How do the structure and shape of proteins, especially enzymes, control their function?
Proteins as chains of amino acids whose sequence sets their shape and function, the range of protein functions, and enzymes as biological catalysts with a specific active site, an optimum temperature and pH, and denaturation outside these conditions.
An SQA National 5 Biology answer on proteins, covering proteins as chains of amino acids, the range of protein functions, and enzymes as biological catalysts with a specific active site, an optimum temperature and pH, and denaturation when conditions become too extreme.
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What this dot point is asking
The SQA wants you to describe proteins as chains of amino acids whose order sets their shape and function, list the main jobs proteins do, and explain in detail how enzymes work as biological catalysts: the active site, specificity, the effect of temperature and pH, the optimum, and what denaturation means.
What proteins are
Because the order of amino acids is set by the base sequence of a gene (see the DNA key area), DNA ultimately controls which proteins a cell makes.
The many jobs of proteins
Proteins carry out a wide range of functions in the body:
- Structural proteins, which form parts of cells and tissues (for example in the cell membrane and in connective tissue).
- Enzymes, which are biological catalysts.
- Hormones, which are chemical messengers carried in the blood.
- Antibodies, which are part of the immune defence against pathogens.
- Receptors on cell membranes, which detect specific molecules such as hormones.
Enzymes as biological catalysts
Enzymes control both synthesis reactions (building large molecules from small ones, such as making starch from glucose) and degradation reactions (breaking large molecules into small ones, such as breaking down hydrogen peroxide).
The active site and specificity
The substrate fits into the active site like a key into a lock. Once the reaction is done, the product leaves and the active site is free to take another substrate.
Optimum conditions and denaturation
Enzyme activity depends on temperature and pH.
- As temperature rises towards the optimum, the reaction speeds up. Above the optimum, the high temperature changes the enzyme's shape.
- Each enzyme also has an optimum pH. Above or below it, activity falls.
The graph of activity against temperature rises to a peak at the optimum, then falls sharply once denaturation begins. The graph against pH peaks at the optimum pH and falls either side.
Examples in context
Example 1. Catalase in liver. Liver cells contain the enzyme catalase, which breaks down toxic hydrogen peroxide into water and oxygen. If liver is boiled first, no oxygen bubbles form because the catalase has been denatured by the high temperature, so its active site no longer fits the hydrogen peroxide.
Example 2. Pepsin and the acidic stomach. Pepsin is a digestive enzyme that breaks down proteins. Its optimum pH is around 2, which is why it works well in the acidic stomach. Move pepsin to the neutral conditions of the mouth and it works poorly, because it is far from its optimum pH.
Try this
Q1. State two functions of proteins other than acting as enzymes. [1 mark]
- Cue. Any two of: structural, hormones, antibodies, receptors.
Q2. Name the part of an enzyme that the substrate fits into. [1 mark]
- Cue. The active site.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA N5 style4 marksDescribe how an enzyme works, using the terms active site, substrate and specific.Show worked answer →
A 4-mark describe answer should explain catalysis, the active site, specificity and the unchanged enzyme.
An enzyme is a biological catalyst, which means it speeds up a chemical reaction in the cell.
Each enzyme has an active site, and the shape of the active site is complementary to one particular substrate, so the substrate fits into it.
Because the active site has a specific shape, each enzyme only works on one type of substrate. This is enzyme specificity.
The enzyme is not used up: it remains unchanged at the end and can be used again.
Markers reward (1) catalyst that speeds up reactions, (2) the active site shape, (3) specificity, and (4) the enzyme being unchanged.
SQA N5 style3 marksExplain why an enzyme stops working when it is heated to a high temperature.Show worked answer →
The key word is denatured, and the explanation must link shape to function.
As temperature rises towards the optimum, enzyme activity increases. Above the optimum, the high temperature changes the shape of the enzyme.
The shape of the active site is changed, so the substrate no longer fits into it.
The enzyme is now denatured, and because the substrate can no longer bind, the reaction stops. This change is permanent.
Markers reward (1) the active site shape changing, (2) the substrate no longer fitting, and (3) the enzyme being denatured.
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Sources & how we know this
- SQA National 5 Biology Course Specification — SQA (2019)