Digestive Enzymes

Enzymes: catalysts for normal spontaneous reactions (slow rate). Catalysts are not permanently affected by the reaction. Enzymes act as organic catalysts. All physiological function intimately linked to enzymes. (digestion, kidney excretion, muscle contraction, tissue growth/breakdown ® turnover, cellular respiration)

Nature & Characteristics:

Protein: All enzymes are proteins

Produced: by living cells

Heat Labile: destroyed by heat

Specificity:

a. 1 bond 1 molecule - maltase

b. >1 bond 1 molecule - lipase

c. cross boundaries?? - No crossing of boundaries, ie. Lipase will not work on CHO or CP

pH: - most have optimum pH: slower (less active) at ¯ or ­ pH

example: pancreatic amylase optimum pH is ~7.0 (neutral). If stomach contents not neutralized, amylase activity would be slow therefore incomplete starch digestion.

Temperature: - like pH, optimum temperature. Temperature is usually constant in warm-blooded animals (98.6 to 102 oC)

Activity:

Secreted form: (proenzyme or zymogen) have to be activated from the form they are secreted as. Pepsinogen in the presence of HCl converted to pepsin

Co-"?": some need help from "other" factors to be active

Cofactor = if enzyme requires a mineral, then the mineral is a cofactor 

Coenzyme = if an enzyme requires a vitamin, the vitamin is a coenzyme

Mechanism of Action:

Substrate: compound to be acted on by the enzyme

Active Site: lactation on the enzyme where it attaches to the substrate

Product: the released result of the reaction

Maltose acted on by maltase to form 2 glucose

Amylose acted on by amylase to form X glucose

Protein acted on by pepsin to form oligopeptides

Fact acted on by lipase to form fatty acids

Hormones and Digestion
• Gastrin
o Secreted by the G-cells in the antrum of the stomach
o Stimulates parietal cells to secrete HCl
• Secretin – released because of acid in duodenum
o Stimulates pancreas to secrete fluid and bicarbonate into the gut
• Cholecystokinin (CCK) - Fats and(or) proteins entering the SI stimulate the release of CCK
o Causes pancreas to secrete digestive enzymes
o Stimulates gall bladder to release bile salts
• Leptin
o Insulin causes secretion
o Secreted from adipocytes
o Acts on hypothalamic leptin receptors to inhibit feed intake
o Increases energy expenditure

 

GI Tract Secretions

Source

Secretion

Species

Substrate(Function)

Mouth

S. Amylase

Pig, man

Glycogen, starch, dextrin

Mucin

All

Lubrication

Pregast. esterase

Young Ruminant

Fat - milk fat

Stomach

Pepsin

All

Protein

HCl

All

Activate pepsin (protein)

Lipase

All

Fats

Mucin

All

Protection & lubrication

Rennin

Young Ruminant

Milk protein (casein)

Intrinsic factor

All

Vit. B12 absorption

Mucin (pyloric)

All

Protection & lubrication

Pancreas

Amylase

All

Starch

Sucrase

All

Sucrose

Trypsin

All

Protein, peptides

Chymotripsin

All

Protein, peptides

Carboxypeptidase

All

Protein, peptides

Lipase

All

Fats

Liver

Bile

All

Fat emulsification

S.I.

Enterokinase

All

Trypsinogen activation

Aminopeptidase

All

Protein

Dipeptidase

All

Dipeptides

Nucleotidase

All

Nucleotides

Nucleosidase

All

Nucleosides

Alkaline Phosphatase

All

Organic phosphates

Lipase

All

Fat

Maltase

All

Maltase

Lactase

All (Avian)

Lactose

Sucrase

All

Sucrose

L.I.

Mucin

All

Protection & lubrication