Chemistry Reference
In-Depth Information
The global outcome of the Krebs cycle is that one molecule of acetyl CoA is converted to 3 molecules of
NADH, 2 of CO
2
, 1 of GTP, and 1 of FADH
2
. The reducing equivalents will be used, as we will see later, to
generate ATP.
AN OVERVIEW OF ANABOLISM
As was pointed out in the introduction, the opposite of catabolism, anabolism involves the biosynthesis of more
complex and generally more highly reduced molecules, from the simpler and more oxidised molecules generated
in the course of catabolism. These biosynthetic pathways require both energy in the form of ATP and reducing
power in the form of NADPH. Whereas most of the NADH is funnelled through the mitochondrial respiratory
chain for use in ATP synthesis, catabolism can also produce reducing equivalents in the form of NADPH (mostly
through a variant of glucose catabolism, the pentose phosphate pathway). Photosynthetic organisms can generate
both NADPH and ATP using light energy.
The tricarboxylic acid cycle plays a key role, in centralising the oxidative metabolism of intermediates from
catabolic pathways. However, the tricarboxylic acid cycle not only enables the oxidation of acetyl CoA but it also
supplies a number of molecules which are used in biosynthetic pathways.
Figure 5.13
shows the positions at which
FIGURE 5.13
The tricarboxylic acid cycle plays a central role in supplying intermediates for biosynthetic pathways as well as receiving
intermediates from catabolic pathways. (Adapted from
Voet & Voet, 2004.
)
intermediates are drawn off for use in anabolic pathways.
-ketoglutarate, can
undergo transfer of an amino group with an amino acid (aminotransferase) to give aspartate and glutamate,
a
-keto acids, like oxaloacetate and
a
