Lecture 16 - Tricarboxylic acid cycle Flashcards
The three main free energy changes in pyruvate formation
Step 1:
Glucose + ATP -> Glucose-6-P + ADP
Step 3:
Fructose-6-P + ATP -> fructose-1,6-bisP + ADP
Step 10:
Phosphoenolpyruvate + ADP -> Pyruvate + ATP
Why are steps 1, 3, and 10 in glycolysis irreversible?
They have high free energy change so, unlike the other steps which have energy changes close to zero, easy conversion does not occur
Since these steps are irreversible, the enzymes for the steps are highly regulated
Glucose regulation: hexokinase
Hexokinase is the enzyme for the conversion of glucose into glucose-6-P
G-6-P inhibits hexokinase, acting as a form of feedback inhibition. This inhibition occurs to prevent excessive G-6-P formation if further steps have glycolysis inhibited
Glucose regulation: phosphofructokinase
Fructose-6-P + ATP -> fructose-1,6-bisP + ADP
Phosphofructokinase-1 (PFK-1) is the most important regulatory enzyme in glycolysis
Allosteric inhibitors of glycolysis
ATP and citrate (in the liver) are glycolysis inhibitors (because if there are high amounts, then either energy is not needed or enough citrate is there for further steps of ATP synthesis)
H⁺ inhibits PFK-1: preventing glycolysis also prevents lactic acid formation
Allosteric activators of glycolysis
AMP and fructose-1,6-bisP (liver) are glycolysis activators (because if there are high amounts, then energy is needed or there is not enough F-1,6-bisP for further steps of ATP synthesis)
Fructose-2,6-bisP is the most important PFK-1 activator
Which is which: fructose-1,6-bisP and ATP as feedback inhibition and feed-froward activation
F-1,6-bisP - feed-forward
ATP - feedback inhibition
Pyruvate kinase activation
When blood glucose levels are high, pyruvate kinase is phosphorylated and inactive
When blood glucose levels are low, pyruvate kinase is dephosphorylated and activated
Pyruvate to Acetyl-CoA conversion
H₃C-CO-COO + CoA-SH -> H₃C-CO-S-CoA + CO₂
The catalysts for this are nicotinamide dinucleotide (NAD) converted to NADH, thiamine pyrophosphate (TPP), and flavin adenine dinucleotide (FAD)
Pyruvate decarboxylation complex: what is it, what is it composed of, what does each part do, and where is the complex found?
Large, composed of 60 subunits of three different enzymes - 24x E1 (oxidative decarboxylation), 24x E2 (transfer of acetyl group to CoA), and 12x E3 (cofactor regeneration)
The mitochondrial matrix
Citric acid cycle recap
For each acetyl CoA which enters the cycle:
* Two molecules of CO2 are released
* Coenzymes NAD⁺ (×3) and FAD (×1) are
reduced to NADH and FADH₂
* One GDP/ADP phosphorylated to GTP/ATP
* Initial molecule (oxaloacetate) reformed
* The carbon atoms entering the cycle are not
lost the first time they go through it
ATP generation
Each NAD⁺ - 2.5 ATP produced
Each FAD - 1.5 ATP produced
Glycolysis: 2 ATP and 2 NADH (3-5 ATP) produced
Link reaction: 2 NADH (5 ATP) produced
TCA cycle: 2 ATP, 6 NADH (15 ATP), and 2 FADH₂ (3 ATP) produced
Total ATP produced = 30-32 ATP
Why is 3-5 ATP produced through NADH during glycolysis?
There are two potential electron carrier pathways: The malate-aspartate shuttle and the glycerol phosphate shuttle
MAS produces 2.5 ATP and is more common
GPS produces 1.5 ATP and is in brown adipose tissue
PDC regulation
Feedback inhibition:
High levels of acetyl-CoA inhibit E2
High levels of NADH inhibit E3
Allosterically activated by fructose-1,6-
bisphosphate
Feed-forward activation:
High levels of NAD⁺
Covalent activation:
Pyruvate dehydrogenation kinase (PDK) reversibly phosphorylates PDC to inactivate it
