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Cell Metabolism
Things to look for in the metabolic process:
1. Energy transformations
Kinetic energy
Examples:
work
Potential energy
2. Chemical reactions transfer energy
What determines how much energy is transferred?
Free Energy
3. Chemical reactions proceed based on several factors.
4. Reactions requiring energy are coupled to reactions releasing
energy.
5. Many metabolic reactions are reversible; many are irreversible
Gibbs Free Energy Changes
Rxn#
Enzyme
DG°'(kJ/mol)
DG(kJ/mol)
1
Hexokinase
-16.7
-33.5
2
Phosphogluco-isomerase +1.7
-2.5
3
Phosphofructokinase
-14.2
-22.2
4
Aldolase
+23.9
-1.3
5
Triose phos. Isomerase
+7.6
+2.5
6
G-3-PDH
+12.6
-3.4
7
Phosphoglycerate kinase -37.6
+2.6
8
Phosphoglycerate mutas
+8.8
+1.6
9
Enolase
+3.4
-6.6
10
Pyruvate kinase
-62.8
-33.4
11
2
3
4
5
6
Identify:
potentially coupled reactions
7
8
9
10
ENZYMES
What are they?
What do they do to help reactions proceed?
How do they do this? (induced fit)
Important characteristics of enzymes:
Specificity
May require:
activation
Cofactors or Coenzymes
Enzyme activity may be modulated.
Possible modulators include:
pH
Temperature
Other molecules
competitive inhibitors
noncompetitive inhibitors
allosteric modulators
covalent modulators
Concentration of enzyme/substrate
Can you explain HOW/WHY each of these modulators results in a change in enzyme activity ?
Km = affinity of substrate for enzyme; defined as
[substrate] that elicits half-maximal reaction velocity
Km
Types of reactions to look for in metabolic pathways:
•Oxidation-reduction reactions
•Transfer chemical groups
phosphate- catalyzed by kinases (+), phosphatases (-)
substrate-level phosphorylation
oxidative phosphorylation
amino acids- catalyzed by aminases (+), deaminases(-)
You have just isolated a new enzyme and have determined the speed of the reaction it catalyzes at
three increasing substrate concentrations. You find that the slope of the product versus time curve
increases as concentration of substrate increases. What can you conclude about the conditions in
the reaction mixture? Explain.
Activity of phosphofructokinase, which catalyzes step 3 of glycolysis, is affected by several
substances that act as allosteric modulators. Effects of some of these modulators are shown in
the following graphs. Which of these substances apparently acts to decrease the affinity of the
enzyme? Explain your answer in terms of the mechanism of action of the substance you’ve
chosen.
Reaction rate
High [AMP]
High [AMP] + citrate
No AMP
Substrate concentration