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Chemical Reactions in Cells. Energetics, Enzymes and Metabolic Reactions. Energy. Energy is the capacity for work or change. Kinetic Energy = energy of movement Potential Energy = stored energy 1 st Law of Thermodynamics

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chemical reactions in cells

Chemical Reactions in Cells

Energetics, Enzymes and Metabolic Reactions

energy
Energy
  • Energy is the capacity for work or change.
  • Kinetic Energy = energy of movement
  • Potential Energy = stored energy
  • 1st Law of Thermodynamics
    • Energy can be transferred and transformed from one form to another but it cannot be created or destroyed.
energy1

usable

usable

usable

usable

Energy
  • 2nd Law of Thermodynamics
    • Energy transfer or transformation increases the entropy of the universe
    • Increase in entropy = randomness
    • Energy conversions result in a loss of useful energy
free energy energy useful for change

Spontaneity of a reaction depends on free

  • energy change

G

reaction = Gproducts – Greactants

G

G

  • If is negative, free energy is released and the reaction is spontaneous

G

  • If is positive, free energy is consumed
Free Energy = Energy Useful for Change
free energy energy useful for change1

Free Energy change depends on changes in

    • total energy (enthalpy)
    • entropy (unusable energy, disorder)

H

S

  • In living systems, entropy changes have substantial influence
    • when is positive, and the term is large, a negative value predicts a spontaneous reaction

S

T

S

G

H

G

=

T

S

Free Energy = Energy Useful for Change
chemical reactions
Chemical Reactions
  • Involve the breaking and formation of chemical bonds
    • Reactants are converted to products.
    • Two types of reactions based on energy use:
      • Exergonic– free energy released
      • Endergonic – free energy consumed
exergonic reactions

Reactants changed to transition-state species

high

Energycontentofmolecules

G

low

Progress of reaction

Exergonic Reactions

Burning glucose (sugar):an exergonic reaction

Activation energy neededto ignite glucose

Glucose + O2

Energy released byburning glucose

C O2 + H2O

endergonic reactions

high

Energycontentofmolecules

G

low

Progress of reaction

Endergonic Reactions

Photosynthesis:an endergonic reaction

Glucose

Net energycaptured bysynthesizingglucose

Activationenergy fromlight capturedby photosynthesis

CO2 + H2O

applying your knowledge
Endergonic Reaction

Exergonic Reaction

D. Which type of reaction would have a positive value for G?

Applying Your Knowledge

A. Which type of reaction would be spontaneous?

B. For which type of reaction will the products have a higher energy than the reactants?

C. Which type of reaction releases energy?

short term energy storage
Short-Term Energy Storage
  • Chemical Energy is stored in the bonds of ATP
    • ATP = adenosine triphosphate
    • ADP = adenosine diphosphate
    • to store energy
      • ADP + Phosphate + Energy ATP
    • to release energy
      • ATP  ADP + Phosphate + Energy
coupled reactions
Coupled Reactions

Pairing of an Exergonic reaction, often involving ATP, with an Endergonic reaction

Note that overall free energy change is negative

metabolic reactions
Metabolic Reactions
  • Anabolic
    • link simple molecules to produce complex molecules (eg. dehydration synthesis of starch)
    • require energy
  • Catabolic
    • break down complex molecules to release simple ones (eg. hydrolysis of starch sugars)
    • release energy stored in chemical bonds
metabolic pathways

D

E

Metabolic Pathways

InitialReactants

Intermediates

FinalProducts

B

C

A

Enzyme 1

Enzyme 2

Enzyme 3

Enzyme 4

Pathway 1

F

G

Pathway 2

Enzyme 5

Enzyme 6

enzymes assist in biological reactions
Enzymes Assist in Biological Reactions

Enzymes are biological catalysts.

biological: composed of protein or, rarely, RNA

catalyst: speeds up a reaction without being changed by the reaction

properties of enzymes

G

–The value of and the ratio of reactants and products at equilibrium is the same as for an uncatalyzed reaction

