PDH & TCH cycle Flashcards
What are the 3 stages of a cellular respiration?
1: Oxidation of fatty acids, glucose, and some amino acids yields acetyl-CoA.
2: Oxidation of acetyl groups in the citric acid cycle includes four steps in which electrons are abstracted
3. Electrons carried by NADH and FADH2 are funneled into the respiratory chain and the production of ATP
What is the significance of a pyruvate translocase transporter?
Problem - Pyruvate is generated in the cytosol by Glycolysis
- Enzymes for conversion of pyruvate to Acetyl-CoA exist in the mitochondria
Solution – Pyruvate is transported into the mitochondria via the Pyruvate Translocase Transporter (PT
What happens to pyruvate under normal aerobic consitions?
Converted to Acetyl CoA
Via Oxidative decarboxylation under aerobic conditions
What oxidizes pyruvate?
Pyruvate is Oxidized to Acetyl-CoA by the Pyruvate Dehydrogenase Complex
The reaction is termed Oxidative Decarboxylation due to the removal of a carboxyl group and two electrons to generate CO2 and NADH
The reaction is irreversible CO2 is removed by exhalation
NADH can be used to generate ATP via oxidative phosphorylation
Acety-CoA enters the Tricarboxylic Acid Cycle (TCA)
What upis pyruvate dehydrigenase complex?
Pyruvate Dehydrogenase Complex (PDH) catalyzes the oxidative decarboxylation of pyruvate
LEO says GER
Losing electrons is oxidation and the substance that loses the electrons is the reducing agent Gain electrons is reduction and the substance that gains the electrons is the oxidizing agent
What are the domains of the multienzyme complex?
3 catalytic domains
- Pyruvate decarboxylase- E1
- Dihydrolipoyl transacetylase- E2
- Dihydrolipoyl dehydrogenase- E3
What is the pyruvate dehydrogenase complex?
- The complex plays a pivotal role in metabolism,
- Limiting the rate of oxidative glucose consumption,
- PDH is highly regulated to respond to all metabolic requirements.
- Several cofactors are required for this regulated mechanism
- Reversible phosphorylation of the complex by associated regulatory enzymes PDH kinases (4 isoforms) and PDH phosphatases (2 isoforms)
What js significant of the E2 N-terminal domain?
E2 N-terminal domain is flexible and can associate with the kinases and phosphatases and can transfer these enzymes to the active sites on E1 like “Tarzan swinging from vines”
How is PDH regulated in muscle?
Ca2+ release during contraction- energy production
Carduac musce- catecholamines
Important regulatory molecules- NADH, Acetyl CoA, Pyruvate, ATP and calcium
How is PDH complex activated?
Activation by phosphatase
• by dephosphorylation of serine residues on E1 PDH by PDH Phosphatases
Phosphatase stimulated by:
• Calcium particularly in skeletal muscle during contraction • Insulin in adipocytes and liver
•Catecholamines in cardiac muscle
How is the PDH complex inhibited?
- Inhibition of PDH by reaction products
- Acetyl CoA & NADH
Kinases are inhibited by:
•Pyruvate (if kinase is inhibited then the complex is NOT inhibited)
Kinases are activated by:
•ATP, Acetyl CoA and NADH (it will then shut down the complex)
What are the bound prosthetic groups and cofactors of thePDH complex?
• Bound prosthetic groups
– Thiamine pyrophosphate (TPP) – from B1
– Lipoic acid – from octanoic acid
– FAD – from B2
• Cofactors
– NAD+ - from B3
– Coenzyme A – from B5
What are the bound prosthetic groups and cofactors of PDH complex?
• Bound prosthetic groups
– Thiamine pyrophosphate (TPP) – from B1
– Lipoic acid – from octanoic acid
– FAD – from B2
• Cofactors
– NAD+ - from B3
– Coenzyme A – from B5
What are the 5 cofactors of PDH & alpha ketoglutarate?
Co-factors for PDH and Alpha Ketoglutarate: "Tender Loving care for Nancy" TPP Lipoic acid Coenzyme A FAD NAD
What is foavin adenine dinucleotide derived from?
from riboflavin (vitamin B2)
Where is NAD+ derived from?
NAD+ derived from niacin (vitamin B3)
Explain the inner workings of PDH Complex
- Pyruvate is decarboxylated to form a hydroxyethyl derivative bound to the reactive carbon of thiamine pyrophosphate, the coenzyme of pyruvate decarboxylase (E1).
- The hydroxyethyl intermediate is oxidized by transfer to the disulfudeform of lipoic acid covalently bound to dihydrolipypoyl transacetylase (E2)
- The acetyl group, bound as a thioester to the side chain of lipoic acid, is transferred to CoA
- The sulfhydryl form of lipoic acid is oxidized by FAD-dependent dihydrolipoyl dehydrogenase (E3), regenerating the disulfude (oxidized) form of lipoic acid
- FASH2 on E3 is reoxidized to FAD as NAD+ is reduced to NADH + H+
What are the deficiencies of PDH activity?
