76 1 Amino Acids, Peptides, Proteins
Table 1.32.Enzymatic reactions affecting proteins
Hydrolysis
- Endopeptidases
- Exopeptidases
Proteolytic induced aggregation
- Collagen biosynthesis
- Blood coagulation
- Plastein reaction
Cross-linking
- Disulfide bonds
Protein disulfide isomerase
Protein disulfide reductase (NAD(P)H)
Protein disulfide reductase (glutathione)
Sulfhydryloxidase
Lipoxygenase
Peroxidase - ε(γ-Glutamyl)lysine
- Transglutaminase
- Aldol-, aldimine condensation and subsequent
reactions (connective tissue)
Lysyloxidase
Phosphorylation, dephosphorylation
- Protein kinase
- Phosphoprotein phosphatase
Hydroxylation
- Proline hydroxylase
- Lysine hydroxylase
Glycosylation
- Glycoprotein-β-galactosyltransferase
Methylation and demethylation
- Protein(arginine)-methyl-transferase
- Protein(lysine)-methyl-transferase
- Protein-O-methyl-transferase
[2pt] Acetylation, deacetylation - ε-N-Acetyl-lysine
endopeptidases varies greatly (cf. Table 1.34).
Trypsin exclusively cleaves linkages of amino
acid residues with a basic side chain (lysyl or
arginyl bonds) and chymotrypsin preferentially
cleaves bonds of amino acid residues which
have aromatic side chains (phenylalanyl, tyrosyl
or tryptophanyl bonds). Enzymes of microbial
origin often are less specific.
1.4.5.2.2 Cysteine Endopeptidases
Typical representatives of this group of enzymes
are: papain (from the sap of a tropical, melonlike
fruit tree,Carica papaya), bromelain (from the
sap and stem of pineapples,Ananas comosus),
ficin (fromFicus latexand otherFicus spp.)and
a Streptococcus proteinase. The range of activity
of these enzymes is very wide and, depending on
the substrate, is pH 4.5–10, with a maximum at
pH 6–7.5.
The mechanism of enzyme activity appears
to be similar to that of serine endopeptidases.
A cysteine residue is present in the active site.
A thioester is formed as a covalent intermediary
product. The enzymes are highly sensitive to
oxidizing agents. Therefore, as a rule they are
used in the presence of a reducing agent (e. g.,
cysteine) and a chelating agent (e. g., EDTA).
Inactivation of the enzymes is possible with oxi-
dative agents, metal ions or alkylating reagents
(cf. 1.2.4.3.5 and 1.4.4.5). In general these en-
zymes are not very specific (cf. Table 1.34).1.4.5.2.3 MetaloPeptidases.......................................
This group includes exopeptidases, carboxypep-
tidases A and B, aminopeptidases, dipeptidases,
prolidase and prolinase, and endopeptidases
from bacteria and fungi, such as Bacillus
cereus, B. megaterium, B. subtilits, B. ther-
moproteolyticus (thermolysin), Streptomyces
griseus(pronase; it also contains carboxy- and
aminopeptidases) andAspergillus oryzae.
Most of these enzymes contain one mole of Zn^2 +
per mole of protein, but prolidase and prolinase
contain one mole of Mn^2 +. The metal ion acts
as aLewisacid in carboxypeptidase A, establish-
ing contact with the carbonyl group of the peptide
bond which is to be cleaved. Figure 1.41 shows
the arrangement of other participating residues
in the active site, as revealed by X-ray structural
analysis of the enzyme-substrate complex.
The enzymes are active in the pH 6–9 range; their
specificity is generally low (cf. Table 1.34).
Inhibition of these enzymes is achieved with
chelating agents (e. g. EDTA) or sodium dodecyl
sulfate.1.4.5.2.4 Aspartic Endopeptidases
Typical representatives of this group are enzymes
of animal origin, such as pepsin and rennin