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Digestive Enzymes

During digestion, four different groups of molecules are commonly encountered. Each is broken down into its molecular components by specific enzymes:
  1. Complex carbohydrates, or polysaccharides, (such as starches) are broken down into oligosaccharides (consisting of two to ten linked monosaccharides), disaccharaides (such as maltose), or individual monosaccharides (such as glucose or fructose). Enzymes called amylases break down starch.
  2. Proteins are broken down into short chains of amino acids (peptides) or individual amino acids by enzymes called proteases.
  3. Fats (lipids) are broken down into glycerol and fatty acids by enzymes called lipases.
  4. Nucleic acids are broken down into nucleotides by enzymes called nucleases.
A summary of enzymes and their substrate (substances upon which enzymes operate) appear in Table 1 .

TABLE 1 Enzymes and Their Substrates
Enzyme Substrate Products of Enzyme Activity
Saliva    
salivary amylase starches maltose, oligosaccharides
Gastric juice (chief cells of stomach)    
Pepsin proteins peptides
Pancreatic juice (acinar cells of pancreas)    
pancreatic amylase starches maltose, oligosaccharides
Trypsin proteins peptides
Chymotrypsin proteins peptides
Carboxypeptidase proteins peptides, amino acids
pancreatic lipase fats fatty acids, monoglycerides
Nucleases RNA & DNA nucleotides
Brush border (absorptive cells of small intestine)    
Dextrinase oligosaccharides glucose
Maltase maltose glucose
Sucrase sucrose glucose & fructose
Lactase lactose glucose & galactose
Aminopeptidase peptides peptides, amino acids
Dipeptidase dipeptides amino acids
nucleosidases nucleotides nitrogen bases, ribose, deoxyribose & phosphates
Phosphatases nucleotides nitrogen bases, ribose, deoxyribose & phosphates