Enzymes: activation energy, properties and structure

In a nutshell

Enzymes are proteins that speed up the rate of chemical reactions in the body. The induced fit model describes how when enzymes and substrates bind, the active shape changes shape slightly to allow a better fit. 

Enzymes

Definition

Enzymes are biological catalysts. They catalyse metabolic reactions at a cellular level (e.g., during respiration) and for the organism as a whole (e.g., digestion in mammals). They play a structural role in organisms by helping in the formation of key molecules like collagen, and also a functional role by helping in processes such as aerobic respiration. They can act intracellularly or extracellularly.

Due to their tertiary structure, enzymes are highly specific. This means their active site, the part of the enzyme where a substrate binds, has a specific shape.

Activation energy 

Enzymes work by lowering the activation energy of a reaction. This is the amount of energy required before a chemical reaction can start. This means enzymes allow reactions to take place at lower temperatures which speed up the rate of reaction.

During a reaction, the enzyme and substrate form an enzyme-substrate complex. This allows substrates to be close together which reduces repulsion between molecules and means they can bond more easily. This is important during reactions where two substrate molecules need to be joined. In reactions where an enzyme is catalysing a breakdown, the active site of the enzyme puts a strain on the bonds in the substrate. This causes it to break up more easily. 

The 'lock and key' model

The lock and key model was an early theory describing how enzymes work. It states that a substrate fits perfectly into the enzyme's active site the same way a key fits into a lock. 

Scientists later found that this is not completely accurate. Whilst it is true that enzymes are specific to certain reactions, the enzyme-substrate complex actually changes shape slightly to fit. This creates a tighter binding of the substrate and active site of the enzyme. This is known as the 'induced fit' model.

The 'induced fit' model

The 'induced fit' model is the current, accepted model of enzyme action. It explains why enzymes are specific to one particular substrate and that substrates need to fit the active site and make it change shape to create a better fit. 

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Enzyme properties and structure

Enzymes are very specific. They typically only catalyse one reaction as only one complementary substrate fits their active site. 

Example

Lactase breaks down lactose into glucose and galactose. 

The active site is determined by the tertiary structure, which itself is determined by the primary structure of the enzyme. If the tertiary structure is modified in any way (e.g., by a change in pH or temperature), then the active site will change and the enzyme will no longer bind with the substrate. Equally, if there is a mutation in a gene, the primary structure of the protein could change as a different amino acid will be present in the sequence. This, in turn, would alter the tertiary structure of the enzyme.

Metabolism

Definition

Metabolism is the all of the chemical reactions that take place in the body to provide energy. Enzymes are key for metabolism. 

​$$metabolism=anabolism+catabolism$$

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