Enzymes in food technology – introduction - Enzymes in Food Technology - page 9

it reaches a maximum. After this point, increases in substrate concentration will not increase
the velocity. It is theorized that when this maximum velocity is reached, all of the available
enzyme is converted to ES, the ES complex. Michaelis constants have been determined for
many of the commonly used enzymes. The size of K m tells us several things about a particular
enzyme:

1. A small K m indicates that the enzyme requires only a small amount of substrate to

become saturated. Hence, the maximum velocity is reached at relatively low substrate
concentrations.

2. A large K m indicates the need for high substrate concentrations to achieve maximum

reaction velocity.

3. The substrate with the lowest K m upon which the enzyme acts as a catalyst is frequently

assumed to be enzyme's natural substrate, though this is not true for all enzymes.

1.5.3 Allostery
Allostery or allosteric regulation is the regulation of an enzyme or other protein by binding an
effector molecule at the protein's allosteric site. The allosteric site is a site other than the active
site of the enzyme. Effectors that enhance the enzyme's activity are referred to as allosteric
activators, whereas those that decrease the protein's activity are called allosteric inhibitors.
Following this mechanism, allosteric inhibition is a form of non-competitive inhibition (see
Section 1.5.6.3).
The term allostery comes from the Greek allos, 'other', and stereos, 'space', referring
to the regulatory site of an allosteric protein being separate from its active site. Allosteric
regulation is a natural example of feedback control.

1.5.4 Cofactors
Many enzymes require the presence of other compounds, called cofactors, which are needed
in order to demonstrate their catalytic activity. This entire active complex is referred to as the
holoenzyme; that is, the apoenzyme (protein portion) plus the cofactor (coenzyme, prosthetic
group or metal-ion-activator) together is called the holoenzyme.
A cofactor may be:

1. a coenzyme - a non-protein organic substance which is dialyzable, thermostable and

loosely attached to the protein part;

2. a prosthetic group - an organic substance which is dialyzable and thermostable which is

firmly attached to the protein or apoenzyme portion; and

a metal-ion-activator - these include K + ,Fe ++ ,Fe +++ ,Cu ++ ,Co ++ ,Zn ++ ,Mn ++ ,
Mg ++ ,Ca ++ and Mo +++ .

3.

Enzymes usually bind these cofactors in or close by the active site. Furthermore, some
enzymes can additionally bind other molecules than substrates or cofactors and these are
often molecules inhibiting the enzyme or are interfering with the catalytic process. In this
way the enzymatic reaction can be regulated by the cell. Some enzymes do not need any
additional components to show full activity. However, others require non-protein molecules
called cofactors to be bound for activity. Cofactors can be either inorganic (e.g. metal ions
or iron-sulphur clusters) or organic compounds, (e.g. flavin or haem). Organic cofactors can