Where in the cell does the Krebs (citric acid) cycle take place?	In the matrix of the mitochondria
What molecule feeds the Krebs cycle its 2 carbons each turn?	Acetyl CoA
What happens to pyruvate before it can enter the Krebs cycle?	It loses one carbon as CO₂, is oxidized (making 1 NADH) and is attached to coenzyme A, forming acetyl CoA
Which enzyme turns pyruvate into acetyl CoA?	Pyruvate dehydrogenase
Per glucose, what does pyruvate oxidation produce?	2 acetyl CoA, 2 CO₂ and 2 NADH (one of each per pyruvate, and 1 glucose gives 2 pyruvates)
Step 1: acetyl CoA joins which 4-carbon molecule, and what does it make?	Acetyl CoA (2C) + oxaloacetate (4C) → citrate (6C); CoA is released
In the Krebs cycle, what comes after citrate?	Isocitrate (step 2)
In the Krebs cycle, what comes after isocitrate?	α-Ketoglutarate (step 3)
In the Krebs cycle, what comes after α-ketoglutarate?	Succinyl CoA (step 4)
In the Krebs cycle, what comes after succinyl CoA?	Succinate (step 5)
In the Krebs cycle, what comes after succinate?	Fumarate (step 6)
In the Krebs cycle, what comes after fumarate?	Malate (step 7)
In the Krebs cycle, what comes after malate?	Oxaloacetate (step 8), ready to take the next acetyl CoA
What comes right before succinate in the Krebs cycle?	Succinyl CoA
What comes right before α-ketoglutarate in the Krebs cycle?	Isocitrate
What comes right before oxaloacetate in the Krebs cycle?	Malate
List the 8 Krebs cycle intermediates in order, starting from citrate.	Citrate, isocitrate, α-ketoglutarate, succinyl CoA, succinate, fumarate, malate, oxaloacetate
How many carbons does citrate have?	6
How many carbons does isocitrate have?	6 (it's an isomer of citrate)
How many carbons does α-ketoglutarate have?	5
How many carbons does succinyl CoA have (in the part that comes from the cycle)?	4
How many carbons does oxaloacetate have?	4
Which two Krebs cycle steps each release a CO₂?	Step 3 (isocitrate → α-ketoglutarate) and step 4 (α-ketoglutarate → succinyl CoA)
Which enzyme joins acetyl CoA and oxaloacetate to make citrate?	Citrate synthase
Which enzyme turns citrate into isocitrate?	Aconitase (by removing and adding back a water, through cis-aconitate)
Which enzyme turns isocitrate into α-ketoglutarate?	Isocitrate dehydrogenase
Which enzyme turns α-ketoglutarate into succinyl CoA?	α-Ketoglutarate dehydrogenase (a multi-enzyme complex)
Which enzyme turns succinyl CoA into succinate?	Succinyl-CoA synthetase
Which enzyme turns succinate into fumarate?	Succinate dehydrogenase
Which enzyme turns fumarate into malate?	Fumarase
Which enzyme turns malate into oxaloacetate?	Malate dehydrogenase
What does citrate synthase make, and from what?	Citrate, from acetyl CoA and oxaloacetate (step 1)
What does fumarase do?	Adds water to fumarate, making malate (step 7)
What does malate dehydrogenase make?	Oxaloacetate (from malate), plus NADH (step 8)
Which Krebs cycle steps make NADH?	Step 3 (isocitrate dehydrogenase), step 4 (α-ketoglutarate dehydrogenase) and step 8 (malate dehydrogenase)
Which Krebs cycle step makes FADH₂?	Step 6: succinate → fumarate (succinate dehydrogenase)
Which Krebs cycle step makes GTP or ATP directly?	Step 5: succinyl CoA → succinate (succinyl-CoA synthetase), by substrate-level phosphorylation
Does step 5 make GTP or ATP?	It depends on the tissue's isoform: ATP in tissues that use a lot of ATP (heart, skeletal muscle), GTP in tissues with many anabolic pathways (liver)
What does one turn of the Krebs cycle produce?	2 CO₂, 3 NADH, 1 FADH₂ and 1 GTP (or ATP)
What does the Krebs cycle produce per glucose?	4 CO₂, 6 NADH, 2 FADH₂ and 2 GTP/ATP (2 turns, one per acetyl CoA)
Counting pyruvate oxidation too, how much CO₂ and NADH come from one glucose after glycolysis?	6 CO₂ and 8 NADH (2 CO₂ and 2 NADH from pyruvate oxidation, plus 4 and 6 from the cycle)
How many turns of the Krebs cycle does one glucose drive?	2: one glucose gives 2 pyruvates, so 2 acetyl CoA
Pattern: which Krebs enzymes make NADH or FADH₂?	The dehydrogenases: isocitrate, α-ketoglutarate and malate dehydrogenase make NADH; succinate dehydrogenase makes FADH₂
Where does succinate dehydrogenase sit, and why does it matter?	In the inner mitochondrial membrane, so its FADH₂ hands electrons straight to the electron transport chain (it's also complex II)
Why is step 1 (citrate synthase) irreversible?	It is highly exergonic
What slows step 1 of the Krebs cycle?	Plenty of ATP (negative feedback); the rate rises when ATP is scarce
What regulates isocitrate dehydrogenase (step 3)?	ATP and NADH slow it; ADP speeds it up
What inhibits α-ketoglutarate dehydrogenase (step 4)?	ATP, succinyl CoA and NADH
Which 3 Krebs cycle steps are irreversible?	Step 1 (citrate synthase), step 3 (isocitrate dehydrogenase) and step 4 (α-ketoglutarate dehydrogenase)
Is oxaloacetate used up by the Krebs cycle?	No: it is regenerated at step 8, so it acts like a carrier that takes in a new acetyl CoA each turn
Name the Krebs cycle's two other names.	The citric acid cycle and the tricarboxylic acid (TCA) cycle