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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
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