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Proteins
6. Proteins as catalysts Page 25
diagram of lock and key Figure 3
Enzyme substrate 'lock and key'.
6.2 How enzymes work (continued)

Enzymes can work by:

  • attracting and sticking to the reacting molecules making it easier for them to meet
  • providing an alternative route for the reaction with a lower activation energy , so that a greater proportion of the collisions have more than enough energy to succeed
  • holding the molecules the right way round so that the reactive groups are brought together.

We can make surface catalysts that work on some reactions in the same way. What makes enzymes special and why are they proteins?

Enzymes have a much greater effect on reaction speeds than the synthetic catalysts we can make. Most importantly, though, enzymes are specific, that is each enzyme will only work with one reaction or one small group of closely related reactions (see Investigations 6a).

For an enzyme and substrate to bind they have to fit together physically. Each enzyme has a region on its surface called the active site (Figure 3). This is a cleft in the protein surface where the substrate binds. It has a shape that fits the substrate like a glove fits a hand or a lock fits a key. Only substrates with a particular molecular shape will have any chance to bind effectively. This lock and key hypothesis does not give the whole picture, though; enzymes are more subtle.

It is not enough for a substrate to just fit into the active site. It needs attractive forces to keep it in place. These attractive forces can involve any of the interactions that you have seen in Chapter 3 except covalent bonds:

  • electrostatic attraction between oppositely charged groups
  • hydrogen bonding
  • permanent dipole-permanent dipole forces
  • instantaneous dipole-induced dipole forces.
graph
Figure 4
Rate of an enzyme catalysed reaction plotted against enzyme concentration [E].
Once the substrate and enzyme have bound together they form what is called an enzyme/substrate complex. The binding process can be so selective that the enzyme discriminates between the two enantiomers (mirror-image isomers ) of a chiral substrate.
The mechanisms of enzyme-catalysed reactions are complicated and all have several steps. One simple model involves:
  1. formation of the enzyme/substrate complex (ES) in an equilibrium reaction
  2. catalysed conversion of substrate into the product (P), held onto the active site in an enzyme/product complex (EP)
  3. release of the product and regeneration of the free enzyme.
graph
Figure 5
Rate of an enzyme catalysed reaction plotted against substrate concentration [S].
This model is used to explain why:
  • the rate of an enzyme catalysed reaction is proportional to the enzyme concentration [E] (see Figure 4)
  • the rate of an enzyme catalysed reaction is proportional to the substrate concentration [S] up to a point, but then becomes independent of [S] (see Figure 5 and Investigations 6a).

This means that step 1 is rate limiting at low [S]. At high [S] all the enzyme active sites have substrate molecules bound (they are saturated) so that the rate of product formation can’t be increased by raising [S] any more.

Questions:
What is the order of the enzyme reaction with respect to substrate concentration at:

a) low substrate concentration

b) high substrate concentration?

Unilever Education Advanced Series: Proteins
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