carbohydrate metabolism 3 Flashcards
conversion of pyruvate to acetyl CoA
Pyruvate , a product of glycolysis , must be converted to acetyl CoA before it enters the Citric acid cycle
in eukaryotes , pyruvate is converted to acetyl CoA after it is transported into the mitochondrion
entry of pyruvate into the mitochondrion
mitochondrion is enclosed by a double membrane
each molecule of pyruvate is transported across the inner mitochondrial membrane into the matrix .
pyruvate translocate transports pyruvate and H+ from the inter membrane to the interior space of the mitochondrion
The role of tricarboxylic acid cycle in metabolism (Krebs cycle)
TCA pathway is amphibole i.e the reactions are involved in catabolic and anabolic processes
glycolysis occurs in the cytosol while TCA cycle in mitochondria
in contrast to glycolysis , none of the intermediates are phosphorylated ; but all are either di or tricarboxylic acids
in contrast to the elaborate allosteric regulation of glycogen metabolism , most of the regulation is provided by substrate availability and production inhibition
The TCA cycle
it is a cycle pathway consisting of 8 enzyme mediated steps
four of the steps are oxidation reactions
the oxidising agent in NAD+ in all except step 6 in which FAD plays the same role
in step 5 , a molecule of GDP (guanosine diphosphate) is phosphorylated to produce GTP
strictly an aerobic process
- only works under aerobic conditions because NAD and FAD must be regenerated by oxidative phosphorylation
occurs in the mitochondrial matrix of eukaryotes/cytosol of prokaryotes
responsible for the conversion of acetyl CoA to 2 co2 while conserving the free energy for ATP production
the energy is stored as 3 NADHS , 1 FADH2 , 1 GTP
redoxidation of NADHand FADH2 by oxygen occurs via the electron transport system
oxidative phosphorylation finished breakdown of fuel sources and drives ATP synthesis
step one of the Krebs cycle
oxidation of acetyl CoA
2 C acetyl CoA condenses with 4C oxaloacetate to form 6C citrate
irreversible condensation reaction
an exergonic reaction, the hydrolysis of a thioester releases energy
citrate is an inhibitor of PFK in glycolysis
step 2 of the Krebs cycle
isomerisation of citrate to isocitrate
citrate is converted into iso-citrate by aconite
the reaction requires Fe2+
reaction proceeds by removal of H2O from citrate to produce cis-aconitate and then H2O is added back to give isocitrate
step 3 of the citric cycle
first oxidation step and formation of a-ketoglutarate
oxidative decarboxylation producing CO2 and NADH
allosteric inhibitors : ATP and NADH
allosteric activators : ADP and NAD+
step 4 of the citric cycle
second oxidation and formation of succinyl coa
oxidative decarboxylation by a-ketoglutarate
dehydrogenase complex produces CO2 and NADH
the succinylcholine-CoA product is a high energy thioester
the complex is similar to the pyruvate dehydrogenase complex and is a key regulatory step in the cycle
step 5 of the citric cycle
formation of succinate
free energy stored in succinyl CoA is conserved through the synthesis of a nucleoside triphosphate - GTP in mammals and ATP in plants and several bacteria
passes high energy phosphate to ADP forming ATP
catalysed by succinylcholine CoA synthase
citric cycle step 6
FAD linked oxidation
Succinate is oxidised to fumarate, catalysed by succinate dehydrogenase complex
Enzyme acceptor is FAD (Flavine Adenine Dinucleotide) rather than NAD+.
FAD is reduced to FADH2
step 7 of the citric cycle
Formation of L-Malate
The newly formed double bond is hydrated
Fumarase catalyses this reaction
The product is L-Malate
step 8 of the citric acid cycle
Final oxidation step and regeneration of oxaloacetate
L-Malate is oxidised to oxaloacetate, catalysed by malate dehydrogenase
NADH is formed during the reaction
Oxaloacetate can react with another molecule of acetyl CoA
