BSCI 230 Today, 2/8/01


Lecture #4
 

Recitation sections max out at 20!

Lots of spots left on our Entertainment Roster!!!
Fund Raising Team, please see Dr. K-I-A after class for a moment.
Todayís topic: ENZYMES

 

Reviews

Two factors determine the spontaneity of a reaction:
Enthalpy: DH
Entropy: DS
The spontaneity of a reaction is expressed as the Gibbs Free Energy or DG
Gibbs Free Energy
DG = DH - T DS

DG and Reactions

C6H12O6 <---> 6CO2 + 6 H2O
To the right, entropy is increasing, bonds are broken to give off energy, DG is negative (metabolism; catabolic)
To the left, entropy is decreasing, bonds are being formed, DG is positive (synthesis, anabolic)

Reactions:

CH4 + 2O2 <--> CO2 + 2 H2O
ATP <---> ADP + Phosphate
2 H2 + O2 <--> 2 H2O
Why doesnít this just happen without a spark??

Energy of Activation with a Catalyst

Proteins as Catalysts
Chemical reaction can occur if it releases free energy
Due to high activation energies, biological reactions occur slowly without catalysts
Enzymes (catalysts) do not alter the equilibrium or the change in free energy
Catalysts decrease activation energy

 

Proteins as Catalysts

Enzymes are proteins (rare exceptions) and end in ìaseî
They act on substrate molecules
An enzyme is not irreversibly changed by the reaction
Enzymes bind substrates to ëactive siteí (see Fig 6-2)

 
 

Properties of Enzymes

Experiment with an Enzyme
Series of test tubes with constant pH, temperature, and [Enzyme]
Each tube has a different [S] (substrate concentration)
We will measure the amount of product formed over time (i.e., the rate of product formation) at each [S]
mmoles/min
What would happen at a higher initial [S] ??

What happens at a very high [S]?

Experimental Results at One VERY HIGH Initial [S]

Experimental Results at One [S]
Experimental Results: Rate vs. [S]INITIAL
Km = [S] at 1/2 Vmax
Km is a measure of the affintiy of the enzyme for the substrate

[S] vs. Velocity plot is not linear and thus not easy to plot or gather information from

Therefore plot


1/[S] vs. 1/Vi
 

Lineweaver-Burk Plot:

1/Vi = Km/Vmax (1/[S]) + 1/Vmax

(note: a straight line!)

y = m (x) + b
 

Can you explain this result? (PH curve)

Can you explain this result? (temperature)

 

Inhibition Is Possible

Similar molecules interfere or compete with substrate for active site of enzyme
Less product is formed in competitive inhibition
Other molecules may inactivate enzyme molecules reducing maximum rate of product formation
Noncompetitive inhibition

 

Other terms:

Allosteric (other site)
inhibition or activation by binding and changing shape of enzyme protein
Feedback Inhibition
product builds up and decreases enzyme activity

 

Metabolism

Catabolic reactions:
Breaking bonds, liberating energy, increasing entropy
a negative DG
Anabolic reactions
Making more complex molecules, requiring energy, decreasing entropy
a positive DG
Metabolic Reaction
C6H12O6 <---> 6CO2 + 6 H2O
To the right, energy is liberated and carbon (C) loses electrons (and protons) and is thus oxidized
To the left, energy is required, C gains electrons (and protons) and is reduced
Capturing the energy form metabolic reactions: