Cell Metabolism - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 518

Hydrogen atoms released in biological oxidation-reduction reactions are carried by nucle-
otide derivatives, especially by nicotinamide adenine dinucleotide (NAD þ ) and nicotin-
amide adenine dinucleotide phosphate (NADP þ ) (see Fig. 10.19 ). This oxidation-reduction
reaction is readily reversible. NADH can donate electrons to certain compounds and accept
from others, depending on the oxidation-reduction potential of the compounds. NADH has
two major functions in biological systems:

1. Reducing power: NADH and NADPH supply hydrogen in biosynthetic reactions such as
CO 2 fixation by autotrophic organisms.

CO

2 þ 4

H CH O þ H O (10.37)

2 / 2 2

2. ATP formation in respiratory metabolism: The electrons (or H atoms) carried NADH are
transferred to oxygen via a series of intermediate compounds (respiratory chain). The
energy released from this electron transport results in the information of up to three ATP
molecules. ATP can be formed from the reducing power in NADH in the absence of
oxygen if an alternative electron acceptor is available (e.g. NO 3 ).

10.7.2. Glucose Metabolism: Glycolysis and the TCA Cycle
The most frequently applied energy source for cellular growth is sugars, in particular
glucose, which are converted to metabolic products (e.g. carbon dioxide, lactic acid, acetic
acid, and ethanol) with concurrent formation of ATP, NADH, and NADPH. NADH is
produced together with NADPH in the catabolic reactions, but whereas NADPH is
consumed mainly in the anabolic reactions, NADH is consumed mainly within the catabolic
reaction pathways, e.g. by oxidation with free oxygen in respiration (see Section 10.7.4 ). Most
sugars are converted to glucose-6-phosphate (G6P) or fructose-6-phosphate (F6P) before
being metabolized. The intracellular isomerization of G6P to F6P is normally in equilibrium,
and G6P can therefore be considered a common starting point in many catabolic pathways. In
some microorganisms, formation of G6P from glucose occurs in the transport process, but in
others this compound is formed from intracellular glucose in a reaction coupled with the
hydrolysis of ATP. The catabolism of sugars from G6P is traditionally divided into glycolysis
and pyruvate metabolism. Glycolysis is defined as the sum of all pathways by which glucose
(or G6P) is converted to pyruvate.
Aerobic catabolism of organic compounds such as glucose may be considered in three
different phases:

1. Glycolysis or Embden-Meyerhof-Parnas (EMP) pathway for fermentation of glucose to
pyruvate.
2. Krebs, tricarboxylic acid (TCA), or citric acid cycle for conversion of pyruvate to CO 2 and
NADH.
3. Respiratory or electron transport chain for formation of ATP by transferring electrons from
NADH to an electron acceptor.

The final phase, respiration, changes reducing power into a biologically useful energy
form (ATP). Respiration may be aerobic or anaerobic, depending on the final electron acceptor.