site and• reduce an enzyme’s productivity.

• Noncompetitive inhibitors• bind to the enzyme somewhere other than the

active site, • change the shape of the active site, and• prevent the substrate from binding.

• A competitive inhibitor• Takes the place of a substrate in the active site

• A noncompetitive inhibitor• Alters an enzyme’s function by changing its shape

Substrate

Enzyme

Active site

Normal binding of substrate

Enzyme inhibition

Noncompetitiveinhibitor

Competitiveinhibitor


PUTTING ALL THIS TOGETHER• Energy Coupling

• ATP Cycle –Anabolic (endergonic) to Catabolic

(exergonic) Reactions• Catalysis

– Enzymes make reactions occur in “real time”• Regulation

– Controlling enzymes allow for turning on/off • Or accelerating / slowing reactions, as needed


Metabolic PathwayEnzymes coordinate to run chemical reactions in tandem- The products of one reaction become the reactants

for the next reaction

http://web.indstate.edu/thcme/mwking/glycolysis.html



• Enzyme inhibitors are important in regulating cell metabolism.

• In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition.


Feedback inhibition

Enzyme 1

Reaction 1A

Startingmolecule

Product

–

Enzyme 2

Reaction 2B

Enzyme 3

Reaction 3C D

CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors

Many beneficial drugs act as enzyme inhibitors, including

• ibuprofen, which inhibits an enzyme involved in the production of prostaglandins (messenger molecules that increase the sensation of pain and inflammation),

• some blood pressure medicines,• some antidepressants,• many antibiotics, and • protease inhibitors used to fight HIV.


Ibuprofen, which inhibits an enzyme “cyclooxygenase”involved in the production of prostaglandins (messenger molecules that increase the sensation of pain and inflammation)

Ditto Aspirin & Acetaminophen

CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors

Enzyme inhibitors have also been developed as

• pesticides and • deadly poisons for chemical warfare.

PUTTING ALL THIS TOGETHER, Again !

• Energy Coupling• ATP Cycle

• Anabolic (endergonic) to Catabolic (exergonic) Reactions

• Catalysis• Enzymes make reactions occur in “real time”

• Regulation• Controlling enzymes allow for turning on/off

• Or accelerating / slowing reactions, as needed

Enzymes are central to the processes that make energy available to the cell

By reacting only one bond in a molecule at each reaction in the pathway, the energy is released gradually.

Can trap energy more efficiently to make ATP !!!


Now we can understand:

How Living Systems

- Obtain and process energy from their surroundings

- Obtain and process energy from “food”

The human body uses energy from ATP for all its activities, both inside & outside cells

Chloroplasts and mitochondria make energy available for cellular work

• Chloroplasts carry out photosynthesis• Using solar energy to produce glucose and oxygen

from carbon dioxide and water

• Mitochondria carry out cellular respiration• Using the energy stored in glucose to make ATP


CELLULAR RESPIRATION

Both photosynthesis and cellular respiration provide energy for metabolism = life’s processes• Photosynthesis traps sunlight energy and converts it to energy in chemical bonds of sugar

Cellular respiration makes ATP for the organism by breaking apart the chemical bonds of sugar

The “big picture” ---


– Photosynthesis uses solar energy• To produce glucose and O2 from CO2 and H2O

CO2

H2O

Glucose

O2

ATP

ECOSYSTEM

Sunlight energy

Photosynthesis in chloroplasts

Cellular respiration in mitochondria

(for cellular work)

Heat energy

+ +

In an Ecosystem:


– Cellular respiration uses glucose to make ATP• Produces CO2 and H2O from glucose and O2

CO2

H2O

Glucose

O2

ATP

ECOSYSTEM

Sunlight energy

Photosynthesis in chloroplasts

Cellular respiration in mitochondria

(for cellular work)

Heat energy

+ +


Cells trap energy in ATP – ADP + Pi ATP by two (2) different processes:

1. By directly coupling ATP synthesis to a specific exergonic reaction

“Substrate-level phosphorylation”

2. By indirectly coupling ATP synthesis to exergonic reactions

“Oxidative Phosphorylation”“Chemiosmosis”“Electron-transport Pathway”

- Trap energy from breaking bonds in glucose

- High-energy electrons carried by NADH & FADH2

- Energy transferred to ATP synthesis enzyme on a membrane


1. ATP made by substrate-level phosphorylationA phosphate group is directly transferred from an organic

molecule to ADP

Enzyme

Adenosine

Organic molecule(substrate)

ADP ATP

P

PP P

P

Directly couple an exergonic reaction to the endergonic reaction than makes ATP, all on an enzyme


2. ATP made byOxidative Phosphorylation / Chemiosmosis

Uses diffusion of a H+ across a membrane to provide energy for the synthesis of ATP

So – Let’s take a look at how membranes transport substances to see how this works:


MEMBRANE STRUCTURE AND FUNCTION

Membranes organize and compartmentalize the chemical reactions in cells

–Membranes• Provide structural order for metabolism

• Provide an efficient way for making ATP using energy from electrons in the chemical bonds of your food

• Use enzyme-like proteins and other organic chemicals for transporting substances across the membrane