objectives
DESCRIPTION
OBJECTIVES. Identify the key steps in citric acid cycle Describe how TCA is regulated Illustrate biomedical importance of TCA Explain energy yield from TCA. CITRIC ACID CYCLE (TCA CYCLE OR KREBS CYCLE) Pyruvate Acetyl- CoA Acetate CO 2. - PowerPoint PPT PresentationTRANSCRIPT
Identify the key steps in citric acid cycle
Describe how TCA is regulated
Illustrate biomedical importance of TCA
Explain energy yield from TCA.
OBJECTIVES
CITRIC ACID CYCLECITRIC ACID CYCLE (TCA CYCLE OR KREBS CYCLE)(TCA CYCLE OR KREBS CYCLE)
PyruvatePyruvate
Acetyl-CoA Acetate CO2
Oxidizing acetyl-CoA from glucose, lipid and protein catabolism in aerobic respiration to maximize energy gain
The cycle supplies precursors for biosynthesis
CITRIC ACID CYCLECITRIC ACID CYCLE
THREE STAGES OF CELLULAR RESPIRATION
STAGE 1 Acetyl CoA production from glucose, fatty acids and amino acids
STAGE 2 Acetyl CoA oxidation =TCA Cycle = yielding reduced electron carriers
STAGE 3 Electron transport and oxidative phosphorylation oxidation of these carriers and production of ATP
MANY CATABOLIC PATHWAYS YIELDMANY CATABOLIC PATHWAYS YIELD ACETYL COA FOR THE TCA CYCLEACETYL COA FOR THE TCA CYCLE glycogenglycogen glucoseglucose lactatelactate PyruvatePyruvate fatty acidsfatty acids
amino acidsamino acids Acetyl-CoAAcetyl-CoA
TCATCA(Note: AA more than one entry point)
Acetyl CoAAcetyl CoA
HS-CoA
Space filledSpace filled
Acetyl
A high energy bond
Pyruvate (PDH) Acetyl-CoA ((PYRUVATE DEHYDROGENASE COMPLEX)PYRUVATE DEHYDROGENASE COMPLEX)
Location = Mitochondrial matrix
CH3 CH3
C=O + NAD++ HS-CoA C=O +NADH+CO2
COO- S-CoA Pyruvate Acetyl-CoA
(A high energy compound)
STAGE 1
IRREVERSIBLE Irreversible means acetyl-CoA cannot be converted backward to Pyruvate
Hence “fat cannot be converted tocarbohydrate”
S--S--
TPP FAD
E1 E2 E3 N A D+
PYRUVATE DEHYDROGENASE COMPLEX
REGULATION OF PYRUVATE DEHYDROGENASE
Irreversible reaction must be tightly controlled-- three ways1.Allosteric Inhibition
Inhibited by products: Acetyl-CoA, NADH ATP
2. Allosteric activation
AMP
Ratio ATP/AMP important
Overall Reaction in the TCA cycleOverall Reaction in the TCA cycle
ACETYL-COA + 3NAD+ + FAD + GDP + Pi+2H2O 2CO2 + 3NADH + FADH2 + GTP + 2H+
+ CoA
Both carbons oxidized One GTPThree NADHOne FADH2
1- CONDENSING ACETYL-COA WITH OXALOACETATE
ACETYL COA
O=C-SCoA-SCoA COO- + CoASH CH3 H2O CH2 + H+
+ O=C-COO CITRATE SYNTHASE HO-C-COO-
CH2 CH2
COO- COO-
OXALOACETATE CITRATEENZYME:CITRATE SYNTHASE
2 - CITRATEISOCITRATE VIA CIS-ACONITATE
CH2-COO- -H2O CH2COO +H2O CH2-COO-
HOC-COO- C-COO H-C-COO-
CH2-COO- H-C-COO- HOC-COO-
CITRATE CIS–ACONITATE ISOCITRATE
ENZYME: ACONITASE
3- OXIDATION OF ISOCITRATE TO -KETOGLUTARATE
First oxidation in TCA cycle
COO- COO-
CH2 NAD+ NADH CH2 + CO2
HC-COO- CH2
HOCH O=C COO- COO-
ISOCITRATE -KETOGLUTARATE
ENZYME = ISOCITRATE DEHYDROGENASE
ISOCITRATE DEHYDROGENASE
Two isoforms
One uses NAD+; other NADP+
Reduction to NADH or to NADPH
Energy is later derived from these
electron carrying molecules -- loss of first CO-- loss of first CO22
-- Note OH to =O -- Note OH to =O
4- OXIDATION OF -KETOGLUTARATE TO SUCCINYL-COA AND CO2
Second oxidation in TCA cycle
-KETOGLUTARATE SUCCINYL COA
COO- COO- + CO2
CH2 NAD+ NADH CH2
CH2 CH2
O=C O=C COO- SCoA
+ CoA-SH ENZYME = - KETOGLUTARATE DEHYDROGENASE COMPLEX
Loss of second of two CO2
Similar to Pyruvate Acetyl-CoA
Enzyme is similar to
Pyruvate dehydrogenase complex
- KETOGLUTARATEDEHYDROGENASE COMPLEX
5- 5- SUCCINYL COA TO SUCCINATESUCCINYL COA TO SUCCINATE
