2.1.4 Enzymes & Metabolism

• Digestive enzymes work outside of cells; they digest large, insoluble food molecules into smaller, soluble molecules which can be absorbed into the bloodstream
• Metabolism is the sum of all the reactions happening in a cell or organism, in which molecules are synthesised (made) or broken down
• Enzymes are biological catalysts made from protein
• Enzymes speed up chemical reactions in cells, allowing reactions to occur at much faster speeds than they would without enzymes at relatively low temperatures (such as human body temperature)
• Substrates temporarily bind to the active site of an enzyme, which leads to a chemical reaction and the formation of a product(s) which are released
• Enzymes remain unchanged at the end of a reaction, and they work very quickly. Some enzymes can process 100s or 1000s of substrates per second

Enzymes are biological catalysts that work in cells, so they randomly move about wherever they are in the cell. They don’t ‘choose’ to collide with a substrate – collisions occur because all molecules are in motion in a liquid

• Enzymes catalyse specific chemical reactions in living organisms – usually one enzyme catalyses one particular reaction:

The enzyme catalase can bind to its substrate hydrogen peroxide as they are complementary in shape, whereas DNA polymerase is not

• The specificity of an enzyme is a result of the complementary nature between the shape of the active site on the enzyme and its substrate(s)
• Enzymes have specific three-dimensional shapes because they are formed from protein molecules
  • Proteins are formed from chains of amino acids held together by peptide bonds
  • The order of amino acids determines the shape of an enzyme
  • If the order is altered, the resulting three-dimensional shape changes

The lock & key model

• The ‘lock and key theory’ is one simplified model that is used to explain enzyme action
• The enzyme is like a lock, with the substrate(s) the keys that can fit into the active site of the enzyme with the two being a perfect fit

Diagram showing the lock and key model

1. Enzymes and substates randomly move about in solution
2. When an enzyme and its complementary substrate randomly collide – with the substrate fitting into the active site of the enzyme – an enzyme-substrate complex forms, and the reaction occurs
3. A product (or products) forms from the substrate(s) which are then released from the active site. The enzyme is unchanged and will go on to catalyse further reactions

The induced-fit model

• Another model that explains enzyme activity is the ‘induced-fit theory’
• In reality when a substrate(s) binds to the active site of the enzyme, the active site and substrate change shape slightly to fit more perfectly together
• This makes it easier for bonds within the substrate to break and new bonds to form, producing product(s)

The effect of temperature

• The specific shape of an enzyme is determined by the amino acids that make the enzyme
• The three-dimensional shape of an enzyme is especially important around the active site area; this ensures that the enzyme’s substrate will fit into the active site enabling the reaction to proceed
• Enzymes work fastest at their ‘optimum temperature’ – in the human body, the optimum temperature is around 37⁰C
• Heating to high temperatures (beyond the optimum) will start to break the bonds that hold the enzyme together – the enzyme will start to distort and lose its shape – this reduces the effectiveness of substrate binding to the active site reducing the activity of the enzyme
• Eventually, the shape of the active site is lost completely and the enzyme is described as being ‘denatured’
• Substrates cannot fit into denatured enzymes as the specific shape of their active site has been lost

Denaturation is largely irreversible – once enzymes are denatured they cannot regain their proper shape and activity will stop

• Increasing temperature from 0⁰C to the optimum increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction
• This means that low temperatures do not denature enzymes, but at lower temperatures with less kinetic energy both enzymes and their substrates collide at a lower rate

This graph shows the effect of temperature on the rate of activity of an enzyme

The effect of pH

• The optimum pH for most enzymes is 7 but some that are produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)
• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed
• This will change the shape of the active site, so the substrate can no longer fit into it, reducing the rate of activity
• Moving too far away from the optimum pH will cause the enzyme to denature and activity will stop

If pH is increased or decreased away from the optimum, then the shape of the enzyme is altered

This graph shows the effect of pH on the rate of activity of an enzyme from the duodenum