Food Biochemistry and Food Processing

7 Enzyme Activities 157

CHEMICALCOMPOSITION OFENZYMES

For many enzymes, protein is not the only compo-
nent required for its full activity. On the basis of the
chemical composition of enzymes, they are catego-
rized into several groups, as follows:

1. Polypeptide, the only component, for example,
   lysozyme, trypsin, chymotrypsin, or papain;


2. Polypeptide plus one to two kinds of metal
   ions, for example,
   -amylase containing Ca^2
   ,
   kinase containing Mg^2
   , and superoxide dis-
   mutase having Cu^2
   and/or Zn^2
   ;


3. Polypeptide plus a prosthetic group, for exam-
   ple, peroxidase containing a heme group;


4. Polypeptide plus a prosthetic group and a metal
   ion, for example, cytochome oxidase (a
   a 3 )
   containing a heme group and Cu^2
   ;


5. Polypeptide plus a coenzyme, for example,
   many dehydrogenases containing NAD
   or
   NADP
   ;


6. Combination of polypeptide, coenzyme, and a
   metal ion, for example, succinate dehydroge-
   nase containing both the FAD and nonheme
   iron.

ENZYMESARESPECIFIC

In the life cycle of a unicellular organism, thousands
of reactions are carried out. For the multicellular
higher organisms with tissues and organs, even more
kinds of reactions are progressing in every moment.
Less than 1% of errors that occur in these reactions
will cause accumulation of waste materials (Drake
1999), and sooner or later the organism will not be
able to tolerate these accumulated waste materials.
These phenomena can be explained by examining ge-
netic diseases; for example, Phenylketonuria (PKU)
in humans, where the malfunction of phenylalanine
hydroxylase leads to an accumulation of metabolites
such as phenylalanine and phenylacetate, and oth-
ers, finally causing death (Scriver 1995, Waters
2003). Therefore, enzymes catalyzing the reactions
bear the responsibility for producing desired metab-
olites and keeping the metabolism going smoothly.
Enzymes have different types of specificities.
They can be grouped into the following common
types:

1. Absolute specificity.For example, urease (Sirko
   and Brodzik 2000) and carbonate anhydrase

(Khalifah 2003) catalyze only the hydrolysis

and cleavage of urea and carbonic acid, respec-

tively. Those enzymes having small molecules

as substrates or that work on biosynthesis path-

ways belong to this category.

2. Group specificity.For example, hexokinase
   prefers D-glucose, but it also acts on other hex-
   oses (Cardenas et al. 1998).


3. Stereo specificity.For example, D-amino acid
   oxidase reacts only with D-amino acid, but not
   with L-amino acid; and succinate dehydroge-
   nase reacts only with the fumarate (transform),
   but not with maleate (cisform).


4. Bond specificity.For example, many digestive
   enzymes. They catalyze the hydrolysis of large
   molecules of food components, such as pro-
   teases, amylases, and lipases. They seem to
   have broad specificity in their substrates; for
   example, trypsin acts on all kinds of denatured
   proteins in the intestine, but prefers the basic
   amino acid residues at the C-terminal of the
   peptide bond. This broad specificity brings up
   an economic effect in organisms: they are not
   required to produce many digestive enzymes
   for all kinds of food components.

The following two less common types of enzyme

specificity are impressive: (1) An unchiral com-

pound, citric acid is formed by citrate synthase with

the condensation of oxaloacetate and acetyl-CoA.

An aconitase acts only on the moiety that comes

from oxaloacetate. This phenomenon shows that, for

aconitase, the citric acid acts as a chiral compound

(Barry 1997). (2) If people notice the rare mutation

that happens naturally, they will be impressed by the

fidelity of certain enzymes, for example, amino acyl-

tRNA synthethase (Burbaum and Schimmel 1991,

Cusack 1997), RNA polymerase (Kornberg 1996),

and DNA polymerase (Goodman 1997); they even

correct the accidents that occur during catalyzing

reactions.

ENZYMESAREREGULATED

The catalytic activity of enzymes is changeable

under different conditions. These enzymes catalyze

the set steps of a metabolic pathway, and their activ-

ities are responsible for the states of cells. Inter-

mediate metabolites serve as modulators on enzyme

activity; for example, at high concentrations of ADP,