Chapt. 20 TCA cycle Ch. 20 Tricarboxylic acid cyle Student Learning Outcomes: • Describe relevance of TCA cycle • Acetyl CoA funnels products • Describe reactions of TCA cycle in cell respiration: 2C added, oxidations, rearrangements-> NADH, FAD(2H), GTP, CO 2 produced • Explain TCA cycle intermediates are used in biosynthetic reactions • Describe how TCA cycle is regulated by ATP demand: ADP levels, NADH/NAD+ ratio
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Lipoate coenzyme:• Made from carbohydrate, aa• Not from vitamin precursor• Attaches to –NH2 of lysine of enzyme• Transfers acyl fragment to CoASH• Transfers e- from SH to FAD
Fig. 10
Energetics of TCA cycle
Fig. 11
Energetics of TCA cycle: overall net -G0’• Some reactions positive; • Some loss of energy as heat (-13 kcal)• Oxidation of NADH,FAD(2H) helps pull TCA cycle forward
Very efficient cycle:• Yield 207 Kcal from1 Acetyl -> CO2 • (90% theoretical 228)• Table 20.1
V. Regulation of TCA cycle
Fig. 12
Many points of regulation of TCA cycle:• PO4 state of ATP (ATP:ADP)• Reduction state of NAD+ (ratio NADH:NAD+)• NADH must enter ETC
Table 20.2 general regulatory mechanisms
Table 20.2 general regulation metabolic paths
• Regulation matches function (tissue-specific differences)• Often at rate-limiting step, slowest step • Often first committed step of pathway, or branchpoint• Regulatory enzymes often catalyze physiological irreversible
reactions (differ in catabolic, biosynthetic paths)• Often feedback regulation by end product• Compartmentalization also helps control access to enzymes• Hormonal regulation integrates responses among tissues:
• Phosphorylation state of enyzmes• Amount of enzyme• Concentration of activator or inhibitor
Citrate synthase simple regulation
Citrate synthase simple regulation:• Concentration of oxaloacetate, the substrate• Citrate is product inhibitor, competitive with S• Malate -> oxoaloacetate favors malate
• If NADH/NAD+ ratio decreases, more oxaloacetate• If isocitrate dehydrogenase activated, less citrate
Allosteric regulation of isocitrate Dehydrogenase
Isocitrate dehydrogenase (ICDH):• Rate-limiting step• Allosteric activation by ADP
• Small inc ADP -> large change rate• Allosteric inhibition by NADH
• Reflect function of ETC
Fig. 13
Other regulation of TCA
Regulation of a-ketoglutarate dehydrogenase:• Product inhibited by NADH, succinyl CoA• May be inhibited by GTP• Like ICDH, responds to levels ADP, ETC activity
Regulation of TCA cycle intermediates:• Ensures NADH made fast enough for ATP homeostasis• Keeps concentration of intermediates appropriate
VI. Precursors of Acetyl CoA
VI. Many fuels feed directly into Acetyl CoA
• Will be completely oxidized to CO2
Fig. 14
Pyruvate Dehydrogenase complex (PDC)
Fig. 15
Pyruvate Dehydrogenase complex (PDC):• Critical step linking glycolysis to TCA• Similar to KGDH (Fig. 20.15)
• Huge complex; • Many copies each subunit:
(Beef heart 30 E1, 60 E2, 6 E3, X)
Regulation of PDC
Fig. 16
PDC regulated mostly by phosphorylation:• Both enzymes in complex
• PDC kinase add PO4 to ser on E1
• PDC phosphatase removes PO4
• PDC kinase:• inhibited by ADP, pyruvate• Activated by Ac CoA, NADH
TCA cycle intermediates and anaplerotic paths
Fig. 17
GABA
TCA cycle intermediates - biosynthesis precursors• Liver ‘open cycle’ high efflux of intermediates:• Specific transporters inner mitochondrial membrane
for pyruvate, citrate, a-KG, malate, ADP, ATP.
Anaplerotic reactions
Fig. 18
Anaplerotic reactions replenish 4-C needed to regenerate oxaloacetate and keep TCA cycling:
• Pyruvate carboxylase• Contains biotin
• Forms intermediate with CO2
• Requires ATP, Mg2+ (Fig. 8.12)
• Found in many tissues
Amino acid degradation forms TCA cycle intermediates
Fig. 19
Amino acid oxidation forms many TCA cycle intermediates:
• Oxidation of even-chain fatty acids and ketone body not replenish
Key concepts
• TCA cycle accounts for about 2/3 of ATP generated from fuel oxidation
• Enyzmes are all located in mitochondrial• Acetyl CoA is substrate for TCA cycle:
• Generates CO2, NADH, FAD(2H), GTP• e- from NADH, FAD(2H) to electron-transport chain.
• Enzymes need many cofactors• Intermediates of TCA cycle are used for
biosynthesis, replaced by anaplerotic (refilling) reactions
• TCA cycle enzymes are carefully regulated
Nuclear-encoded proteins in mitochondria
Nuclear-encoded proteins enter mitochondria via translocases:
• Proteins made on free ribosomes, bound with chaperones
• N-terminal aa presequences
• TOM complex crosses outer• TIM complex crosses inner• Final processing
• Membrane proteins similar
Fig. 20
Review question
Succinyl dehydrogenase differs from other enzymes in the TCA cycle in that it is the only enzyme that displays which of the following characteristics?
a.It is embedded in the inner mitochondrial membrane
b.It is inhibited by NADHc.It contains bound FADd.It contains fe-S centerse.It is regulated by a kinase