Trypsin homologous sequences

The folding pattern of cytochrome b5 is also found in the complex heme protein flavocytochrome b2 from yeast (Chapter 15)133 and probably also in liver sulfite oxidase134,135 Both are 58-kDa peptides which can be cleaved by trypsin to 11-kDa fragments that have spectroscopic similarities and sequence homology with cytochrome b5. Sulfite oxidase also has a molybdenum center (Section H). The 100-residue N-terminal portion of flavocytochrome b2 has the cytochrome b5 folding pattern but the next 386 residues form an eight-stranded (a / P)8 barrel that binds a molecule of FMN.133,136 All of these proteins pass electrons to cytochrome c. In contrast, the folding of cytochrome... 

For instance, the mammalian serine proteases — trypsin, chymotrypsin, and elastase—are very similar in structure and conformation. If a new mammalian serine protease is discovered, and sequence homology with known proteases... 

The trypsin family of serine proteases includes over 80 well-characterized enzymes having a minimum sequence homology of >21%. Two amino acid residues are absolutely conserved (Cysl82, Glyl96) within their active sites [26,27]. These proteases have similar catalytic mechanisms that lead to hydrolysis of ester and amide bonds. This occurs via an acyl transfer mechanism that utilizes proton donation by histidine to the newly formed alcohol or amine group, dissociation and formation of a covalent acyl-enzyme complex. 

Induction of mRNAs for several other specific rat hepatic proteins by GH has also been demonstrated [81-83]. The effect could be demonstrated in vivo and in vitro and involved a relatively rapid induction with a 5-fold increase in mRNA levels within 4 h of the administration of GH, although synergism with cortisol (possibly and/or thyroxine) was necessary for a maximal response [83]. cDNAs corresponding to two of the induced proteins have been cloned [82,83] and found to have sequences homologous to those of a known family of serine protease inhibitors. One of these proteins was shown to be secreted as a heavily glycosylated serum protein, and to have potent anti-trypsin activity [83]. Regulation of the production of this protein by GH was shown to occur mainly at the transcriptional level [83]. 

Sequence 1, in Table I, is the sequence of trypsin, chymotrypsin, pancreatic elastase, thrombin, and other mammalian proteases, and it occurs throughout the animal kingdom down to invertebrates as primitive as the sea anemone (4). The Streptomyces griseus enzymes are from Pronase, a commercial enzyme preparation. Two of its components, Streptomyces griseus trypsin and protease A, not only have the Asp.Ser.Gly sequence, but they show several other homologies in sequence with the mammalian enzymes (5, 6), The same is true for the sequence of a-lytic protease, the Myxobacter 495 enzyme (7). 

The evolutionary pathways leading to such a scheme are, in part, reflected in the apparent structural homology which exists between these proteases. Sequence information points to a close relationship between trypsin and chymotrypsin (8), and there are marked similarities in the structures of carboxypeptidases A and B, both to each other and to pepsin. Such relationships seem to exist but are beyond the scope of this article. 

Many other proteins have subsequently been found to contain catalytic triads similar to that discovered in chymotrypsin. Some, such as trypsin and elastase, are obvious homologs of chymotrypsin. The sequences of these proteins are approximately 40% identical with that of chymotrypsin, and their overall structures are nearly the same (Figure 9.12). These proteins operate by mechanisms identical with that of chymotrypsin. However, they have very different substrate specificities. Trypsin cleaves at the peptide bond after residues with long, positively charged side chains—namely, arginine and lysine—whereas elastase cleaves at the peptide bond after amino acids with small side chains—such as alanine and serine. Comparison of the Sj pockets of these enzymes reveals the basis of the specificity. 

No feature of primary structure, such as repetition of particular amino acid sequences, is common to all enzyme molecules. However, considerable homologies of sequence are found between enzymes that appear to share a common evolutionary origin, such as the proteases trypsin and chymotiypsin, and similarities of sequence are even more marked among the members of a family of isoenzymes. The amino acid sequence in the immediate neighborhood of the active center of the enzyme (discussed later) is often closely similar in enzymes of related function (e.g., the serine proteases are so called because they all have this amino acid in the active center). 

Serine hydrolases are enzymes that play a key role in diverse physiological systems. They all use a serine side-chain hydroxy group as a nucleophile in their enzymatic reaction. In contrast to the serine proteases of the trypsin/elastase family discussed above, the two esterases discussed here belong to a different mechanistic class that shares no sequence homology or structural similarity. Lipases digest nutritional fat triglycerides and acetylcholinesterase degrades the synaptic neurotransmitter, acetylcholine. 

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