1. Chemical Reactions and Enzymes

2. Chemical Reactions
A chemical reaction is a process that changes one set of chemicals into another set of chemical.
The elements or compounds that enter into a chemical reaction are known as the reactants.
The elements or compounds produced by a chemical reaction are known as products.

3. Energy in Reactions
Chemical reactions can either release energy or absorb energy.
An exergonic reaction releases energy and will often occur spontaneously.
An endogonic reaction absorbs energy and will not occur spontaneously.
Which of the following reactions are exergonic?

4. Activation energy is the energy required to get a chemical reaction started.
Burning glucose (sugar): an exergonic reaction
high
high
activation energy needed
to ignite glucose
glucose
activation
energy from
light captured
by photosynthesis
glucose + O2
energy
content of
molecules
energy
content of
molecules
net energy
captured by
synthesizing
glucose
energy released
by burning glucose
Figure :6-2
Title:
Energy relations in exergonic and endergonic reactions
Caption:
(a) An exergonic (downhill) reaction, such as the burning of sugar, proceeds from high-energy reactants (here, glucose) to low-energy products (CO2 and H2O). The energy difference between the chemical bonds of the reactants and products is released as heat. To start the reaction, however, an initial input of energy—the activation energy—is required. (b) An endergonic (uphill) reaction, such as photosynthesis, proceeds from low-energy reactants (CO2 and H2O) to high-energy products (glucose) and therefore requires a net input of energy, in this case from sunlight. Question In addition to heat and sunlight, what are some other potential sources of activation energy?
CO2 + H2O
CO2 + H2O
low
low
progress of reaction
progress of reaction
Photosynthesis: an endergonic reaction

5. high activation energy without catalyst activation energy energy
A Catalyst is a substance that speeds of the rate of a chemical reaction
high
activation energy
without
catalyst
activation energy
with catalyst
energy
content of
molecules
reactants
Figure :6-8
Title:
Catalysts lower activation energy, increasing the rate of reactions
Caption:
A high activation energy (black curve) means that reactant molecules must collide very forcefully in order to react. Only very fast-moving molecules will collide hard enough to react, so reactions with high activation energies proceed slowly at low temperatures, where most molecules move relatively slowly. Catalysts lower the activation energy of a reaction (red curve), so a much higher proportion of molecules move fast enough to react when they collide. Therefore, the reaction proceeds much more rapidly. Question Can a catalyst make a non-spontaneous reaction occur spontaneously?
Enzymes are proteins that act as biological catalyst
products
low
progress of reaction

6. Enzymes provide a site where reactants can be brought together and aligned properly to react.
- the reactants of an enzyme catalyzed reaction.

7. substrates active site of enzyme enzyme Figure :6-9 Title:
The cycle of enzyme–substrate interactions
Caption:
Question How would you modify reaction conditions if you wanted to increase the rate at which an enzyme-catalyzed reaction produced its product?

8. “on” or “off” keys (binding proteins)
Regulation of Enzyme Activity
allosteric
regulator
molecule
Allosteric
inhibition pH
Temperature
“on” or “off” keys (binding proteins)
Up: Enzyme “off”
Down: Enzyme “on”
Competitive
inhibition
Enzyme
structure
active site
substrate
enzyme
allosteric
regulatory site
Figure :6-11
Title:
Enzyme regulation by allosteric regulation and competitive inhibition
Caption:
(a) Many enzymes have an active site and an allosteric regulatory site on different parts of the molecule. (b) When enzymes are inhibited by allosteric regulation, binding by a regulator molecule alters the active site so the enzyme is less compatible with its substrate. (c) During competitive inhibition, a molecule somewhat similar to the substrate fits into the active site and blocks entry of the substrate.
Competitive- key
Fits lock, but wont
Open door
Up: Enzyme “off”