2009_Book_FoodChemistry

1.4 Proteins 79
Fig. 1.41. Carboxypeptidase A active site. (according
to Lowe and Ingraham, 1974)
(called Lab-enzyme in Europe), active in the
pH range of 2–4, and cathepsin D, which has
a pH optimum between 3 and 5 depending on the
substrate and on the source of the enzyme. At
pH 6–7 rennin cleaves a bond of κ-casein with
great specificity, thus causing curdling of milk
(cf. 10.1.2.1.1).
Aspartic proteinases of microbial origin can be
classified as pepsin-like or rennin-like enzymes.
The latter are able to coagulate milk. The
pepsin-like enzymes are produced, for example,
by Aspergillus awamori, A. niger, A. oryzae,
Penicillium spp. and Trametes sanguinea. The
rennin-like enzymes are produced, for example,
by Aspergillus usamii and Mucor spp., suchas
M. pusillus.
There are two carboxyl groups, one in undissociated
form, in the active site of aspartic proteinases.
The mechanism postulated for cleavage
of peptide bonds is illustrated in Reaction 1.159.
The nucleophilic attack of a water molecule on
the carbonyl carbon atom of the peptide bond
is catalyzed by the side chains of Asp-32 (basic
catalyst) and Asp-215 (acid catalyst). The numbering
of the amino acid residues in the active site
applies to the aspartic proteinase from Rhizopus
chinensis.
(1.159)
Inhibition of these enzymes is achieved with
various diazoacetylamino acid esters, which apparently
react with carboxyl groups on the active
site, and with pepstatin. The latter is isolated from
various Streptomycetes as a peptide mixture with
the general formula (R: isovaleric or n-caproic
acid; AHMHA: 4-amino-3-hydroxy-6-methyl
heptanoic acid):
R − Val − Val − AHMHA − Ala − AHMAH
(1.160)
The specifity of aspartic endopeptidases is given
in Table 1.34.
1.4.6 Chemical and Enzymatic Reactions
of Interest to Food Processing
1.4.6.1 Foreword
Standardization of food properties to meet nutritional/physiological
and toxicological demands