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Enzymes

Key Concepts

Enzymes are globular proteins that act as biological catalysts.
Most enzymes are named after the reaction they catalyse and the name ends in ase.
Enzymes are more efficient and faster acting catalysts than most laboratory or industrial catalysts.
Most enzymes are unique to a particular biochemical reaction.
Enzyme operation is often pH and temperature dependent.
Enzymes usually function over a narrow range of temperature and pH.
Enzymes are believed to work in two ways: induced fit and lock and key
Enzymes can be denatured by changes in temperature or pH.
Denaturation refers to the disruption of the tertiary and secondary structure of the enzyme preventing it from acting as a catalyst.
Denaturation does NOT effect the primary structure of an enzyme.

Examples of Enzymes

Enzyme	Catalysed Reaction
amylase	hydrolysis of starch
maltase	hydrolysis of maltose
invertase (sucrase)	hydrolysis of sucrose
succinate dehydrogenase	dehydrogenation of succinic acid
ascorbic acid oxidase	oxidation of ascorbic acid
protease	hydrolysis of proteins
polymerase	polymerisation reaction
reductase	reduction reaction

Enzyme Structure

Enzymes are proteins that act as catalysts for biological reactions.

Primary Structure :

The sequence of amino acids making up the enzyme.
Peptide or amide bonds are the covalent bonds between C=O and N-H groups of the amino acids in the sequence.

Secondary Structure :

Shape of the enzyme caused by hydrogen bonding between C=O and N-H groups in the chain.
Common shapes are helix and sheet.

Tertiary Structure :

Folding and bending of the enzyme molecule caused by the interaction of alkyl (R) groups.
Disulfide links, C-S-S-H, are a common interaction causing tertiary structure.

How Enzymes Work

Lock and Key Model

The enzyme has a hollow active site where the reaction can take place.
The shape of the active site is determined by the tertiary structure of the enzyme.
The molecule on which the enzyme works (the substrate) attaches to the active site on the enzyme, just like a key fitting into a lock.
The bonding between the active site and the substrate may be hydrogen bonding, dipole-dipole interactions, or weak intermolecular forces (Van der Waals, Dispersion or London forces).
The enzyme functions by weakening the bonds in the substrate molecule causing a reaction to take place faster.

Induced Fit Model

The product is not the same shape as the reacting substrate.
The different shape of the product causes the enzyme-substrate complex to dissociate.
After the dissociation the enzyme's active site is ready to accept another molecule of substrate.

enzyme + substrate ---> enzyme-substrate complex ---> enzyme-product complex ---> enzyme + product

Conditions For Enzyme Function

Temperature

Enzymes are highly sensitive to changes in temperature.

In humans, most enzymes function best at body temperature (approximately 37^oC).

In general, heating a reaction mixture speeds up the rate of reaction by increasing the number of successful collisions between reactant molecules. This is shown on the graph to the right before the optimum temperature for enzyme activity is reached.

If the temperature of the reaction is raised too high past the optimum temperature, the reaction rate decreases as the enzyme becomes denatured and loses its ability to function as a catalyst for the reaction.

pH

Some enzymes can only act within a certain pH range.

Outside this pH range the enzyme may become denatured, lose its tertiary structure, and therfore lose its ability to function as a catalyst for the reaction.