Question

The enzymes catalyzing breakdown without addition of water are called
A) Lyases
B) Hydrolases
C) Ligases
D) Oxidoreductases

Answer 1

The proteinaceous substances that do not undergo any change but are capable of catalysing biochemical reactions are known as enzymes. In 1961, the International Union of Biochemistry (IUB) introduced the modern system of enzyme classification. It groups enzymes into six categories according to the type of chemical reaction they catalyse.

Complete answer:
The six categories of enzymes are- oxidoreductase, transferases, hydrolases, lyases, isomerases, ligases. Now let us discuss some classes in detail.
Lyases – These catalyse the cleavage of specific covalent bonds and removal of groups without hydrolysis, e.g., histidine decarboxylase (cleaves C-C bond in histidine to form histamine and carbon dioxide).
Hydrolases – These catalyse splitting of larger molecules into smaller molecules by addition of water (hydrolysis) and breaking of bonds like ester, ether, peptide, glycosidic, C-C, C-halide, P-N, etc. which are formed by dehydration condensation.
Ligases – These catalysed by covalent bonding of two substrates to form a large molecule. The energy for the reaction derived from the hydrolysis of ATP, e.g., pyruvate carboxylase combines pyruvate and carbon dioxide to form oxaloacetate at the expansion of ATP.
Oxidoreductase – These catalyse oxidation or reduction of their substrates by removing or adding electrons (and/or $H^+$) from or to substrates. These are of three types – oxidases, dehydrogenase and reductases.

So, the correct answer is option (A) lyases.

Note: In the past, enzymes were given arbitrary names. Some were named after the substrates on which they acted, according to the source from which they were obtained, some enzymes like DNA polymerase indicate its specific action. Presently the enzymes are categorized and named by an ‘International code of enzyme nomenclature’. The name of each enzyme consists of two parts which ends with an ‘ase’. The first part of enzyme indicates its substrates and the second part of enzyme indicates the reaction catalysed.