Page 155 - Color Atlas of Biochemistry

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146 Metabolism

Respiration and fermentation ylic acid cycle and the pyruvate dehydro-
genase reaction (see p. 144). β-Oxidation
and the malate shuttle, which are dependent
A. Aerobic and anaerobic oxidation of +
on free NAD , also come to a standstill. Since
glucose
amino acid degradation is also no longer able
In thepresenceof oxygen (i. e.,in aerobic to contribute to energy production, the cell
conditions), most animal cells are capable of becomes totally dependent on ATP synthe-
“respiring” various types of nutrient (lipids, sized via the degradation of glucose by
amino acids, and carbohydrates)—i. e., using glycolysis. For this process to proceed contin-
+
oxidative processes to break them down com- uously, the NADH+H formed in the cyto-
pletely. If oxygen is lacking (i. e., in anaerobic plasm has to be constantly reoxidized. Since
conditions), only glucose can be used for ATP this can no longer occur in the mitochondria,
synthesis. Although in these conditions glu- in anaerobic conditions animal cells reduce
cose breakdowninanimals already ends in pyruvate to lactate and pass it into the blood.
lactate and only produces small quantities of This type of process is called fermentation
ATP, it is decisively important for the survival (see p. 148). The ATP yield is low, with only
of cells at times of oxygen deficiency. two ATPs per glucose arising during lactate
synthesis.
In aerobic conditions (left), ATP is derived
almost exclusively from oxidative phosphor- To estimate the number of ATP molecules
ylation (see p. 140). Fatty acids enter the mi- formed in an aerobic state, it is necessary to
tochondria with the help of carnitine (see know the P/O quotient—i. e., the molar ratio
p. 164),and arebrokendownthere into CoA- between synthesized ATP (“P”) and the water
bound acetyl residues. Glucose is converted formed (“O”). During transport of two elec-
+
into pyruvate by glycolysis (see p. 150) in the trons from NADH+H to oxygen, about 10 pro-
cytoplasm. Pyruvate is then also transported tons are transported into the intermembrane
into the mitochondrial matrix, where it is space, while from ubiquinol (QH 2 ), the num-
oxidatively decarboxylated by the pyruvate ber is only six. ATP synthase (see p. 142) prob-
+
dehydrogenase complex (see p. 134) to yield ably requires three H to synthesize one ATP,
acetyl-CoA. The reducing equivalents (2 so that maximum P/O quotients of around 3
+
NADH+H per glucose) that arise in glycolysis or 2 are possible. This implies a yield of up to
enter the mitochondrial matrix via the malate 38 ATP per mol of glucose. However, the ac-
shuttle (see p. 212). The acetyl residues that tual value is much lower. It needs to be taken
are formed are oxidized to CO 2 in the tricar- into account that the transport of specific me-
boxylic acid cycle (see p. 136). Breakdown of tabolites into the mitochondrial matrix and
amino acids also produces acetyl residues or the exchange of ATP 4– for ADP 3– are also
products that can directly enter the tricarbox- driven by the proton gradient (see p. 212).
ylic acid cycle(seep. 180). Thereducing TheP/O quotients for theoxidation of
+
equivalents that are obtained are transferred NADH+H and QH 2 are therefore more in the
to oxygen via the respiratory chain as re- range of 2.5 and 1.5. If the energy balance of
quired. In the process, chemical energy is re- aerobicglycolysisiscalculatedon thisbasis,
leased, which is used (via a proton gradient) the result is a yield of around 32 ATP per
to synthesize ATP (see p. 140). glucose. However, this value is also not con-
stant, and can be adjusted as required by the
In the absence of oxygen—i. e., in anaerobic cell’s own uncouplers (UCPs; see p. 144) and
conditions—the picture changes completely. other mechanisms.
Since O 2 is missing as the electron acceptor
+
for the respiratory chain, NADH+H and QH 2
can no longer be reoxidized. Consequently,
not only is mitochondrial ATP synthesis
halted, but also almost the whole metabolism
in the mitochondrial matrix. The main reason
+
forthisisthe high NADH+H concentration
+
and lack of NAD , which inhibit the tricarbox-

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