Digestive Enzymes

Enzymes are catalysts, or substances that speed up the rate of reaction but are not changed or used up in the reaction. They are proteins and highly specific for the substrates, or molecules/nutrients, that they can cat on.

Enzyme Reaction Process

1. Enzyme and substrate are available
2. Substrate binds to enzyme
3. Substrate is converted and products are released

Step-by-Step Digestion

In the Mouth

In the mouth food is chewed and mixed with saliva, softening it so that it can be swallowed easily

Ptyalin is an enzyme released by the salivary glands, which acts on some starch, converting it to sugar. It is a type of alpha-amylase (salivary amylase)

In the Stomach

In the stomach food is acted on by gastric juice

Hydrochloric acid (HCl) breaks down proteins, preparing them for digestion
HCl activates the enzyme, pepsin, which is secreted by gastric glands in an inactive form (called pepsinogen)

Pepsin is the first enzyme to digest nearly every type of protein into smaller peptide fragments

Other minor gastric enzymes (those enzymes secreted in the stomach):

Geltinase breaks down type I and type V gelatin and type IV and type V collagen. In humans geltinases are called matrix metalloproteinases MMP2 and MMP9
Gastric amylase degrades starch, but is not significant
Gastric lipase digests fats, but not relevant in adults
Chymosin (or Rennin) is an enzyme that changes liquid milk to a solid

Produced by gastric chief cells in infants in order to curdle the milk they ingest, allowing for greater absorption of nutrients

Food, gastric juice, and mucus combine to form chyme, which travels to the small intestine

In the Intestines

Most digestion occurs in the small intestine

In the duodenum chyme is mixed with bile delivered from the liver and gallbladder through the common bile duct

Bile contains salts that emulsify (break-up) fats

Pancreatic juice is also delivered to the small intestine from the pancreas. It contains sodium bicarbonate, which neutralizes the acidic chyme form the stomach, along with the enzymes:

Lipase – digests almost all of the fat that the bile has emulsified

Fats are broken down into glycerol and fatty acids, which are more absorbable
If pancreatic lipase is absent, fats are expelled with the feces in an undigested form

Amylase – converts starch to maltose (a simple sugar)
Trypsin& Chymotrypsin – specific for certain amino acids, split proteins into small peptides
Nucleases – digest nucleic acids DNA and RNA

Note: when pancreatic juice is absent, serious digestive disturbances always occur
Peptidases complete the digestion of peptides to amino acids
The small intestine also produces the enzymes maltase, sucrase, and lactase which act on the disaccharides maltose, sucrose, and lactose respectively to convert them to simpler, more absorbable sugars

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Summary

Organ	Enzyme/Chemical Secreted	Action
Salivary glands	Salivary amylase	Begins starch digestion (polysaccharides to maltose)
Stomach	Hydrochloric acid (HCl)*	Breaks down proteins
Stomach	Pepsin	Begins protein digestion (proteins to peptides)
Small Intestine	Peptidases	Digests peptides to amino acids
Small Intestine	Lactase, maltase, sucrase	Digests disaccharides to monosaccharides or glucose (carbohydrate digestion)
Pancreas	Sodium bicarbonate*	Neutralizes HCl
	Amylase	Digests starch
	Trypsin	Digests proteins to peptides
	Lipases	Digests fats to fatty acids and glycerol
	Nucleases	Digests nucleic acids
Liver/Gallbladder	Bile salts*	Emulsifies fats