Biology Times (^) March 19
- Hydration: A molecule of water gets added to fumarate to form malate in the presence of enzyme called
fumerase. - Dehydrogenation: Malate is dehydrogenated through the enzyme of malate dehydrogenase to produce
oxaloacetate (OAA), and one NADH 2.
OAA picks up another molecule of activated acetyl CoA to repeat the cycle. As two molecules of acetyl
CoA are formed from one glucose.
Therefore net gain of kreb’s cycle is:NADH 2 and FADH 2 are reduced co-enzymes which are further oxidised in electron transport chain (ETC)
in the presence of O 2.Kreb’s cycle is an amphibolic pathway. It is the one which is used for both breakdown (catabolism) and
build-up (anabolism) reactions.
Respiratory pathway is mainly a catabolic pathway which provides energy for living system.
Many of the intermediates in Kreb’s cycle are involved in synthesis (anabolism) of other important organic
molecules.
Examples: Acetyl coA involves in synthesis of fatty acids, Cutin and aromatic compounds. a-ketoglutarate
involves in synthesis of amino acid called glutamic acid.
Succinyl coA takes part in synthesis of pyrrole compounds of chlorophyll, cytochrome and phytochrome.
OAA forms aspartic amino acid and produces pyrimidines and alkaloids.
Thus being catabolic, Kreb’s cycle provides a number of intermediates which are used in different anabolic
pathways to form important biomolecules like glutamic acid, aspartic acid etc. So it is called amphibolic
pathway.
m Products of this step are for one molecule of pyruvic acid
i. 3 molecules of CO 2
ii. 4 molecules of NADH+H+
iii.1 molecule of FADH 2
iv. 1 molecule of ATP
m For the complete oxidation of one glucose molecule it needs
i. 1 turn of glycolysis
ii. 2 turns of Link Reation
iii.2 turns of Kreb’s cycle
m All enzymes of Kreb’s cycle are soluble in mitochondrial matrix, but succinate dehydrogenase (SDH) is
found attached to the inner mitochondrial membrane.
m Ketoglutaric acid is the first dicarboxylic acid formed during Kreb’s cycle and is the only 5 carbon
compound of Kreb’s cycle.Net gain of ATP via krebs cycle:
m Six molecules of NADH+H+ and two molecules of FADH 2 produced in Kreb’s cycle enter into ETS and
generate 22 ATP (3ATP for each molecule NADH+H+ and 2ATP for each molecule FADH 2 ) by oxidative
phosphorylation.
m Hence Kreb’s cycle generates a net of 24 ATP (SLP-2; OP-22).