succinyl-CoA COO-
+ GDP + Pi CH2 + CoA-SH +
CH2 GTP COO-
SUCCINATE -- SUBSTRATE LEVEL PHOSPHORYLATION
-- GTP is equivalent to ATP; GTP to ATP by NUCLEOSIDE DIPHOSPHOKINASE ENZYME = SUCCINYL COA SYNTHETASE
6- OXIDATION OF SUCCINATE TO FUMARATE
FLAVIN DEPENDENT OXIDATIONThird oxidation of TCA cycle, FAD in flavoprotein reduced to FADH2
COO- COO-
CH2 + E3-FAD CH + E3-FADH2
CH2
COO- HC-COO-
SUCCINATE FUMARATE
Dehydrogenation; note double bondENZYME = SUCCINATE DEHYDROGENASE
7- 7- HYDRATIONHYDRATION
COO- COO-
CH +H2O HOCHHC HCH COO- -H2O COO-
FUMARATE L-MALATE
ENZYME = FUMARASE
8- 8- OXIDATION OF MALATE TO OXALOACETATEOXIDATION OF MALATE TO OXALOACETATE
COO- COO-
HO HO CH NAD+ NADH C=O=O CH2 CH2
COO- COO-
MALATE OXALOACETATE FOURTH OXIDATION: another pair ofelectrons is made available in NADH
ENZYME = MALATE DEHYDROGENASE
SUMMARYSUMMARYFIRST HALF Introduction of two carbon atoms and their loss, yielding 2 NADH and a GTP (ATP) SECOND HALF Partial oxidation of succinate to oxaloacetate. Another NADH is produced as well as a reduced FADH2
OXALOACETATE IS REGENERATED FOR NEXT CYCLE
Overall Reaction
Acetyl-CoA+3NAD++FAD+GDP+Pi+2H2O2CO2 + 3NADH + FADH2 +GTP+2H++CoA
One high energy compound made
Four pairs of electrons are madeavailable to the respiratory chain andoxidative phosphorylation. These areused to generate most of the ATPneeded.
What is the maximum yield of highenergy ATP in the aerobic catabolismof glucose?
GlycolysisGlycolysis::glucose 2pyruvate + 2NADH+2ATP 8 ATPs
Pyruvate Dehydrogenase:Pyruvate Dehydrogenase:2pyruvate 2acetyl CoA + 2NADH 6 ATPs
TCA cycle:TCA cycle:acetyl CoA2CO2+3NADH+FADH2+GTP 2x12ATPs OVERALL YIELD FROM GLUCOSE 38 ATPs
ENERGY RELATIONSHIPS This represents 41% 41% conservation of
the potential energy available in
glucose as ATP
REGULATION OF CITRIC ACID CYCLEREGULATION OF CITRIC ACID CYCLE FOUR WAYS
1- PYRUVATE DEHYDROGENASE -- Inhibited by acetyl-CoA and NADH2- CITRATE SYNTHASE -- Substrate = oxaloacetate -- limited3- ISOCITRATE DEHYDROGENASE-- Activated allosterically by ADP -- Inhibited allosterically by NADH4- 4- - KETOGLUTARATE DEHYDROGENASE- KETOGLUTARATE DEHYDROGENASE-- Inhibited allosterically by products = succinyl-CoA and NADH
Major regulator is intramitochondrial
NAD+/NADH ratio
REGULATION OF CITRIC ACID CYCLEREGULATION OF CITRIC ACID CYCLE
REPLACEMENT OF INTERMEDIATES
Intermediates are removed for
biosynthesis
1- AMPHIBOLIC reactions (Removal of intermediates) 2- ANAPLEROTIC reactions (Replacing cyclic intermediates)
AMPHIBOLIC PATHWAYS
A- TRANSAMINASESoxaloacetate Asp removes 4C-ketoglutarate Glu removes 5Cpyruvate Ala removes 6C
B- FATTY ACID BIOSYNTHESIScitrate Acetyl CoA and oxaloacetate acetyl CoA can build fatty acids
C- HEME BIOSYNTHESISsuccinyl CoA + glycine Porphyrins
ANAPLEROTIC REACTIONS
A-A- PYRUVATE CARBOXYLASE –Replaces oxaloacetate- most important,especially in liver and kidney
OCH3-C-COO- + CO2 + ATP O -OOC-CH2C-COO- + ADP + Pi
oxaloacetate
B- MALIC ENZYME Replaces malate-- pyruvate + CO2 +NADPHmalate + NADP+
C- FROM AMINO ACIDS
Reversals of transaminations--restores oxaloacetate or a-ketoglutarate with abundant Asp or Glu
Glutamate dehydrogenase Glu + NAD(P)+ a-ketoglutarate + NAD(P)H + NH4
+
NADH acetyl CoA NAD+ oxalo- citrate synthase MDH acetate l-malate citrate H2O fumarase aconitase H2O fumarate 2-step FADH2 succinate dehydrogenase isocitrate FAD NAD+
succinate TCATCA IDH NADHCoASH GTP succinate-CoA synthetase CO2
GDP+ Pi
succinyl CoA NADH NAD+ CO2 -ketoglutarate
-KGDH CoASH