PDH Complex Flashcards
Where does the PDH complex reaction take place
Mitochondrial matrix
After pyruvate is shuttled from cytosol —> matrix
Characteristics of outer mitochondrial membrane
Contains porin molecules —> large channels that allow free diffusion of most all low MW metabolits
Cytosolic content is essentially contiguous with the IMS content
Characteristics of inner mitochondrial membrane (IMM)
Very impermeable to charged molecules
Transporters are required
Enzymes for with processes are contained in the matrix
PDH-complex
CAC
FA oxidation
Pyruvate transport from cytosol —> matrix
Pyruvate is a charged molecule … so cannot cross IMM
Active symport transport with H+ ions
H+ = move down gradient
pyruvate = moves against gradient
Effect of pyruvate transport into matrix on ATP production
H+ gradient is crucial for ATP production
With each pyruvate transport…[H+] in cytosol decreases
Thus, ATP production is decreased
The 5 vitamins that are required as cofactors in the PDH complex
- Niacin (B3)
- Riboflavin (B2)
- Thiamine (B1)
- Lipoic acid (lipoate)
- Pantothenic acid (B5)
Niacin (B3)
Structural component of NAD+
NADH is produced in the final enzyme reaction and is the end product of the PDH complex reaction
Riboflavin (B2)
Structural component of FAD
4th reaction = FADH2 produced
Ultimately will become oxidized in the next reaction and NADH is made
FAD/FADH2 = also cofactor for CAC enzyme (succinate dehydrogenase)
Thiamine (B1)
Thiamine pyrophosphate (TPP)
Cofactor covalently bound to PDH complex
Involved in the acetyl-transferase function the first 2 reactions
**also cofactor in transketolase reaction of PPP
Lipoic acid (lenoate)
Bioactive form = covalently conjugated to the complex via amide bond (lipoamide)
Oxidation/reduction acetyl-transferase function of the 2nd and 3rd reactions
Pantothenic acid (B5)
Structural component of CoA
Used in 3rd reaction to generate Acetyl-CoA
**CoA used in FAS and oxidation as well
Overall reaction that the PDH complex catalyzes
Pyruvate —> Acetyl-CoA
In mitochondrial matrix
Three subunits E1, E2, E3
The reaction is thermodynamically favorable
As the PDH reaction progresses…the substrate is linked to the complex and channeled …..
Towards the center of the complex
Moving from E1 —> E3
Pyruvate dehydrogenase subunit (E1)
1st reaction
Pyruvate becomes oxidatively decarboxylated
CO2 produced
Resulting 2C hydroxyl-ethyl group is transferred to the TPP cofactor (which is covalently attached to E1
—> forms hydroxyethyl-TPP
Second reaction of PDH complex
Hydroxyethyl group is oxidized to an acetyl group by (E1)
Dihydrolipoyl transacetylase (E2) then catalyzes the transfer of the acetyl group from TTP —> lipollysine (cofactor lipoate covalently attached to a lysine molecule in the transacetylase)
Electrons donated = reduce a disulfide bone in lipoate
Acetyl group is transferred to lipoate through a thioester linkage —>
Producing acyl-lipollysine
3rd reaction of PDH complex
Acetyl group is transferred from the thioester linkage in lipollysine to another thioester linkage in CoA
—> Acetyl-CoA
(Lipollysine is still in reduced state —> disulfide bond is still separated)
After the production of acetyl-CoA in the 3rd reaction…what is the purpose of the remaining PDH reactions?
Reoxidizing lipollysine…so that it can accept electrons from antoher hydroxyethyl-TPP
4th reaction of PDH complex (post Acetyl-CoA)
Dihydrolipoyl dehydrogenase (E3)
Reoxidizes lipollysine
FAD —> FADH2
Final (5th) reaction of PDH complex
E3 subunit
Electrons transferred from FADH2 —> NAD+
To make NADH
Regulation of PDH complex
Phosphorylation/dephosphorylation regulation
PDH kinase phosphorylates and INACTIVATES PDH….a phosphatase does the opposite
Build up of end products (Acetyl-CoA and NADH) —> stimulate PDH kinase
Pyruvate —> inactivate PDH kinase
High ADP, NAD+, CoA—> inactivate PDH kinase
LCFAs —> stimulant PDH kinase
Phosphatase activty:
Primarily activated by Ca2+ —> leads to activated PDH
Think of muscle cells - increase Ca influx…need ATP for muscle contraction
Beriberi
Most common PDH disorder
Deficiency in thiamine
Can be caused by malnourishment
Also in alcoholics
No thiamine —> disrupt PDH complex, and CAC enzyme a-KG dehydrogenase
Disruption in PDH complex and alpha-KGDH —>
Neuro and cardio disorders
Nerves prefer glucose…no PDH complex and cannot get a lot of energy from glucose anymore
No PDH complex —> less Acetyl-Coa —> less FAS
Lactic acidosis
Treatment = eating ketogenic amino acids (leucine and lysine) and fat…bypass the
PDH complex and generate Acetyl-CoA by other mechanisms
Arsenite and Mercury poisoning
Heavy metals that bind tightly to SH groups in PDH complex
Disrupts function of E2 lipoate cofactor (lipollysine) in PDH and a-KGDH
Symptoms like beriberi
Leigh Disease
Defects in one of the genes encoding the PDH complex (usually E1)
Progressive CNS degeneration
Can’t metabolize glucose —>—> CO2
Hypotonia, seizures, eye issues, ataxia
Treatment = no carbs, increase ketogenic aas, supplement TPP
