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Step 1
C C CH 3
O O
O -
C CH 3
O CO (^2) CoA
NAD+ NADH CoA
C C CH 2 C
O
O -
O
O O - Oxaloacetate (4C) Citrate (6C)
CoA
C C CH 2 C
O
O -
O O -
HO CH 2 C O
O -
C C CH C
O
O -
O O -
CH 2 C O
O - HO
H
Isocitrate (6C)
NAD+
NADH
Step 2
C CH 2 CH 2 C
O
O -
O C O O -
CO 2
Step 3
α‐Ketoglutarate (5C)
C
CH 2
CH 2
C
O
O -
O
CoA
C
CH 2
CH 2
C
O
O -
O
O -
C
CH
O
O -
CH
C O
O -
C
HC
CH 2
C
O
O -
O
O -
OH
NAD+
NADH
CoA
CO 2
Step 4
Succinyl CoA (4C)
CoA GTP GDP
ATP
ADP
Pi
Succinate (4C)
FADH
FAD
Step 5
Furmate (4C)
Step 6
H 2 O
Malate (4C)
Step 7
NAD+
NADH
Step 8
The figure above represents the Citric Acid Cycle (also called the “Krebs Cycle”). The part of the molecule that becomes carbon
dioxide is highlighted in a blue box. Notice that a 4 carbon molecule called Oxaloacetate picks up 2 more carbons when it is joined
with an acetyl group from Acetyl CoA. Through the beginning steps of the cycle, 2 carbons are lost as carbon dioxide and the
molecule is again restored to a 4 carbon state, ready to pick up another acetyl group.
The details of the eight steps above are shown in the following pages. This time, the part of the molecule that undergoes a change
is highlighted in blue and the name of the enzyme that catalyzes the reaction is in a green box.
Citric Acid Cycle
Step 1
Step 2
The CH 3 end of the acetyl CoA loses a proton and becomes bonded to the second carbonyl carbon (C=O) of oxyloacetate. The coenzyme (CoA) is subsequently lost with the input of water.
Citrate
Synthase
Acetyl CoA Oxaloacetate
An isomerization reaction takes place. This involves the removal of a water molecule and then the insertion of a water molecule. The hydroxyl (OH) group changes position to a different carbon as a result. isocitrate
Step 3
This is the first of 4 oxidation steps in the cycle. The carbon carrying the hydroxyl group (OH) is converted to a carbonyl group (C=O). CO2 is lost from the intermediate and alpha ketoglutarate is produced. NADH is produced.
Isocitrate
deyhdrogenase
GDP
Step 4
Another oxidation step that results in another loss of CO2. This reaction is very complex and is similar to the reaction that converts pyruvate to acetly CoA. NADH is produced.
Step 5
CoA is displaced when an inorganic phosphate replaces CoA. Then the phosphate is used to phosphorylate GDP to make GTP. Later the high energy phosphate on GTP can be used to phosphorylate ADP to make ATP.
citrate
Aconitase
Isocitrate
α –Ketoglutarate dehydrogenase
Succinyl‐CoA
GTP Succinate
C CH 3
O CoA
C C CH 2 C
O
O -
O
O O -
CoA
H 2 O
S‐Citryl‐CoA intermediate Citrate
C C CH 2 C
O
O-
O O -
HO CH 2 C O
CoA
C
C
CH 2
C
O
O -
O
O -
HO CH 2 C
O
O -
C C CH C
O
O -
O O -
HO CH 2 C O
O - H
C C CH C
O
O -
O O -
CH 2 C O
O - HO
H
C C
O
O -
CH 2 C O
O - C C O O -
H
H 2 O H 2 O
Cis‐aconitate intermediate
C C CH C
O
O -
O O -
CH 2 C O
O - HO
H
CO 2
C
C
C
C
O
O -
O
O -
CH 2 C
O
H O^ -
O
C CH 2 CH 2 C
O
O -
O C O NAD+ NADH O - Oxylosuccinate intermediate (^) α‐Ketoglutarate
C CH 2 CH 2 C
O
O -
O C O O - α‐Ketoglutarate
+ CoA
C
CH 2
CH 2
C
O
O -
O
CoA
CO 2
NAD+ NADH
Pi
C
CH 2
CH 2
C
O
O -
O
CoA Succinyl‐CoA
C
CH 2
CH 2
C
O
O -
O
O -
+ CoA
H 2 O
Step 6
In this, the third oxidation reaction, two hydrogens are removed from succinate. FAD+ becomes reduced to FADH2.
Succinate Furmate
ATP
ADP
Succinyl ‐ CoA‐ synthase
C Succinate dehydrogenase
C
C
C
O
O -
O
O -
H H
H H
C C
O
O -
C C O O -
H H
FAD FADH
