D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 11
CHEMICAL REACTIONS/ENZYMES
1. Chemical Reactions:
• general chemical reaction: A + B
 ---------
 C + D
- A & B represent reactants; C & D are products (ppt. 1)
- all reactions are bidirectional, and proceed to an equilibrium point (equal forward & backward changes produce no net change = no work)
- biological (steady state) reactions operate as open systems, i.e. never reach
equilibrium because reactants are replenished & products are removed (fig. 8 – 7 & ppt.
2)
• free energy changes: exergonic reactions lose energy to surroundings - proceed spontaneously, in accordance with laws of thermodynamics (figs. 8 - 3, 8 - 5 & ppt. 3);
endergonic reactions require energy from surroundings (fig. 8 – 6, 8 - 12 & ppt. 4)
- spontaneous tendency: decrease in free energy content or increase in
entropy (loss of order); exergonic reactions are likely; endergonic are unlikely
- activation energy: energy needed to initiate reaction; furnished by heat
(kinetic energy of molecules) (fig. 8 - 12 & ppt. 4)
- catalysts: substances which lower activation energy (fig. 8 – 13 & ppt. 5); speed rate of reaction; enzymes are catalysts in living systems: (fig. 8 – 14 & ppt. 6)

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- coupled reactions: two types: 1) product of one reaction becomes reactant for the next (open system)
2) energy surrendered from an exergonic reaction, e.g. ATP hydrolysis (fig. 8
- 8 & ppt. 7) may be used to drive an endergonic reaction (fig. 8 - 9 & ppt. 8)
• energy flow in living cell: many reactions of the cell's metabolism serve to
facilitate endergonic processes by coupling with exergonic processes via ATP (ppts. 9
& 10)
2. Enzymes:
• enzymes are proteins: many have requirement for cofactors (metals or organic groups) that must be bound for functional activity (fig. 8 - 14 & ppt. 11)
• substrate specificity: enzymes bind substrate(s) at active site; highly specific
“lock-and-key” or induced fit relationship (fig. 8 – 14 & ppts.11 & 12); enzyme-substrate
complex formed is temporary, releasing product and free enzyme (recycles) (fig. 8 – 15 & ppt. 13)
• product specificity: unique product(s) result(s)
• enzyme activity: rate of enzyme-catalyzed reaction (substrate consumption or
product formation)
- dependence on temperature & pH: temperature optimum – temperature that supports maximal rate of enzyme-catalyzed reaction; pH optimum – pH that supports maximal rate of enzyme-catalyzed reaction; these values differ for different enzymes
(fig. 8 – 16 & ppt. 14)

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- competitive inhibition: molecular analog excludes substrate by competing for active site; slows activity
- non-competitive inhibition: inhibitor binds at allosteric site; disrupts configuration of active site resulting in loss of activity
(fig. 8 – 17 & ppt. 15)
• enzyme regulation: enzymes can be regulated through activation or inhibition
(figs. 8 – 19 & ppt. 16)
- end-product (feedback) inhibition: accumulation of end product of a reaction sequence (metabolic pathway) imposes allosteric inhibition upon enzyme catalyzing initial step of sequence (fig. 8 – 21 & ppt. 17)
• summary of enzyme action: (movie - ppt. 18)