Several vitamins required for PDH activity:
– Vit B5→Co-enzyme A,
– Vit B3→NAD,
– Vit B2→FAD,
– Vit B1→TPP
– Deficiency in any of these vitamins will disrupt PDH activity
What are the PDH consequences for thiamine deficiency?
• Wernicke-Korsakoff syndrome characterized by Ataxia, Ophthalmolplagia, Memory loss, Cerebral Hemorrhage, Confabulation
• At risk: Alcoholics & malnourished individuals
• Beriberi: wet and dry based on presence/absence of oedema
– Wet Beriberi: Heart failure, decrease ATP, increased cardiac output, (dilated cardiomyopathy)
– Dry Beriberi: systemic muscle wasting, polyneuritis
Aside from PDH, what effects does thiamine deficiency cause?
Other thiamine –requiring enzymes:
• α-ketoglutarate DH (KG DH) part of the TCA cycle
• Branched chain α–ketoacid DH (BCKDH)
What is the metabolic effec of PDH deficiency?
Increase in pyruvate with concomitant increase in lactic acid and alanine (via transamination), in production of acetyl CoA; severe reduction in ATP production
Whatvare the clinical features of PDH?
lactic acidosis in blood and CSF, increased blood pyruvate and alanine, neurologic defects, myopathy;
usually fatal at early age
Whaat are the therapies of PDH?
Dichloroacetate inhibits PDH kinase so that PDH complex remains unphosphorylated and catalytically as active as possible Supplementation with thiamine, lipoic acid and carnitine
High fat and low carbohydrate diet
What are the symptoms of PDH enzyme deficiency?
• Frontal prominence • Wide nasal bridge
• Flared nares
• Long philtrum
• Brain malformations
– Corpus callosum agenesis
– Cerebral and basal ganglia cystsHow does heavy metal poisoning affect pyruvate metabolism?
- Arsenate, Mercury, and lead have high affinity for -SH
- Lipoic acid is one of the cofactors in PDC
- PDH Complex becomes inactive when lipoic acid is bound to heavy metals.
- CNS solely depends on Glucose Metabolism through to oxidative phosphorylation for energy, (can not use fatty acids as a fuel source) therefore effected by heavy metal poisoning.
Other names of the TCA cycle…
Tricarboxylixc acid cycle
Krebs cycle
Citric acid cycle
What are anaplerotic reactions?
Inputs into TCA cycle
How is the TCA cycle an amphibolic pathway?
• The TCA cycle is an amphibolic pathway, it serves in both catabolic and anabolic processes
– Provides precursors for many biosynthetic pathways
– It is necessary to replace the ones used in biosynthesis of molecules
Explain what are anaplerotic reactions
Reactions which replenish TCA acid cycle intermediates are called anaplerotic reactions (to fill up)
– Dynamic balance between:
• Reactions where intermediates are consumed
• Intermediates which are replenished by the anaplerotic reactions.
– So that the concentrations of the citric acid cycle intermediates remain almost constant
What are the steps in the TCA cycle?
- Condensation of acetyl CoA and oxaloacetate to form citrate (tricarboxylic acid) by
Citrate synthase
High citrate will inhibit citrate synthase and phosphofructokinase - Citrate is isomerized to isocitrate by
Aconitase - Irreversible oxidative decarboxylation of isocitrate by
Isocitrate dehydrogenase
Yields 1 NADH (1 of 3) Allosteric activation - Conversion of a-ketoglutarate to succinyl CoA by a-ketoglutarate dehydrogenase
Releases second CO2 and forms second NADH (2 of 3) - Succinate thiokinase (succinyl CoA synthetase)
Cleaves high energy thioester bond of succinyl CoA to yield succinate also
Yields 1 GTP - Succinate is oxidized to fumarate by
succinate dehydrogenase
FAD is reduced to FADH2 - Fumarate is hydrated to malate by
Fumarase - Malate is oxidized to oxaloacetate by
malate dehydrogenase
Reaction has positive 🔼G0’ but reaction is pulled forward in the direction of oxaloacetate because of the highly exergonic citrate synthase reaction
Third NADH formed (3 of 3)
What enzymes regulate TCA?
- Citrate Synthase
- Isocitrate Dehydrogenase
- A-KG Dehydrogenase
Describe the regulation of citrate synthase
- irreversible reaction
- inhibited by its product
- Activated primarily by substrate availability
Describe the regulation of isocitrate dehydrogenase
- Irreversible reaction
- Allosterically activated by ADP & calcium
- Inhibited by ATP, NADH
Describe the regulation of a-ketoglutarate dehydrogenase
- Structure similar to PDH with E1, E2 & E3
- 5 co-enzymes required, TPP, lipoic acid, FAD, NAD+ and CoA
- Activated by Ca2+
- Inhibited by NADH and succinyl CoA
What is produced by one TCA cycle revolution produce?
One TCA cycle revolution produces:
3 NADH (9 ATP)
1 GTP (1ATP) 1 FADH2 (2 ATP)
12 ATP per acetyl CoA oxidized
