Digestion, primary structure

Fig. 2. Primary structure of hamster PrP (Stahl et al., 1993). The first 22 residues at the N-terminus are the signal sequence. PrPc is completely digested by proteinase K, whereas the N-terminal sequence of PrPSc to residue 89 (arrow, closed head) is digested. — CHO indicates the glycosylation sites at residues 181 and 197 Gpi the glycosylpho-sphatidylinositol anchor at 231 and the N-terminal octarepeats. In one case of human prion disease, a stop codon was found at 145 (arrow, open head) (Kitamoto et al., 1993). HI, H2, H3, and H4 denote the predicted a-helices (Huang et al, 1994, 1996), and A-C denote the a-helices and SI, S2 the /(-strands determined by solution NMR (James et al, 1997).

Whey proteins contain opioid-like sequences, namely a-la (both bovine and human) f(50-53) and (3-1 g (bovine) f(102-105), in their primary structure. These peptides have been termed a- and (3-lactorphins (Chiba and Yoshikawa, 1986). Studies by Antila et al (1991) indicated that proteolysis of a-la with pepsin produces a-lactorphin, and that digestion of (3-lg with... 

Twelve ACE-inhibitory peptides have been identified from sardine muscle hydrolysate, revealing that a dipeptide, Val-Tyr, acts as a key inhibitor (Matsufuji et al., 1994). Of the identified ACE-inhibitory peptides, the tripeptides (Leu-Arg-Pro, lie-Val-Tyr) and the dipeptide (Val-Tyr) show strong inhibitory activity. Moreover, two inhibitory peptides (myopentapep-tides A and B) have been purified from a thermolysin digest of porcine skeletal muscle proteins. The sequences were found in the primary structure of the myosin heavy chain (Arihara et al., 2001). 

The most widely accepted theory of prion diseases suggests that the infectious prion protein has the same primary structure as a normal protein found in nerve cells, but it differs in its tertiary structure. In effect, it is a misfolded, denatured version of a normal protein that polymerizes to form the amyloid protein plaques seen in the brains of infected animals. When an animal ingests infected food, the polymerized protein resists digestion. Because it is simply a misfolded version of a normal protein, the infectious prion does not provoke the host s immune system to attack the pathogen. 

D. B. Kassel, B. D. Musselman, and J. A. Smith, Primary structure dtermination of peptides and enzymatically digested proteins using capillary Uquid chroamtogra-phy/mass spectrometry and rapid linked-scan techniques, A a/. Chem. 63 (1991), 1091-1097. 

Advances in enzymatic digestion methodology and nucleotide separation have made it possible to study the primary structures of the messenger RNAs — 3000-4000 residues) as well as of DNAs (10 -10 residues). 

Production of pepsin and other proteolytic digestive enz)unes must be carefully controlled because the active enzymes would digest and destroy the cell that produces them. Thus, the stomach lining cells that make pepsin actually synthesize and secrete an inactive form called pepsinogen. Pepsinogen has an additional forty-two amino acids in its primary structure. These are removed in the stomach to produce active pepsin. 

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