Digestion and absorption of proteins

Proteins are large polymers. Unlike starch and glycogen, which are polymers of only a single type of monomer unit (glucose), proteins consist of a variety of amino acids. 

There is an almost infinite variety of proteins, composed of different numbers of the different amino acids (between 50 and 1,000 amino acids in a single protein molecule), in different order. There are some 30-50,000 different proteins and polypeptides in the human body. Each protein has a specific sequence of amino acids. 

Small proteins have a relative molecular mass of about 50—100 X 10, whereas some of the large complex proteins have a relative molecular mass of up to 10. In addition to proteins, smaller polymers of amino acids, containing up to about 50 amino acids, are important in the regulation of metabolism. Collectively these are known as polypeptides. 

Twenty-one amino acids are involved in the synthesis of proteins, together with a number that occur in proteins as a result of chemical modification after the protein has been synthesized. In addition, a number of amino acids occur as metabolic intermediates but are not involved in proteins. 

Proteins are composed of linear chains of amino acids, joined by condensation of the 

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FIG. 1 Digestion and absorption of proteins in the small intestine. (1) Brush-border peptidases, (2) brush-border amino acid transport systems, (3) brush-border peptide transport systems, (4) cytoplasmic peptidases, (5) basolateral amino acid transport systems, (6) basolateral peptide transport systems. 

Figure 23.1. Digestion and Absorption of Proteins. Protein digestion is primarily a result of the activity of enzymes secreted by the pancreas. Aminopeptidases associated with the intestinal epithelium further digest proteins. The amino...

The name given to two amino acids chemically linked together, it is a final stage in the digestion and absorption of proteins or an initial step in protein synthesis. 

Digestion and absorption in the small intestine. Most digestion and absorption of carbohydrates, proteins, and lipids occurs in the small intestine. A summary of the digestive enzymes involved in these processes is found in Table 18.3. 

The digestion and absorption of fat is considerably more complex than that of carbohydrate or protein because it is insoluble in water, whereas almost aU enzymes catalyse reactions in an aqueous medium. In such media, fat can form small droplets, an emulsion, which is stable in this medium. Formation of an emulsion is aided by the presence of detergents these possess hydrophobic and hydrophilic groups, so that they associate with both the fat and the aqueous phases. Such compounds are known as emulsifying agents and those involved in digestion are mainly the bile salts and phospholipids. 

Abnormalities in protein digestion In individuals with a deficiency in pancreatic secretion (for example, due to chronic pancreatitis, cystic fibrosis, or surgical removal of the pancreas), the digestion and absorption of fat and protein is incomplete. This results in the abnormal appearance of lipids (called steatorrhea, see p. 175) and undigested protein in the feces. 

Availability of Starting Materials for Maillard Reaction in Vivo Not all starting materials present in the body are available for Maillard reaction. For example, trypotophan in blood, formed by digestion and absorption of tryptophan-rich proteins, is easily bound by plasma albumin, making it unavailable for further transformation. 

The digestion and absorption of organic and inorganic nutrients, as well as all other biochemical processes in living organisms, are influenced by the unique properties of water. Water is an interactive liquid or solvent. Its chemical interactions with solutes are called hydration. Hydration involves weak associations of water molecules with other molecules or ions, such as Na+, Cl , starch, or protein. Because hydration bonding is weak and transitory, the number of water molecules associated with an ion or molecule at any particular moment is approximate and difficult to measure. However, typical indicated hydration numbers are Na", 1-2 K+, 2 Mg2+, 4-10 4r-8 Zn2+, 4-10 Fe, 10 Q- 1 and F-, 4 (Conway, 1981). 

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