Chapter 8  Metabolism

 

 Outline

•     Metabolic pathways

•      Energy

•      ATP & Exergonic and Endergonic Reactions

•      Enzymes

•      Control of Metabolism

 

 

Metabolism

•     All of the chemical reactions that occur within an organism.

•     Intricately branched pathways that alter molecules one step at a time.

•     Each step is controlled by an enzyme.

 

 

Metabolic Pathways

•     Catabolic pathways break down complex molecules and release energy.

–  uphill

•     Anabolic pathways build complicated molecules and use energy.

–  Downhill

•     Energy released from the downhill pathways fuels the uphill pathways

–  energy coupling

 

 

 Energy  Definitions

•     “The capacity to do work.”

•     Two states

–  Kinetic energy: The energy of motion

–  Potential energy: Stored energy

•     Forms: mechanical, heat, sound, electric, light, radioactive and chemical energy.

•     Units:

–   kilocalorie (kcal)

•   1 calorie = the heat to raise the temp. of a gram of water one degree Celsius.

–   Joule ( 1 joule = 0.239 cal

 

 

Figure 8.2 Transformations between kinetic and potential energy

 

Chemical Energy

 potential energy stored in molecules as a result of the arrangement of the atoms.

–   Includes the molecules of food, coal, gasoline, and other fuels

–   Very important in biology.

 

 

The Laws of Thermodynamics

•      The First Law of Thermodynamics

–   “Energy cannot be created nor destroyed.”

–   Energy can be changed from form to form.

 

•      The Second Law of Thermodynamics

–   “Entropy in the universe is continuously increasing.”

–   (Read – “With every change in energy form, some energy is lost as heat, so that there is less usable energy available”.

 

 

The order of life requires constant input of energy.

 Free Energy

•     The amount of energy available to break and form chemical bonds is called the “free energy of the molecule”.  (i.e. energy available for work)

•     ∆G = ∆ H – T∆ S

–   Where

•   G = “Gibbs free energy”

•   H = enthalpy (energy in the chemical bonds)

•   T = absolute temperature

•   S = entropy

•    ∆  = change

 

 

Figure 8.5  The relationship of free energy to stability, work capacity, and spontaneous change

 

 Endergonic Reactions

•      The reaction requires the input of energy in order to occur.

•      Will not proceed spontaneously.

•     ∆ G is positive.

–   Products contain more free energy than the reactants.

 

 

Figure 8.6  Energy changes in exergonic and endergonic reactions

 

Exergonic Reactions

•      The reaction releases energy.

•      Will proceed spontaneously.

•     ∆ G is negative.

–   Reactants contain more free energy than the products.

 

 

Figure 8.6  Energy changes in exergonic and endergonic reactions

 

Equilibrium and Metabolism

•     Reactions in a closed system eventually reach equilibrium and can no longer do work.

Equilibrium and Metabolism

•     The reactions in our body occur in an open system.  The constant flow of material in and out of the cell keeps the chemical reactions in metabolic pathways from reaching equilibrium.

•     Therefore, differences in free energy are maintained (- ∆G) and the chemical reactions can proceed.

 

 

Equilibrium and Metabolism

•     Most chemical reactions in the cell occur in multistep metabolic pathways.

•     The product of one reaction is used in the next step, so products do not accumulate.

 

 

Equilibrium and Metabolism

•     As longer as there is a constant source of fuel (glucose!), the metabolic pathways do not reach equilibrium, and the cell can continue to do work.

 ATP

•     Energy currency used by all cells

•     Powers almost every energy-requiring process.

 

 

Figure 8.9  The structure and hydrolysis of ATP

 

 How ATP Powers Reactions

•      Energy is stored in the high energy phosphate bonds.

•      If ATP is converted to ADP +P, energy is released.

•      The cell couples the breakdown of ATP with an endergonic reaction.

•      ATP is not useful for long term storage because it breaks down too easily.