Properties of Enzymes
  • Enzymes speed up biological reactions by lowering the activation energy for the reaction.
    • Enzymes provide a surface where the catalysis takes place
    • The reaction reaches equilibrium more rapidly
activation energy controls rate of reaction

high

Energycontentofmolecules

G

low

Progress of reaction

Activation Energy: Controls Rate of Reaction

Amount of energy required for reaction to occur

transition state

Activationenergy withoutcatalyst

Activationenergy withcatalyst

properties of enzymes1
Properties of Enzymes
  • Enzymes are SPECIFIC for the reactants (substrates) in the reactions that they catalyze.
  • Only substrates that fit the active site of the enzyme can bind and complete the reaction
    • active site: region on enzyme where substrates bind
enzyme substrate interactions

induced fit

Enzyme-Substrate Interactions

Substrate

Substrate

1 Substrates enter active site

ActiveSite

2 Shape change promotes reaction

Enzyme

Product released;enzyme ready again

chemical events at active sites
Chemical Events at Active Sites
  • Enzymes hold substrates in the proper orientation for the reaction to take place
chemical events at active sites1
Chemical Events at Active Sites
  • Enzymes induce strain in the substrate to produce a transition state favorable to reaction
  • Active site provides a microenvironment that favors the chemical reaction
chemical events at active sites2
Chemical Events at Active Sites
  • Active site directly participates in the reaction
    • covalent bonding can occur between enzyme and substrate
    • R groups of the enzyme’s amino acids can temporarily add chemical groups to the substrates
molecules that assist enzymes
Molecules that Assist Enzymes
  • Cofactors: inorganic ions that bind to enzymes, eg. zinc
  • Coenzymes: small organic factors that temporarily bind to enzymes, eg. biotin, NAD, ATP
  • Prosthetic groups: non-protein factors that are permanently bound an enzyme, eg. heme
factors influencing reaction rate
Factors Influencing Reaction Rate

Rate no longer increases since the active sites of all enzymes are saturated with substrate

  • Substrate Concentration

Rate is more rapid

Rate is proportional to substrate concentration

factors influencing reaction rate1

DIPF

Factors Influencing Reaction Rate
  • Competitive Inhibitors: Bind at the active site, compete for binding with substrate
    • Irreversible: form covalent bond with amino acids in the active site
factors influencing reaction rate2
Factors Influencing Reaction Rate
  • Competitive Inhibitors: Bind at the active site, compete for binding with substrate
    • Reversible: molecule similar to substrate occupies active site but does not undergo reaction
factors influencing reaction rate3
Factors Influencing Reaction Rate
  • Non-Competitive Inhibitors: Bind to a different site, cause a conformational change in the enzyme that alters the active site
    • Reversible
factors influencing reaction rate4
Factors Influencing Reaction Rate
  • Allosteric Regulation
    • Conversion between active and inactive forms of an enzyme due to binding of regulatory molecules at an allosteric site
      • Activators stabilize the active form
      • Allosteric inhibitors stabilize the inactive form
factors influencing reaction rate5
Factors Influencing Reaction Rate
  • Allosteric Regulation
    • Cooperativity: a substrate causing induced fit in one enzyme subunit can cause a change to the active form in all the other subunits
enzyme regulation feedback inhibition

CH3

CH3

CH2

OH

H

C

CH3

H

C

NH3

H

C

NH3

H

C

COOH

COOH

Feedback InhibitionIsoleucine allosterically inhibits enzyme 1

Enzyme Regulation: Feedback Inhibition

Commitment step

A

B

C

D

Enz. 1

Enz. 2

Enz. 3

Enz. 4

Enz. 5

Threonine(substrate)

Isoleucine(end product)

Feedback Inhibition: The product of a pathway inhibits an initial step in the pathway to decrease its own production

properties of enzymes2
Properties of Enzymes
  • Three dimensional structure of an enzyme preserves its ACTIVE SITE
  • Conditions that can affect three dimensional structure include: heat, pH (acid/base balance) and other chemicals (salt, charged ions)
applying your knowledge1
Where can an inhibitor bind to stabilize the inactive form of an enzyme?

Where do the substrates bind?

Enzymes (raise or lower) the (1, 2, 3, 4 or 5) of a reaction.

What is the model for a shape change caused by substrate binding to the enzyme?

Active Site

Activation Energy

Allosteric Site

Commitment step

Induced fit

Applying Your Knowledge
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