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    Enzymes are globular proteins that control biological reactions. Digestive enzymes speed up the breakdown (hydrolysis) of food molecules into their ‘building block’ components. These reactions occur outside of the cells lining the gut.

    Naming and classification of enzymes

    There are 2 systems used for naming enzymes:

    • The suffix ‘-ase’ is used with the root name of the substance being acted upon, for example, when sucrose (sugar) is digested, it is acted upon by an enzyme called sucrase.
    • The type of chemical reaction involved as the enzyme functions, for example, when sucrase acts on sucrose, it breaks it into a molecule of glucose and a molecule of fructose. This reaction involves adding a water molecule to break a chemical bond and so the enzyme is a hydrolase. All digestive enzymes belong to this hydrolase class.

    Enzymes are classified according to the type of chemical reaction catalysed. All digestive enzymes are hydrolases, whereas most of the enzymes involved in energy release for muscular contraction are oxidation-reduction enzymes such as oxidases, hydrogenases and dehydrogenases.

    Chemical structure of enzymes

    Enzymes are large protein molecules, all of which have their own specific 3D shape. Embedded within the shape is a region known as the ‘active site’, which can attract other suitably shaped molecules to bind to the site. The analogy that is often used to describe this mechanism is that of a key fitting into a lock. The enzyme serves as the lock and the attracted molecule (called the substrate) is the key.

    Once the chemical reaction within this lock and key arrangement has been completed, the products are released and the enzyme is free to attract another substrate molecule.

    The rate of reaction for such a process is thousands of substrate molecules per minute. If a solution of sugar is left in a sealed container, it breaks down into glucose and fructose extremely slowly. In the presence of a small amount of the enzyme sucrase, the rate of breakdown is millions of times faster.

    Sometimes, chemical substances other than substrates can bind with the active sites of enzymes, blocking their normal function. For example, water-soluble compounds of arsenic and mercury are extremely poisonous because they can permanently bind to some enzyme systems, markedly reducing their efficiency. Depending on the dose, the end result could be death.

    Digestive enzymes

    Digestive enzymes all belong to the hydrolase class, and their action is one of splitting up large food molecules into their ‘building block’ components. Another unique property is that they are extracellular enzymes that mix with food as it passes through the gut. The majority of other enzymes function within the cytoplasm of the cell.

    The chemical digestion of food is dependent on a whole range of hydrolase enzymes produced by the cells lining the gut as well as associated organs such as the pancreas. The end goal is to break large food molecules into very much smaller ‘building block’ units. These can then be readily and rapidly absorbed through the gut wall and into the bloodstream for transport to the liver and from there to other parts of the body.

    The main enzyme-producing structures of the human digestive system are the salivary glands, stomach, pancreas, liver and small intestine.

    Digestive juices and enzymes

    Substance digested

    Product formed

    Saliva
    Amylase

    Starch

    Maltose

    Gastric juice
    Protease (pepsin) and hydrochloric acid

    Proteins

    Partly digested proteins

    Pancreatic juice
    Proteases (trypsin)
    Lipases
    Amylase

    Proteins
    Fats emulsified by bile
    Starch

    Peptides and amino acids
    Fatty acids and glycerol
    Maltose

    Intestinal enzymes
    Peptidases
    Sucrase
    Lactase
    Maltase

    Peptides
    Sucrose (sugar)
    Lactose (milk sugar)
    Maltose

    Amino acids
    Glucose and fructose
    Glucose and galactose
    Glucose

    Bile from the liver
    Bile salts

    Fats globules

    Fat droplets

    The following pathway summarises how starch present in a food like bread is broken down chemically into glucose, which can then be absorbed through the intestinal wall and into the bloodstream for transport to the liver and from there to other parts of the body.

    Mouth and duodenum

    Starch hydrolysed into maltose through the action of the enzyme amylase.

    Jejunum

    Maltose hydrolysed into glucose through the action of the enzyme maltase.

      Published 13 July 2011 Referencing Hub articles