Amino acid degradation digestive enzymes

Essential amino acids must be acquired in the diet nonessential amino acids can be synthesized by the body. Complete proteins contain all the essential and nonessential amino acids. Incomplete proteins are missing one or more essential amino acids. Protein digestion begins in the stomach, where proteins are degraded by the enzyme pepsin. Further digestion occurs in the small intestine by enzymes such as trypsin and chymotrypsin. 

Mammals, fungi, and higher plants produce a family of proteolytic enzymes known as aspartic proteases. These enzymes are active at acidic (or sometimes neutral) pH, and each possesses two aspartic acid residues at the active site. Aspartic proteases carry out a variety of functions (Table 16.3), including digestion pepsin and ehymosin), lysosomal protein degradation eathepsin D and E), and regulation of blood pressure renin is an aspartic protease involved in the production of an otensin, a hormone that stimulates smooth muscle contraction and reduces excretion of salts and fluid). The aspartic proteases display a variety of substrate specificities, but normally they are most active in the cleavage of peptide bonds between two hydrophobic amino acid residues. The preferred substrates of pepsin, for example, contain aromatic residues on both sides of the peptide bond to be cleaved. 

Rapid degradation of proteins is often induced at certain stages of differentiation. For example, sporeforming bacteria contain a protease that becomes activated upon germination of the spore.131 Within minutes this enzyme digests stored proteins to provide amino acids for the synthesis of new proteins during growth. 

Conventionally, the first attribute known about an enzyme used to be its function, usually in a crude extract. This property was screened for in microbial cultures or in tissue samples. The crude extract was then purified to homogeneity and the protein subjected to biochemical studies to learn of its pH and T profiles, its pi and subunit composition, catalytically important residues, and other properties. Proteolytic digestion of the protein with subsequent Edman degradation led to the primary sequence, but no information on the secondary structures such as a-heli-ces and [5-sheets or the folding in three dimensions of the polypeptide chain. The primary sequence could have been used to deduct the gene sequence but, with the degeneration of the code, several possibilities for certain amino acids occur, which makes prediction of the gene sequence a risk. 

There are also a number of enzymes that degrade organic N compounds, inside or outside the cell, thereby making N available for assimilation or uptake, respectively. Examples we will consider include urease, amino acid oxidases, and extracellular peptidases (MulhoUand et al., 2002 MulhoUand and Lee, in revision. Chapter 7 by MulhoUand and Lomas, Fig. 2, this volume Table 32.1). In addition, for heterotro-phic organisms, digestive enzymes, especially proteases, are important for internal N cycling, recouping cellular N, and excretion. 

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