Transition Reaction and Krebs Cycle Flashcards
this step of aerobic respiration converts the two molecules of the 3-carbon pyruvate from glycolysis (and other pathways) into two molecules of the 2-carbon molecule acetyl Coenzyme A (acetyl-CoA) and 2 molecules of carbon dioxide.
- takes place in the matrix
- One carbon is lost (in the form of CO2) to form an acetyl molecule
transition reaction/pyruvate oxidation
what happens in the transition reaction
As the two pyruvates undergo oxidative decarboxylation, two molecules of NAD+ become reduced to 2NADH + 2H+ which carry protons and electrons to the electron transport chain to generate additional ATP by oxidative phosphorylation. Along this, 2CO2 from pyruvate is produced and a coenzyme A attaches to the now 2-carbon molecule– forming acetyl CoA
- The two molecules of acetyl-CoA can now enter the citric acid cycle.
overall reaction for transition reaction
2 pyruvate + 2 NAD+ + 2 coenzyme A yields 2 acetyl-CoA + 2 NADH + 2 H+ + 2 CO2.
enzyme for transition reaction
pyruvate dehydrogenase
this cycle harnesses the available chemical energy of acetyl coenzyme A (acetyl CoA) into the reducing power of nicotinamide adenine dinucleotide (NADH).
it is part of the larger glucose metabolism whereby glucose is oxidized to form pyruvate, which is then oxidized and enters this cycle as acetyl-CoA.
krebs cycle/citric acid cycle/tricarboxylic acid cycle
who proposed the citric acid cycle
Hans Adolf Krebs
Step 1 of Krebs cycle
In the first step of the citric acid cycle, _______ (2C) joins with a four-carbon molecule, ___________, releasing the
______ and forming a six-carbon molecule called _______.
enzyme: ?
acetyl CoA; oxaloacetate; CoA; citrate; citrate synthase
Step 2 of Krebs cycle
In the second step, citrate is converted into its isomer, ___________. This is actually a two-step process, involving first the removal (forming ________) and then the addition of a water molecule, which is why the citric acid cycle is sometimes described as having nine steps—rather than the eight listed here
enzyme: ?
isocitrate; cis-aconitate; aconitase
Step 3 of Krebs cycle
In the third step, isocitrate is oxidized and releases a molecule of carbon dioxide, leaving behind a five-carbon molecule—_____________. During this step, _____ is reduced to form ______
enzyme: ? important in regulating the speed of the citric acid cycle.
α-ketoglutarate; NAD+; NADH; isocitrate dehydrogenase
Step 4 of Krebs cycle
the fourth step is similar to the third. In this case, it’s α-ketoglutarate that’s oxidized, reducing ________ to ______ and releasing a molecule of carbon dioxide in the process. The remaining four-carbon molecule picks up _________, forming the unstable compound ________
enzyme: ? important in regulating the speed of the citric acid cycle.
NAD+; NADH; Coenzyme A; succinyl CoA; α-ketoglutarate dehydrogenase
Step 5 of Krebs cycle
In step five, the _____ of succinyl CoA is replaced by a ___________, which is then transferred to ____ to make ATP. In some cells, GDP—____________—is used instead of ADP forming GTP—______________—as a product (which will then produce ATP). The four-carbon molecule produced in this step is called _________.
enzyme: ?
CoA; Phosphate Group; ADP; ATP; guanosine diphosphate; guanosine triphosphate; succinate; succinyl CoA synthase
Step 6 of Krebs cycle
In step six, succinate is _______, forming another four-carbon molecule called ________. In this reaction, two hydrogen atoms—with their electrons—are transferred to ____ producing ________. The enzyme that carries out this step is embedded in the______________ of the mitochondrion, so ________ can transfer its electrons directly into the electron transport chain.
enzyme: ?
oxidized; fumarate; FAD; FADH2; inner membrane; FADH2; succinate dehydrogenase
in step 6, why is FAD+ the electron acceptor instead of NAD+?
FAD is a better electron acceptor than NAD+, meaning that it has a higher affinity, or “hunger”, for electrons. Succinate is not a great electron donor, meaning that it has a fairly high affinity for electrons itself and is not eager to give them up.
NAD+ is not electron-hungry enough to pull electrons away from succinate, but FAD+
Step 7 of Krebs cycle
In step seven, ______ is added to the four-carbon molecule fumarate, converting it into another four-carbon molecule called ______.
enzyme: ?
water; malate; fumarase
Step 8 of Krebs cycle
In the last step of the citric acid cycle, ________—the starting four-carbon compound—is regenerated by _______ of malate. Another molecule of NAD+ is reduced to NADH in the process
enzyme: ?
oxaloacetate; oxidation; malate dehydrogenase
