Science · Biology

Enzymes

1877 (term coined by Wilhelm Kühne)

Biology — biochemistry

Enzymes are biological catalysts, almost always proteins, that accelerate chemical reactions by binding substrates at an active site and lowering the activation energy. They are astonishingly specific and astonishingly fast: catalase handles millions of molecules a second, and some enzymes speed reactions by a factor of 10^17. Wilhelm Kühne coined the name in 1877; Eduard Buchner showed in 1897 that fermentation needs no living cell, and James Sumner crystallised the first one, urease, in 1926, proving they are proteins.

How to spot them

▸Biological catalysts, almost always proteins, with names usually ending in -ase
▸Bind substrates at an active site; lower activation energy without being consumed
▸Described by the lock-and-key model of 1894 and the induced-fit model of 1958
▸Kinetics described by the Michaelis–Menten equation, with constants Km and Vmax
▸The first one to be crystallised was urease, in 1926

Key facts

The word was coined in 1877 by Wilhelm Kühne, from the Greek for 'in leaven' (1877)

It replaced the vaguer 'ferment', and the catalyst inside yeast was distinguished from the yeast itself.

Eduard Buchner won the 1907 Nobel Prize for showing that a cell-free yeast extract could still ferment sugar (1897)

It demolished the vitalist idea that fermentation required a living cell.

James Sumner crystallised urease from jack beans in 1926, proving these catalysts are proteins (1926)

He shared the 1946 Nobel Prize in Chemistry with John Northrop and Wendell Stanley.

Emil Fischer's 1894 'lock and key' picture, softened by Daniel Koshland's 'induced fit' in 1958

The binding site moulds itself around the substrate rather than fitting rigidly.

Lysozyme, discovered by Alexander Fleming in his own nasal mucus, was the first of these to have its structure solved (1965)

By David Phillips at the Royal Institution, using X-ray crystallography.

Sidney Altman and Thomas Cech won the 1989 Nobel Prize in Chemistry for showing that RNA can do this job too (1989)

Catalytic RNAs are called ribozymes; the ribosome itself is one.

Catalase, the fastest known, processes millions of hydrogen peroxide molecules a second — which is why a cut fizzes

Its turnover number is of the order of 10 million per second.

Key ideas

They do not change the equilibrium of a reaction or make an unfavourable reaction favourable; they lower the activation energy barrier, so equilibrium is simply reached faster in both directions.
Michaelis–Menten kinetics: reaction rate rises with substrate concentration then plateaus at Vmax as active sites saturate; Km, the substrate concentration giving half Vmax, is a rough measure of binding affinity.

Quick-fire facts

•Named by Wilhelm Kühne in 1877 from Greek en zyme, 'in leaven'.
•Names usually take the suffix -ase after the substrate or reaction: lactase, protease, DNA polymerase. Older names survive — pepsin, trypsin, ptyalin, rennin.
•Classified by the Enzyme Commission into seven classes with EC numbers: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases and translocases.
•Orotidine 5'-monophosphate decarboxylase gives the largest known rate enhancement — around 10^17, turning a reaction that would take 78 million years into milliseconds.
•Most work best near body temperature and a particular pH; heat 'denatures' them by unfolding the protein, which is why cooking destroys them.
•Cofactors and coenzymes — metal ions, or vitamin-derived molecules such as NAD+ and coenzyme A — are often required; the protein alone is the apoenzyme, the complete unit the holoenzyme.
•Competitive inhibitors compete for the active site; non-competitive and allosteric inhibitors bind elsewhere and change the shape. Many drugs and poisons work this way, from aspirin to nerve agents.
•Everyday uses: biological washing powders (proteases and lipases), rennet in cheese-making, lactase in lactose-free milk, Taq polymerase in PCR.
•Common misconception: they are not all proteins. Ribozymes are made of RNA, and the peptide bond in every protein you own is made by one.

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Sources

David L. Nelson and Michael M. Cox — Lehninger Principles of Biochemistry
Robert A. Copeland — Enzymes: A Practical Introduction to Structure, Mechanism and Data Analysis