Digestive enzymes

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:

  1. 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.
  2. 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.

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