Food Chemistry

1.4 Proteins 79

Fig. 1.41.Carboxypeptidase A active site. (according
toLoweandIngraham, 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 ofκ-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.andTrametes sanguinea.The
rennin-like enzymes are produced, for example,
byAspergillus usamiiandMucor spp.,suchas
M. pusillus.
There are two carboxyl groups, one in undisso-
ciated form, in the active site of aspartic pro-
teinases. 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 num-
bering of the amino acid residues in the active site
applies to the aspartic proteinase fromRhizopus
chinensis.

(1.159)

Inhibition of these enzymes is achieved with

various diazoacetylamino acid esters, which ap-

parently react with carboxyl groups on the active

site, and with pepstatin. The latter is isolated from

variousStreptomycetesas 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 nutri-

tional/physiological and toxicological demands