Download Metabolism: sum of all chemical rxns

Chapter 8: Introduction to Metabolism
Metabolism: sum of all chemical reactions & life functions in an organism.
Catabolism: (catabolic pathway) releases energy by breaking down large molecules to simpler ones as in
cellular respiration or digestion.
Anabolism: (anabolic pathway) consumes energy (stores it) by building complicated molecules from
simpler ones as in photosynthesis or dehydration synthesis.
Energy is the ability to do work.
Kinetic energy: energy of motion as in heat (random
molecular movement) or light (powers photosynthesis)
Potential energy: energy which matter possesses due to
location or molecular arrangement as in chemical energy
stored in bonds.
Energy transformations:
KE of sunlight  PE of chemical bonds in glucose during
photosynthesis
PE in ATP  KE to drive cellular activities
First law of thermodynamics: Energy of the universe is
constant; it cannot be created or destroyed, only
transformed.
Second law of thermodynamics: Every process increases
entropy (disorder that is proportional to randomness).
Closed system: matter isolated from its surroundings.
Open system: energy can be transferred between a system and the environment.
Entropy and living organisms: Animals maintain highly ordered structure at the expense of increased
entropy in their surroundings. They take in complex high energy molecules as food and extract chemical
energy to create and maintain order. They return to the environment simpler, low energy molecules such as
CO2 and H2O and heat (unavailable to do work, usually). Organisms therefore are open systems.
The free-energy concept:
The amount of energy available to do work is called free energy (G).
G= Gibbs free energy (available to do work)
H= enthalpy or total energy
T= temperature in Kelvin (C+273)
S= entropy (randomness)
G=H-TS
The most useable energy harvested from a reaction is the system’s free energy  from the initial to final
state. Energy must be absorbed by bonds to break them for a reaction to occur; then atoms are rearranged.
When bonds reform, energy is released. The net energy is the difference between energy needed to break
bonds and amount released while forming new ones.
Why does G matter?
1. It tells the maximum amount of a system’s energy available to do work.
2. It indicates whether a reaction will occur spontaneously (without additional energy added).
3. It tells of a system’s stability or tendency to change to a more stable state.
Chapter 8: Introduction to Metabolism
Looking Closer:
1. Exergonic Reaction: net release or loss of energy. This is spontaneous (downhill) and G is
negative because energy is released.
2. An example is cellular respiration: G = -686 kcal/mol. C6H12O6 + 6O2  6H2O + 6CO2 +36 ATP
3. Endergonic Reaction: energy requiring reaction that absorbs free energy from the
surroundings. This has a net gain of free energy (G is +) and is not spontaneous (uphill)
because energy is absorbed.
4. An example is photosynthesis: G= +686 kcal/mol.
5. As a reaction approaches equilibrium, the G of the system decreases (for exergonic reaction).
6. When a reaction is pushed away from equilibrium, the G of the system increases.
7. When a reaction reaches equilibrium, G=0 because there is no net change in the system.
8. The strategy for cellular metabolism is to couple endergonic with exergonic reactions.
ATP and Cellular Work:
ATP drives 3 types of cellular work:
1. mechanical work: beating cilia, muscle contractions, cytoplasmic flow, chromosome movement.
2. transport work: pumping substances across membranes.
3. chemical work: pushing endergonic reactions that would not occur spontaneously.
ATP is Adenosine Triphosphate. It is a nucleotriphosphate similar to RNA.
Adenine is a nitrogenous base, ribose is a sugar, and there are three
phosphate groups.
Phosphates are unstable (too many negative charges); the phosphates
hydrolyze (split) readily in an exergonic reaction to release energy
(G =-7.3 kcal/mol).
How ATP Performs Work :
1. Controlled by enzymes
2. exergonic hydrolysis of ATP is coupled with
endergonic process of transferring a phosphate to
another molecule.
3. the molecule that gains a phosphate becomes
phosphorylated, becoming more reactive and
unstable.
ATP is continually regenerated by the cell:
1. The process is rapid (107 molecules used &
generated /sec/cell).
2. Reaction is endergonic:
3. +7.3 kcal/mol.
4. energy comes from cellular respiration.
5. Respiration controls removal of H2O and CO2
ensuring ATP production.
Enzymes:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Control the speed of reaction (G cannot predict speed).
They are organic catalysts changing the speed of reaction without being consumed.
They are proteins, composed of amino acids.
They absorb energy, break bonds & lower the activation energy to control the speed of a
reaction
Activation energy is the amount of energy molecules must absorb to start a reaction.
Transition state occurs when reactant molecules absorb enough energy to react.
React with a specific substrate (the substance being broken down).
The fit between enzyme & substrate depend on an enzyme’s 3D shape.
Active site: where enzyme & substrate meet (usually a pocket on enzyme surface & can
change shape).
Induced fit: change in enzyme active site shape, caused by the substrate.
Chapter 8: Introduction to Metabolism
Steps in catalytic cycle of enzymes:
1. Enzyme-substrate complex: substrate
binds to enzyme using H-bonds or ionic
bonds.
2. Substrate converted to products in
induced fit.
3. Products depart active site, enzyme
used again.
4. 1 enzyme converts 1000 substrates per
second.
How enzymes lower activation energy:
1. Active sites can hold 2 or more
substrates.
2. Induced fit may distort substrates’
chemical bonds lowering amount of
energy needed to break them.
3. Active site provides lower pH needed to
break bonds more effectively (caused by acidic amino acids).
How initial [substrate] determines the rate of reaction.:
1. The higher the [substrate], the faster the reaction
2. If [substrate] is high enough, the enzyme becomes saturated (all active sites are used up).
3. When an enzyme is saturated, the reaction rate depends on how fast the active site can
convert substrate to product (digest nutrients).
What affects enzyme activity:
1. Temperature: best temp is 37C (greatest # of enzyme-substrate collisions occurs here).
2. pH: best pH is 6-8 (neutral; exception is pepsin in stomach w/ pH =2).
3. ionic []: salts interfere w/ ionic bonds w/in enzymes.
4. Cofactors: small non-protein molecules which bind to active sites and are required for proper
enzyme function (ie: Zn, Fe, Cu (all inorganic), vitamins (organic coenzymes)).
5. Enzyme inhibitors: certain chemicals can selectively inhibit enzyme activity (covalent bonds
are irreversible, weak H-bonds are reversible).
6. Competitive inhibitors: resemble enzyme’s normal substrate & block the active site from the
substrate.  [substrate] to overcome competitive inhibitors.
7. Noncompetitive inhibitors: bind to another part of enzyme, not active site, changing enzyme’s
shape.
a. DDT inhibits nervous system.
b. penicillin blocks bacterial cell wall production.
Chapter 8: Introduction to Metabolism
Controlling Metabolism:
1. Feedback inhibition: regulation by end products.
2. This prevents the cell from wasting chemical resources.
3. Use of allosteric sites to change enzyme shape & shut it down.
4. Structural Order and Metabolism:
a. Dissolved enzymes and substrates may be localized in chloroplasts or mitochondria.
b. Some enzymes have fixed locations in the cell membrane.
c. Multienzyme complexes produce a sequence of reactions based on physical
arrangement.
Chapter 8: Introduction to Metabolism