Krebs cycle - Univerzita...
Transcript of Krebs cycle - Univerzita...
Krebs cycle Energy - 215
Petr Tůma
Eva Samcová
Overview of Citric Acid Cycle
Key Concepts • The citric acid cycle (Krebs cycle) is a multistep catalytic process that converts acetyl groups derived from carbohydrates, fatty acids, and amino acids to CO2, and produces NADH, FADH2, and GTP.
Overview: Oxidative Fuel Metabolism
Reactions of Citric Acid Cycle
Tricarboxylic acid cycle (TCA)= Krebs
cycle – the hub of the metabolic system
• It accounts for the major portion of carbohydrate, fatty acid, amino acid oxidation and generates numerous biosynthetic precursors
• TCA is therefore amphibolic, that is, it operates both catabolically (biosynthesis of ATP) and anabolically (biosynthesis of amino acids, heme, glucose, lipids)
Starting compounds
• Acetyl-coenzyme A (acetyl-CoA)
• Oxaloacetate
• Cataplerotic reactions which utilize and therefore drain TCA
• Anaplerotic reactions replenish citric acid cycle intermediates
General features of CAC
• Oxidizes the acetyl-CoA not only from pyruvate
• The net reaction :3NAD+ + FAD + HS-CoA + GDP + Pi
→ 3NADH + H+ + FADH2 + GTP + CoA + 2CO2
● All enzymes of CAC are in mitochondrion, so all
substrates including NAD+ and GDP must be generated in
the mitochondrion or be transported to mitochondrion from
cytosol. Similarly all the products of CAC must be
consumed in mitochondrion or transferred to cytosol.
• The net effect of each round of CAC is oxidation of 1
acetyl group to 2CO2
Synthesis of Acetyl-Coenzyme A
Key Concepts • Pyruvate dehydrogenase is a multienzyme complex that catalyzes a five-part reaction in which pyruvate releases CO2 and the remaining acetyl group becomes linked to coenzyme A. • The reaction sequence requires the cofactors TPP, lipoamide, coenzyme A, FAD, and NAD+.
Cofactors of PDH Complex
Enzymes of the Citric Acid Cycle
Key Concepts • The eight enzymes of the citric acid cycle catalyze condensation, isomerization, oxidation–reduction, phosphorylation, and hydration reactions. • Two reactions produce CO2, one reaction produces GTP, and four reactions generate the reduced coenzymes NADH or FADH2.
Reaction 2: Aconitase
Reaction 3: Isocitrate Dehydrogenase
Releases 1st CO2
Reaction 4: α-Ketoglutarate
Dehydrogenase Resembles PDH
Complex—2nd NADH
Reaction 6: Succinate Dehydrogenase
Produces FADH2
Malonate Competitively Inhibits
Succinate Dehydrogenase
Reaction 7: Fumarase
Reaction 8: Malate Dehydrogenase
Produces 3rd NADH
Products of Citric Acid Cycle
Energy review
Reaction Cofactor ATP
isocitrate → 2-oxoglutarate NADH + H+ 3 (2,5)
2-oxoglutarate → succinyl-
CoA
NADH + H+ 3 (2,5)
succinyl-CoA → succinate - 1
succinate → fumarate FADH2 2 (1,5)
malate → oxaloacetate NADH + H+ 3 (2,5)
12 (10)
acetyl-CoA + 3NAD+ + Q + GDP + P + H2O →
→ 2CO2 + 3NADH+H+ + FADH2 + GTP +CoA
Regulation of Citric Acid Cycle
Regulation of TCA cycle activation – low rate of ATP/ADP and NADH/NAD+
inhibition – high rate of ATP/ADP and NADH/NAD+
The flux controlling enzymes
– Citrate synthase
– Isocitrate dehydrogenase
– 2-ketoglutarate dehydrogenase
• specific inhibitors
– fluoroacetate – aconitase
– Arsenic compounds inactivates lipoamide enzymes
(pyruvate dehydrogenase and 2-ketoglutarate
dehydrogenase
– malonate – succinate dehydrogenase
3 Enzymes Function Far From
Equilibrium
Amphibolic Functions of CAC
Metabolic Sources of Acetyl-CoA
• Acetyl-CoA (high energy compound)
the common product of carbohydrate,
fatty acids, and amino acids breakdown
• Pyruvate Dehydrogenase reaction
– pyruvate + NAD+ + CoA → acetyl-CoA + NADH/H+
+ CO2
– Irreversible reaction
– CoA, thiamine diphosphate, lipoamide, FAD, NAD+
• β-oxidation of fatty acids
• Amino acids (ketogenic AA, glucogenic AA)
Anabolic
reactions • citrate → fatty acids,
steroids
• 2-oxoglutarate → AAs of glutamate family
• succinyl-CoA → heme
• malate, oxaloacetate → glucose, AAs of pyruvate family
Anaplerotic Reactions
Replenish CAC Intermediates
Anaplerotic reactions replenishment of intermediates of KC
• pyruvate + CO2 + ATP → OAA + ADP – Pyruvate carboxylase
• Amino acids – Asp, Asn – OAA
– Glu, Gln, His, Pro, Arg – 2-oxoglutarate
– Ile, Val, Met, Trp – succinyl-CoA
– Ala, Ser, Thr, Cys, Gly - pyruvate
• Degradation of fatty acids with uneven (odd) number of carbon atoms:
Propionyl-CoA → succinyl-CoA
• Acetyl-CoA has no anaplerotic effect
The Mitochondrion
• The site of eukaryotic oxidative
metabolism with enzymes that mediate
this process
• Is bounded by a smooth outer membrane
and contains invaginated inner membrane
• The inner mitochondrial membrane is
impermeable to most hydrophilic
compounds therefore contain specific
transport systems (proteins 80%)
Mitochondrion - metabolic functions
Mitochondrion - structure
Transport of substances through
inner mitochondrial membrane
Shuttles Transfer of NADH + H+ from
cytoplasm into mitochondrion
1. The malate-aspartate shuttle
heart, liver, kidney
2. The glycerolphosphate shutles
– Glycerol-3-phosphate /Dihydroxyacetone phosphate
– brain, skeletal muscles
The malate-aspartate shuttle
The glycerolphosphate shuttle