How are the chemical reactions of a cell organised into pathways and controlled by enzymes?
Metabolic pathways as integrated, enzyme-controlled chains of reactions (anabolic and catabolic), the role of membranes and cell compartments, and the control of enzyme activity by induced fit, activation energy, and competitive and non-competitive inhibition including feedback inhibition.
An SQA Higher Human Biology answer on metabolic pathways, covering anabolic and catabolic reactions, the role of membranes and reversible and irreversible steps, how enzymes lower activation energy by induced fit, and the control of enzyme activity through competitive, non-competitive and feedback inhibition.
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What this dot point is asking
The SQA wants you to describe metabolic pathways as enzyme-controlled chains of reactions, distinguish anabolic and catabolic reactions, explain how enzymes lower activation energy by induced fit, and describe how competitive, non-competitive and feedback inhibition control enzyme activity.
Metabolic pathways
Pathways are not random: they are organised by membranes, which compartmentalise the cell and channel substrates from one enzyme to the next. Some steps in a pathway are reversible, able to run in either direction depending on conditions, while others are effectively irreversible and may have alternative routes around them. Pathways come in two kinds:
- Anabolic reactions build up large molecules from smaller ones and require energy (for example, building proteins from amino acids).
- Catabolic reactions break down large molecules into smaller ones and release energy (for example, the breakdown of glucose in respiration).
How enzymes control the reactions
Each enzyme is specific to its substrate because of the shape of its active site. By lowering the activation energy, enzymes allow reactions that would otherwise be far too slow at body temperature to happen quickly, which is what makes controlled metabolism possible.
Control by inhibition
The activity of enzymes, and therefore the flow through a pathway, is controlled by inhibitors:
- Competitive inhibition. The inhibitor has a shape similar to the substrate and binds to the active site, blocking the substrate. Its effect is reversible by increasing the substrate concentration, because more substrate out-competes the inhibitor.
- Non-competitive inhibition. The inhibitor binds at a site away from the active site and changes the shape of the active site, so the substrate no longer fits. Adding more substrate does not reverse this.
- Feedback inhibition. The end product of a metabolic pathway acts as an inhibitor of an enzyme earlier in the pathway. When enough end product has accumulated, it switches the pathway off, preventing the cell wasting energy and raw materials; when the product is used up, inhibition stops and the pathway switches back on.
Feedback inhibition is an elegant self-regulating control, because the pathway adjusts its own output to match what the cell needs.
Examples in context
Example 1. Statins as enzyme inhibitors. Statin drugs inhibit an enzyme in the metabolic pathway that makes cholesterol, reducing cholesterol production in the liver. This is a medical use of enzyme inhibition to control a metabolic pathway and links directly to cardiovascular health.
Example 2. Feedback control of an amino-acid pathway. When a cell has made enough of a particular amino acid, the amino acid inhibits the first enzyme in its own synthesis pathway. Production stops until the amino acid is used up, showing feedback inhibition matching output to demand.
Try this
Q1. State whether building protein from amino acids is anabolic or catabolic. [1 mark]
- Cue. Anabolic (it builds up a larger molecule and requires energy).
Q2. Explain why adding more substrate reverses competitive but not non-competitive inhibition. [1 mark]
- Cue. A competitive inhibitor competes for the active site so extra substrate out-competes it, whereas a non-competitive inhibitor changes the active site and is unaffected by substrate concentration.
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 Higher 20183 marksExplain how an enzyme lowers the activation energy of a reaction, using the term induced fit in your answer.Show worked answer →
A 3-mark answer needs activation energy, the active site and induced fit.
Activation energy is the energy needed to start a reaction. An enzyme works by binding its specific substrate at the active site, forming an enzyme-substrate complex. This lowers the activation energy so the reaction can proceed more easily and at a faster rate.
Induced fit means the active site is not a rigid shape: when the substrate binds, the active site changes shape slightly to fit closely around it. This close fit puts the substrate under strain and brings reacting groups together, which is how the enzyme reduces the activation energy.
Award (1) activation energy is the energy to start a reaction, (2) substrate binds the active site forming a complex, and (3) induced fit, the active site changing shape, lowers the activation energy.
SQA Higher 20214 marksDistinguish between competitive and non-competitive inhibition, and explain how feedback inhibition controls a metabolic pathway.Show worked answer →
This is a 4-mark distinguish-and-explain question.
In competitive inhibition, the inhibitor has a similar shape to the substrate and binds to the active site, blocking the substrate. Its effect can be reversed by raising the substrate concentration, because more substrate out-competes the inhibitor for the active site.
In non-competitive inhibition, the inhibitor binds to a site away from the active site and changes the shape of the active site, so the substrate no longer fits. Raising the substrate concentration does not reverse this.
In feedback inhibition, the end product of a metabolic pathway acts as an inhibitor of an enzyme earlier in the pathway. When enough end product has been made, it switches off the pathway, preventing waste; when the product is used up, the pathway switches on again.
Award (1) competitive binds the active site and is reversible with more substrate, (2) non-competitive binds elsewhere and changes the active site, (3) feedback uses the end product as inhibitor, and (4) it inhibits an earlier enzyme to control output.
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