Chemical Reactions

To understand enzymes, it helps to first be clear about what a chemical reaction actually is, and how it differs from a merely physical change.

Physical changes versus chemical reactions

Chemical compounds can undergo two broad kinds of change:

• A physical change does not break any chemical bonds. A simple change in shape is a physical change. A change in the state of matter is also physical — for example ice melting into water, or water turning into vapour. In all of these, the substance is the same; only its form or state has changed.
• A chemical reaction is different. Here bonds are actually broken and new bonds are formed during the transformation, so new substances appear. For instance, barium hydroxide reacting with sulphuric acid to give barium sulphate and water is an inorganic chemical reaction:

Ba(OH)₂ + H₂SO₄ → BaSO₄ + 2H₂O

In the same way, the hydrolysis of starch into glucose is an organic chemical reaction.

Rate of a reaction

The rate of any physical or chemical process is the amount of product formed per unit of time:

rate = δP / δt

If the direction of the change is also specified, the rate may be called the velocity of the reaction. Several factors affect these rates, and temperature is a major one. As a rough rule of thumb, the rate roughly doubles for every rise of ten degrees Celsius and roughly halves for every fall of ten degrees Celsius.

Catalysts and the astonishing speed of enzymes

A catalysed reaction runs vastly faster than the same reaction with no catalyst. This is dramatically true of enzyme-catalysed reactions. A striking example is the joining of carbon dioxide with water to form carbonic acid, brought about by the enzyme carbonic anhydrase present in the cytoplasm:

CO₂ + H₂O ⇌ H₂CO₃ (Carbonic anhydrase)

• Without any enzyme, this reaction crawls along, producing only about 200 molecules of carbonic acid in a whole hour.
• With carbonic anhydrase, it races: roughly 600,000 molecules of carbonic acid are formed every second.

That means the enzyme has speeded the reaction up by about ten million times — a demonstration of the incredible power of enzymes.

Many enzymes and metabolic pathways

There are thousands of different enzymes, and each one catalyses its own particular chemical or metabolic reaction. Very often a chemical change is not a single step but a series of steps, with each step catalysed by an enzyme (either the same enzyme complex acting repeatedly or a set of different enzymes). Such a multistep, enzyme-catalysed sequence is called a metabolic pathway. …