Enzymes may be described as biocatalysts synthesised by living cell. They are protein in nature, colloidal and thermoliable in character and specific in their action. Enzymes regulate the rate of biochemical reactions without being utilized in these reactions. Enzymes accelerate biochemical reactions without affecting equilibrium. The term enzyme was coined by w.kuhne. The study of enzymes is called Enzymology. Edward Buchner who is considered as father of enzymology, accidentally discovered that a juice extracted using the yeast cells could bring out fermentation. He coined the term Zymase to designate the active principle involved in fermentation. James B. Sumner, isolated the enzyme urease from Jack bean (Canavalia ensiformis) in pure crystalline form, and established the protein nature of enzymes. John Northrop and his associates isolated pepsin and trypsin in crystalline form. So far more than 200 enzymes have been purified and characterised.
Endoenzymes and Exoenzymes:
Enzymes which act with in the cells in which they are synthesized are called Endoenzymes. Most of the enzymes are endoenzymes Eg:

  1. Chloroplasts contain enzymes that bring about phosphorylation, synthesis of photosynthetic pigments, carbon fixing etc,.

  2. Mitochondria contains enzymes of kerbs cycle and terminal oxidation.

  3. Lysosomes contains lytic enzymes.

  4. Glyoxisomes contain enzymes of B-oxidation of fatty acids to acetyl co-A.

  5. Endoplasmic reticulum is associated with enzymes of lipid metabolism.

  6. Nucleus contains enzymes of DNA and RNA metabolism.

  7. Cytoplasm contains enzymes involved in glycolysis and many other biosynthetic pathways.

Enzymes which act outside the cells of their origin are called exo-enzymes.

  1. Enzymes produced by fungi and bacteria for digesting the substances on which they grow.

  2. Enzymes produced by insectivorous plants like Drosera and nepenthes.

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