Download Outline05 Enzymes - Napa Valley College

Biology 219 – Human Physiology
Clemens
Metabolism, Energy and Enzymes
Ch. 4
A. Metabolism
 total of all chemical reactions in the body
catabolic reactions - break down large molecules into smaller ones
anabolic reactions - build larger molecules from smaller ones (synthesis)
exergonic reactions release energy
endergonic reactions require energy
Major classes of metabolic reactions:
1. Hydrolysis and Dehydration (Condensation)
hydrolysis - adds H2O to break bonds between monomer units, catabolic
A–B + H2O → A–OH + H–B
e.g., sucrose + H2O → glucose + fructose
dehydration (condensation) - removes H2O to join monomers, anabolic
A–OH + H–B → A–B + H2O
e.g., peptide bond formation: amino acid1 + amino acid2 → dipeptide + H2O
2. Phosphorylation and Dephosphorylation
phosphorylation – adds a phosphate group (Pi)
C + Pi → C–P + H2O
e.g., ATP synthesis: ADP + Pi + energy → ATP + H2O
Transfer of phosphate from ATP: C + ATP → C–P + ADP; catalyzed by kinase enzymes
dephosphorylation – removes a phosphate group
C–P + H2O → C + Pi
e.g., ATP hydrolysis: ATP + H2O → ADP + Pi + energy
3. Oxidation-Reduction (Redox) Reactions
 electron transfer reactions: Oxidation Is Loss, Reduction Is Gain of electrons
- major energy source for cells: oxidation of sugars, fatty acids, and amino acids
- redox reactions are coupled: one molecule is oxidized, another is reduced
- redox reactions in cells often involve transfer of H atoms (not H+ ions)
e.g., reduction of pyruvate to lactate: C=O + 2 H → H-C-OH
- coenzymes are temporary carriers of H atoms and their electrons:
NAD+ + 2 H → NADH + H+
oxidized
reduced
FAD + 2 H → FADH2
oxidized
reduced
e.g., pyruvate + NADH + H+ → lactate + NAD+
- oxygen (O2) is the final electron acceptor in cellular respiration: ½ O2+ 2 H → H2O
B. Energy Metabolism
- cells use chemical energy to do biological work: movement, synthesis, transport
- energy is released in exergonic reactions that convert high-energy to lower-energy molecules
e.g., oxidation of glucose C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + energy
(high energy)
(lower energy)
- ATP is the “energy currency” of cells
energy captured from oxidation of substrates is used to make ATP (ADP + Pi → ATP)
energy released from ATP hydrolysis powers energy-requiring processes
Biology 219 – Human Physiology
Clemens
C. Enzymes
 biochemical catalysts – greatly speed up chemical reactions
- most enzymes are proteins
- increase reaction rate by lowering activation energy
or provide an alternative chemical pathway for the reaction
Rate
Functional properties of enzymes:
1. Substrate specificity
- substrate (reactant) binds reversibly to the active site of the enzyme
- specific fit between substrate and active site (shape and charge/polarity)
- induced fit: enzyme pulls on chemical bonds of the substrates
2. Sensitivity to temperature and pH
- effects on tertiary structure of proteins
3. Saturation kinetics
- reaction rate depends on substrate concentration (non-linear):
saturation
at low [S], reaction rate increases with increasing [S]
at high [S], reaction rate reaches a maximum level
- at the saturation point, all active sites are occupied
- maximum rate is limited by number of available enzymes
[Substrate]
4. Regulation
a. covalent regulation
- regulation of enzyme via covalent binding of a chemical group
- usually involves addition of phosphate group which activates the enzyme
E (inactive enzyme) + ATP → E~P (active enzyme) + ADP
- protein kinase enzymes catalyze phosphorylation of other enzymes;
important in cell signaling
b. allosteric regulation
- regulation by non-covalent binding of a modulator to a regulatory site on the enzyme
- can be either allosteric activation or allosteric inhibition
- reaction rate depends on concentration of the modulator as well as the substrate
c. feedback inhibition
- regulation of a multi-step metabolic pathway
- a product of the reaction pathway acts as a modulator that inhibits an enzyme in
an earlier step (via allosteric inhibition) to regulate formation of products
enzyme 1 enzyme 2
A X → B
→
C
more enzymes
→ → → Z
Examples of enzymes:
catalase
2 H2O2 → 2 H2O + O2
carbonic anhydrase
H2O + CO2 
H2CO3  H+ + HCO3hexokinase
glucose + ATP → glucose-6-phosphate + ADP
(first step in glycolysis)
phosphofructokinase (PFK) - key regulatory enzyme in glycolysis; feedback inhibition by ATP
Clinical applications:
1. abnormal enzyme levels may indicate disease (e.g., liver enzymes)
2. genetic deficiency of an enzyme → inborn errors of metabolism (e.g., PKU)