Download Chapter 6: An introduction to metabolism

Chapter 6: An introduction to metabolism
Metabolism Energy and Life
1. The chemistry of life is organized into metabolic pathways
i. Metabolism is all the chemical reactions in an organism
1. All about managing materials and energy resources in the
cell
2. Catabolic pathways are about degradation
a. Major pathway is respiration
3. Anabolic pathways are about synthesis
a. Example is synthesis of proteins form amino acids
ii. Energy coupling
1. Energy produced in catabolism is used to drive anabolism
2. Organisms transform energy
i. Chemical energy is a form of potential energy stored in molecules
as a result of the arrangement of the atoms in those molecules
3. The energy transformations of life are subject to two laws of thermodynamics
i. First Law: principle of conservation of energy
1. Organisms are energy transformers
ii. Second Law: every energy transformation makes the universe
more disordered
1. Energy flows to living systems in the from of light and
leaves in the form of heat
4. Organisms live at the expense of free energy
i. Exergonic reaction proceed with a net release of free energy
ii. Endergonic reactions adsorb free energy for the environment
iii. ATP is responsible for most energy coupling in cells
5. ATP powers cellular work
i. Cells do three types of work
1. Mechanical work (moving parts of the cell)
2. Transport work (moving molecules across membrane
against concentration gradients)
3. Chemical work (synthesis activities)
ii. The 3 P tail of ATP is unstable due to all the negative charges
close to each other
1. ATP readily gives up a P to reduce the instability
2. The recipient of the P is phosphorylated which is also less
stable than the original molecule, unstable molecules are
more reactive
3. Regeneration of ATP from ADP requires coupling to an
exergonic reaction
Enzymes
1. Enzymes speed up metabolic reactions by lowering energy barriers
i. An enzyme is a catalytic protein
ii. Starting a reaction involves activation energy so the reactants can
reach the transition state, which is unstable (nice graphs on page
97)
iii. Enzymes lower the activation energy of a specific reaction
2. Enzymes are substrate specific
i. The substrate fits the active site
1. Induced fit means the substrate can make the active site fit
the substrate closely
3. The active site is the catalytic center of the enzyme
i. The induced fit can apply stress to the substrate molecule,
stretching or bending critical chemical bonds
ii. The active site can provide a micro-environment conductive to
particular reactions
iii. The active site can temporarily participate in the reaction
4. A cells physical and chemical environment affects enzyme activity
i. 3 D structure of a protein is sensitive to temperature and pH
ii. Reaction velocity increases with increasing temperature up to a
point
iii. Enzymes also have an optimal pH
The control of metabolism
1. Metabolic control often depends on allosteric regulation
i. Competitive inhibitor mimics the substrate and blocks the active
site
ii. Noncompetitive inhibitor binds to a location (allosteric site) away
from the active site but changes the conformation so that the active
site does not function
iii. Allosteric enzymes are usually made of multiple polypeptides and
fluctuate between active forms and inactive forms
iv. Allosteric activators can stabilize the active form
v. Allosteric inhibitors can stabilize the inactive form
vi. Feedback inhibition happens when the product of a multiple step
process inhibits the enzyme at the beginning of the process
vii. An enzyme made of subunits can show cooperativity when the
binding of a substrate to one active site can change the
conformation of the other active sites to amplify the enzyme
2. The localization of enzymes in a cell helps order metabolism
i. Enzymes for several steps in a pathway are assembled into multienzyme complex
ii. Enzymes can be located together in fixed locations in the cell or in
the membrane
Example is enzymes for respiration are in the mitochondria