 

 

Figure 8.10  Energy coupling by phosphate transfer

 

 

Figure 8.12 The ATP cycle

 

 

 Enzymes

•      Biological catalysts

•      Highly specific

•      Act as control points for the chemical reactions that occur in organisms.

•      Not changed or used in the reaction, can be used over and over.

•      Most enzymes are proteins, but some are RNA molecules.

 

 

Figure 8.13 Example of an enzyme-catalyzed reaction: Hydrolysis of sucrose

 

 

Activation Energy

•     Most spontaneous reaction require activation energy to get them started.

•     Rate of a reaction depends on the activation energy.

•     Reactions with a large energy of activation will proceed more slowly.

•     Catalysts lower the activation energy.

•     Can increase the rate of an exergonic reaction, but cannot make an endergonic reaction “go”: 

 

 

Figure 8.14  Energy profile of an exergonic reaction

 

 

Figure 8.15  Enzymes lower the barrier of activation energy

 

 

How Enzymes Work

•      Enzymes are globular proteins with a cleft called an “active site”.

•      Substrates bind to the enzyme at the active site, forming an enzyme-substrate complex.

•      The binding of the substrate changes the conformation of the enzyme so that they fit together better (induced fit).

 

 

Figure 8.16  The induced fit between an enzyme and its substrate

 

 

An Enzyme Catalyzed Reaction

•      The unique 3-D shape of the enzyme allows it to bind to the substrates (reactants).

•      Substrates are brought together in a favorable orientation, or chemical bonds are stressed, which lowers the activation energy.

 

Figure 8.17  The catalytic cycle of an enzyme

 

 

Factors Affecting Enzyme Activity

•      Temperature

•      pH

•      Cofactors

•      Inhibitors and Activators

 

 

Temperature

•     Heat increases the rate of an uncatalyzed reaction.

•     The rate of an enzyme –catalyzed reaction increase with temperature, up to the temperature optimum. 

–   With ↑ temp, get ↑ flexibility of H-bonds, hydrophobic interactions.

–   After the temperature optimum, increased random movement of molecules leads to denaturation of the enzyme.

 

 

Figure 8.18 Environmental factors affecting enzyme activity

 

 

 pH

•      Ionic interaction between charged residues hold enzymes together.

•      These bonds are sensitive to the [H+] concentration.

•      Most enzymes have a pH optimum between ph 6-8.

 

 

Figure 8.18  Environmental factors affecting enzyme activity

 

 

 Enzyme Cofactors

•      Cofactors assist enzyme function.

•      Type of cofactors

–   Inorganic metal ions (Zn2+ , Mg2+ , Mn2+ )

–   Coenzymes – non-protein organic molecules

–     Ex. NAD+

 

 Inhibitors and Activators

•     Enzyme activity is highly regulated by inhibitors and activators.

•     Types of inhibitors

–   Competitive inhibitors compete with the substrate for binding at the active site.

–   Noncompetitive inhibitors bind to an allosteric site that changes the shape of the enzyme so that it is less active.

•     Activators bind to allosteric site and keep the enzyme in its active configuration, increasing active.

 

 

Figure 8.19  Inhibition of enzyme activity

 

 

The Control of Metabolism

•     Regulation of enzyme activity helps control metabolism.

•     Often involves allosteric regulation.

 

 

Figure 8.20 Allosteric regulation of enzyme activity

 

 

The Control of Metabolism

•     Feedback inhibition is the switching off of a metabolic pathway by its end product which acts as an inhibitor of an enzyme near the beginning of the pathway.

 

 

Figure 8.21 Feedback inhibition

 

 

Specific Localization of Enzymes Within the Cell

•     Cellular structure organize metabolic pathways.

–  The enzymes in a pathway can be organized into a multienzyme complex.

–  Enzymes can be embedded in certain cellular membranes.

–  Enzymes can be located in particular organelles.

 

 

 The End.