What are the three metabolic enzymes involved in the PDH complex?
E1: Pyruvate Dehydrogenase
E2: Dihydrolipoyl Transacetylase
E3: Dihydrolipoyl Dehydrogenase
What is the first enzyme in the PDH complex and its prosthetic group?
- prosthetic group: Thiamine Pyrophosphate
What is the second enzyme in the PDH complex and its prosthetic group?
- prosthetic group: lipoic acid
Alone, PDH complex's second enzyme's prosthetic group has a different name than when it is bound to the enzyme.
What is it referred to as when bound and why?
- PDH complex-bound lipoic acid is lipoamide
...because it binds to the amide group of a lysine side chain of the enzyme dihydrolipoyl transacetylase
What is the functional significance of how dihydrolipoyl transacetylase's prosthetic group is bound?
the long lysine side chain to which lipoic acid is bound acts as a swinging arm...
it carries products and between the other two enzymes of the PDH complex
What is the 3rd enzyme of the PDH complex and what is its prosthetic group?
- prosthetic group: FAD
What is the first step of pyruvate's oxidative decarboxylation via PDH complex?
Enzyme + prosthetic group?
Substrates: Pyruvate + H+
Enzyme/P Grp: Pyruvate Dehydrogenase + Thiamine Pyrophosphate (TPP)
Products: Hydroxyethyl-TPP + CO2
The 2nd step in the PDH complex contains two smaller steps.
What first happens to hydroxyethyl-TPP formed in step one?
Via what enzyme + P-group?
HETPP is oxidized via lipoamide (p-group)...
leaving a reduced dihydrolipoamide
What is the second part of step 2 of the PDH complex reactions, acting on the dihydrolipamide + acetyl-TPP formed in part one?
What kind of special bond forms?
Acetyl group of acetyl-TPP transfers to...
...dihydrolipoamide, forming a high energy thio-ester bond and...
...creating acetyl dihydrolipoamide
What is the 2nd reaction which occurs associated with dihydrolipoyl transacetylase (E2 of PDH complex)?
An acetyl group transfer (hence transacetylase !!!)
Substrates: Acetyl-dihydrolipoamide + CoA-SH
Products: Acetyl-CoA + Dihydrolipoamide
What is the reaction associated with enzyme 3 of the PDH complex?
Enzyme + P Group?
Re-oxidation of lipoamide, reduction of FAD...
Enzyme/P-grp: Dihydrolipoyl Dehydrogenase + FAD
- Lipoamide (oxidized)
- FADH2 (not exactly a product, but rather the reduced version of the prosthetic group)
How is the prosthetic group of dihydrolipoyl dehydrogenase re-oxidized so that it can continue to function in the PDH complex cycle?
NAD+ accepts its electrons and is thus reduced to NADH
Where does the NADH formed by the PDH complex reactions go?
What pre-requirement does this place on PDH complex activity?
NADH fuels respiration
(no other reactions in the matrix significantly re-oxidize it)
- this means PDH complex activtiy is coupled to respiration and is thus aerobic (even though it doesn't use O2)
What happens to the acetyl-CoA formed by the PDH complex reactions?
(2 possible uses)
What is important about this acetylation step?
- further oxidation in the citric acid cycle
- lipid biosynthesis
- this step can not be undone
How and to which enzymes of the PDH complex does allosteric regulation occur?
Dihydrolipoyl acetyltransferase is inhibited by increased levels of acetyl-CoA
Dihydroplipoyl dehydrogenase is inhibited by increased levels of NADH
How and to which enzyme(s) of the PDH complex does covalent modification regulation occur?
Pyruvate dehydrogenase (E1) is either phosphorylated (decreased activity) or dephosphorylated (increased activity) by...
What activates PDH Kinase (and thus inhibits pyruvate dehydrogenase activity)?
increased levels of ATP, NADH, or Acetyl-CoA
What inhibits PDH Kinase (and thus activates pyruvate dehydrogenase)?
increased levels of NAD+, Pyruvate, ADP, or AMP
What activates PDH Phosphatase (and thus activates pyruvate dehydrogenase) ?
Ca++ (as from working muscle)