Digestion, primary structure proteins

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. 

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. 

The values of the molecular weights calculated from the primary structures of Torpede a, B, y and a subunits from the nucleotide sequences of cloned cDNA [27, 28] are about 20% higher than those obtained by us from SEC/LALLS. We cannot clearly explain the discrepancy. However, as pointed out by Noda et al. [27], it is likely that a carboxy-terminal peptide is eliminated by post-transitional cleavage. Furthermore, we cannot entirely eliminate the possibility that partial proteolytic digestions may occur in the process of preparation of the receptor proteins. 

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