Characteristics and mechanism of enzyme action

Biology · Enzymes. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

Enzymes are biological catalysts. Almost all are globular proteins. They speed up reactions in the cell, are needed in very small amounts, and are unchanged at the end, so they can be used again.

The substrate fits into a small pocket on the enzyme called the active site, forming an enzyme-substrate complex. Because of the shape of the active site an enzyme is specific: it works on one substrate or one group of related substrates.

Every reaction needs a starting push of energy called the activation energy. An enzyme lowers this energy, so the reaction goes quickly at body temperature. It does not change the products or the equilibrium of the reaction.

Rules to remember

Enzyme + substrate → enzyme-substrate complex → enzyme + product.
Active site = binding site (holds the substrate) + catalytic site (changes it into product).
Cofactor = non-protein part needed by some enzymes. Activator: a detachable inorganic ion such as Mg²⁺. Prosthetic group: covalently (firmly) bonded. Coenzyme: loosely attached organic molecule, often made from a vitamin.
Apoenzyme (protein part) + cofactor = holoenzyme (the complete, active enzyme).
Lock and key model (Emil Fischer): the active site is rigid and the substrate fits it exactly, like a key in a lock.
Induced fit model (Koshland): the active site is flexible and changes shape to fit the substrate when it binds.
Enzymes lower the activation energy; they are not used up and do not change the end products.

Common mistake

Students mix up coenzyme and prosthetic group. Both are non-protein, but a coenzyme is loosely attached and a prosthetic group is covalently bonded to the enzyme; an activator is an inorganic ion.

Worked example

An enzyme stops working when Mg²⁺ ions are removed from its solution, and works again when the ions are added back. What is Mg²⁺ for this enzyme?

The enzyme needs a non-protein helper, so Mg²⁺ is a cofactor.
Mg²⁺ is an inorganic ion, so it is not a coenzyme (coenzymes are organic).
It can be taken away and put back, so it is detachable, not covalently bonded like a prosthetic group.

Answer: An activator (a detachable inorganic cofactor).

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Chapter 3: Enzymes

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