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Bioprocess Engineering
Chap 5 Major Metabolic Pathways
I Introduction
1. Even the same species may produce different products when grown under different
nutritional and environmental conditions
2. Metabolic pathways are subgroups as aerobic and anaerobic metabolism
3. Catabolism: the intracellular process of degrading a compound into smaller amd
simpler products (ex: glucose to CO2 and H2O) and produces energy for the cell
4. Anabolism: involves in the synthesis of some complex compounds (ex: glucose to
glycogen) and requires energy
II Bioenergetics
1. Energy is obtained from the catabolism of carbon compounds (carbohydrates)
2. Metabolic rxns can be classified in 3 catagories:
(1) degradation of nutrients
(2) biosynthesis of small molecules (a.a., nucleotides)
(3) biosynthesis of large molecules
3. Energy in biological system is stored and transferred via adenosine triphosphate
(ATP), which contains high-energy phosphate bonds (p.130)
(1) active form of ATP is complexed with Mg2+
(2) standard free-energy charge for the hydrolysis of ATP is 7.3kcal/mol
(Eq.5.1, 5.2, p.130)
(3) Fig5.2 (p.130)
4. Hydrogen atoms released in biological oxidation-reduction rxns are carried by
nucleotide derivatives, especially by nicotinamide adenine dinucleotide (NAD+ )
and nicotinamide adenine dinucleotide phosphate (NADP +)
(1) NADH can donate electrons to certain compounds and accept from others
(2) NADH has two major functions in biological systems:
a. reducing power: NASH and ANDPH supply hydrogen in biosynthetic
rxns
b. ATP formation in respiratory metabolism by the energy released from
the electron transport by NADH
I. Glucose metabolism: Glycolysis and the TCA cycle
1. The catabolism of glucose by glysolysis (Embden-Meyerhof-P)arnas (EMP)
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pathway) is the primary pathway in many organisms
2. Aerobic catabolism of organic compounds such as glucose may be considered
in three different phases:
(1) EMP pathway for fermentation of glucose to pyruvate
(2) Krebs, tricarboxylic acid (TCA), or citric acid cycle for conversion of
pyruvate to CO2 and NADH
(3) Respiration or electron transport chain for fo rmation of ATP by
transferring electrons from NADH to an electron acceptor
a. aerobic respiration: when O2 used as final electron
acceptor
b. anaerobic respiration: when NO3 -, SO 43-, Fe3+, Cu2+, and
S0 used as electron acceptors
3. Glycolysis or the EMP pathway results in the breakdown of glucose to 2
pyruvate molecules (Fig.5.4, p.133)
(1) under anaerobic conditions, pyruvate may be converted to lactic acid,
ethanol, acetone, butanol, and acetic acid
(2) under aerobic conditions, pyruvate is converted to CO2 and NADH
through TCA cycle
(3) the overall glycolysis rxn:
glucose+2ADP+2NAD++2Pi→2pyruvate+2ATP+2(NADH+H+)
(4) pyruvate produced in EMP pathway transfers its reducing power to
NAD+ via Kreb cycle
(5) Glycolysis takes place in cytoplasm
(6) The site for Kreb cycle is the matrix of mitochondria in eukaryotes, but
is associated with membrane-bound enzymes in prokaryotes
(7) TCA cycle: succinate and α-ketoglutarate produced during TCA cycle
are used as precursors for the synthesis of certain amino acids
(8) Major roles of TCA cycle:
a. to provide electrons (NADH) for the electron transport
chain and biosynthesis
b. to provide C skeletons for amino acid synthesis
c. to generate energy
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