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To illustrate the effect of a Competitive Enzyme Inhibitor

Succinic acid forms fumaric acid in the presence of the enzyme succinic dehydrogenase and a suitable hydrogen acceptor.
                  COOH                                        COOH
                  I                                           I
                  CH2                                         CH           
                  I                                           II   +  2H
                  CH2         Succinic dehydrogenase          CH
                  I                  -->                      I
                  COOH                                        COOH
                Succinic acid                             Fumaric acid

Tetrazolium chloride is a hydrogen acceptor and in the reduced form is pink.
Succinic dehydrogenase catalyses the dehydrogenation of succinic acid only. It is possible however, to block the active site of the enzyme by using a molecule which closely resembles the substrate.
Malonic acid is such a molecule.
Succinic dehydrogenase having combined with malonic acid rather than succinic acid can neither dehydrogenate it or readily lose it again. The enzyme site is thus blocked. The process may be reversed by adding an excess of succinic acid which competes with malonic acid for the action site.
Peas contain succinic acid dehydrogenase in the mitochondria. Succinic acid forms during metabolism.


4 Petri dishes containing:
1) 5 peas soaked in 1% malonic acid   )  Set up prior
2) 5 peas soaked in 5% malonic acid   )  to experiment,
3) 5 peas soaked in distilled water   )  soak overnight.
4) 5 peas soaked in 0.5% malonic acid )
Dropping pipette
1% Tetrazolium chloride solution. 5% succinic acid.


1. Cut the peas in dishes 1,2 and 3 in half. Add enough of tetrazolium chloride to cover the peas in dishes 1,2 and 3. Cover - leave for half an hour in a dark place.
2. At the end of the time record the colour of the peas. Sketch to show distribution of colour.
3. Add 5% succinic acid to the peas in dish 4, leave for half an hour then repeat steps 1 and 2.

For consideration

1. Which areas show maximum coloration - why?
2. Explain the different coloration of the soaked peas.
3. What effect did the addition of succinic acid have - explain.

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