Page 321 - Biology_F5
P. 321
The second step is the formation of isocitrate. In the presence of aconitase enzyme, citrate is
converted into isocitrate. This process is accomplished by dehydration and rehydration to yield
an isomer called isocitrate.
Biology for Advanced Level Secondary Schools
The third step of reaction is oxidation of Isocitrate. In this reaction, the Isocitrate
The third step of reaction is oxidation of Isocitrate. In this reaction the Isocitrate is oxidatively
is oxidatively decarboxylated to form α- ketoglutarate. The enzymes involved are
decarboxylated to form α- ketoglutarate. The enzymes involved are isocitrate dehydrogenase
isocitrate dehydrogenase and oxalosuccinate decarboxylase together with NAD, and
and oxalosuccinate decarboxylase together with NAD, and the products are NADH and CO .
2
2
the products are NADH and CO .
2
2
FOR ONLINE READING ONLY
The fourth step is the oxidation of α- Ketoglutarate to form Succinyl – CoA. In the
The fourth step is the oxidation of α- Ketoglutarate to form Succinyl – CoA. In the presence of
presence of α- ketoglutarate dehydrogenase enzyme, α- ketoglutarate is oxidatively
α- ketoglutarate dehydrogenase enzyme, α- ketoglutarate is oxidatively decarboxylated to form
decarboxylated to form succinyl CoA (4C). During this reaction NAD is reduced to
succinyl CoA (4C). During this reaction NAD is reduced to NADH2. The products are
+
+
NADH and CO .
NADH . The products are NADH and CO .
2
2
2
2 2
The fifth step is the conversion of Succinyl CoA into succinate. This reaction is
The fifth step is the conversion of Succinyl CoA into succinate. This reaction is catalysed by
The fifth step is the conversion of Succinyl CoA into succinate. This reaction is catalysed by
catalysed by an enzyme succinyl CoA synthetase. It involves the removal or loss of
an enzyme succinyl CoA synthetase. It involves the removal or loss of CoA from succinyl –
an enzyme succinyl CoA synthetase. It involves the removal or loss of CoA from succinyl –
CoA. The CoA is replaced by phosphate group which is then removed and attached to
CoA from succinyl – CoA. The CoA is replaced by phosphate group which is then
CoA. The CoA is replaced by phosphate group which is then removed and attached to
guanosine diphosphate (GDP), thereby forming guanosine triphosphate (GTP). GTP is an
removed and attached to guanosine diphosphate (GDP), thereby forming guanosine
guanosine diphosphate (GDP), thereby forming guanosine triphosphate (GTP). GTP is an
energy yielding molecule and is used to generate ATP when it donates a phosphate group to
energy yielding molecule and is used to generate ATP when it donates a phosphate group to
triphosphate (GTP). GTP is an energy yielding molecule and is used to generate ATP
ADP.
ADP.
when it donates a phosphate group to ADP.
There are two forms of the enzyme, called isoenzymes, for this step, depending upon the
There are two forms of the enzyme, called found. One form is found in tissues that
There are two forms of the enzyme, called isoenzymes, for this step, depending upon the
type of animal tissue in which they are found. One form is found in tissues that use large
isoenzymes, in this step, depending on the use large amounts of ATP, such as heart
type of animal tissue in which they are found. One form is found in tissues that use large
amounts of ATP, such as heart and skeletal muscle. The second form of the enzyme is found
amounts of ATP, such as heart and skeletal muscle. The second form of the enzyme is found
type of animal tissue in which they are and skeletal muscle. The second form of
in tissues that have a high number of anabolic pathways, such as liver. This form produces
in tissues that have a high number of anabolic pathways, such as liver. This form produces
guanosine triphosphate (GTP). GTP is energetically equivalent to ATP; however, its use is
guanosine triphosphate (GTP). GTP is energetically equivalent to ATP; however, its use is
more restricted. In particular, protein synthesis primarily uses GTP.
more restricted. In particular, protein synthesis primarily uses GTP. Form Five Student’s Book
314 The sixth step is the oxidation of succinate to form fumarate. The conversion of succinate into
The sixth step is the oxidation of succinate to form fumarate. The conversion of succinate into
fumarate involves removal of hydrogen and is catalysed by succinate dehydrogenase. In this
fumarate involves removal of hydrogen and is catalysed by succinate dehydrogenase. In this
reaction FAD is reduced to FADH .
reaction FAD is reduced to FADH . 2
2