chapter  8
Enzymes
Pages 18

The history of enzymes is closely connected with the

knowledge obtained in the fields of foodstuff chemistry

and technology (1,2). It started around 1526 with

Paracelsus’ studies of fermentation products. The

enzyme levels of a certain food are ascertained to

determine its degree of freshness (as in the case of

oxidative enzymes in vegetables), to detect particular

treatments, such as pasteurization (easily monitored in

milk by measuring the levels of phosphatase and

lactoperoxidase), or to see whether decay or microbial

contamination has started. Enzyme levels can be

measured easily by observing how they act on their

substrates, since most of these enzyme-catalyzed

reactions have an absolute specificity. The first satis-

factory mathematical analysis of the course of an

enzyme-catalyzed reaction was made by Michaelis and

Menten in 1913, who suggested that the rate of

transformation of a substrate (v) is a function, over a

certain range, of the substrate [S ] and enzyme [E ]

concentration:

v ¼ kcat½E½S KM þ ½S ð1Þ

where kcat and KM are constants. Assuming that this equation is valid for most enzyme-catalyzed reactions,

the enzyme concentration (or activity) is proportional

to the rate of appearance of a product or disappear-

ance of a substrate, and independent of substrate

concentration, only when [S ] KM for which the above expression may be reduced to:

v ¼ kcat½E ð2Þ For this reason, it is desirable to know the Km (Michaelis constant) of an enzyme for a particular substrate in a particular food. This can easily be calculated through the Lineweaver-Burk (doublereciprocal) plot of Eq. (1). The variability in the Km value makes it difficult to describe a universal method of analysis for a certain enzyme, since KM values vary greatly for the same enzyme, depending on the substrate used and the food analyzed. This means that if enzyme activity/concentration is to be accu-

rately estimated, prior knowledge of the KM value is necessary.