1
General Aspects of Enzyme Analysis

1.1 Introduction and Essentials for Enzyme Assays

The book is intended toward supporting manipulation with enzymes. It starts with a concise presentation of theoretical aspects of enzyme reactions, followed by a description of the general features of enzymes. The observation of these features is indispensable for any manipulation with enzymes. A broad space is assigned to a detailed specification of enzyme assays. They are important because of two reasons: on the one hand, they are the tools to detect and to identify a distinct enzyme; on the other hand, they give a measure of the quantity and activity of the enzyme. First, the general requirements for enzyme assays are described, which must be regarded when performing a special assay and likewise for developing a new assay procedure. This is followed by a presentation of a series of special enzyme tests. The criterion for selection was mainly the frequency of application, but also different techniques and procedures, such as spectroscopic, radioactive, continuous, stopped, and coupled assays. Complementary to enzyme assays is the study of binding processes for the characterization of a distinct enzyme. Its interaction with substrates, products, cofactors, activators, and inhibitors is essential for understanding its mechanism of action. Such studies need special theoretical considerations and distinct methods, and provide different information. Finally, a survey of practical applications of enzymes in technical processes, therapy, and medicine is presented.

Enzymes as very efficient biocatalysts fulfill two essential functions in the living organism. Speeding up of reactions permits even virtually improbably reactions to become accessible to the metabolism, and tuning its catalytic efficiency via inhibition or activation enables precise regulation of the metabolism. The protein nature of enzymes1 provides the ideal precondition to accomplish this challenge: the keen specificity of the enzymes for their ligands - the substrates, activators, or inhibitors - which is indispensable to perform the multifaceted reactions within the cell and their compartments simultaneously in a controlled manner; the capability to construct distinct structural regions with subtle steric and electrostatic configuration to form an efficient catalytic center; as well as their ability to switch between distinct states of different structure and activity. Nevertheless, the protein structure alone cannot accomplish all types of reactions; frequently non-proteinogenic components, metal ions, dissociable coenzymes, or non-dissociable prosthetic groups are included.2

The highly developed structure of the enzymes calls for a special differentiated treatment. In this chapter, the principle of enzyme reactions will be examined both from the theoretical and practical viewpoints. By an enzyme reaction, one or more substrates are converted into one or more products. It is assumed that the reaction runs from substrates to products. However, due to the general principle of reversibility of chemical reactions, both directions are possible, but depending on the energy state frequently one direction is favored and usually, but not in any case, this direction is chosen for testing the enzyme. The task of the operator is to examine the respective compounds both qualitatively and quantitatively. The respective type of the substrate and the product is determining for the special type of the enzyme under study and is a prerequisite for further analysis. The enzyme assay serves to quantify the enzyme with respect to both its concentration and activity. The progress of the enzyme reaction can be observed by the formation of the product, or likewise by the disappearance of the substrate. Owing to the stoichiometric rules, both approaches must yield the same result. This is also valid in the case where more substrates or products are involved in the reaction; it is sufficient to observe only one substrate or product to quantify the reaction.3 So one reaction partner can optionally be selected to observe the course of the reaction; from the viewpoint of the reaction, it makes no difference which of the respective compounds will be chosen. Therefore, practical aspects determine the choice of the observed component. The most significant aspect is the existence of a specific signal to discern the respective component. The signal should be intense and clear and easily detectable with an appropriate and easily accessible technique. The absolute signal intensity is not only crucial but it must also be different from that of the unobserved reaction components. For example, it is not sufficient if a product shows a high signal when the substrate possesses a similar signal. Therefore, often various assays have been developed for the same enzyme and the assay that can be most easily realized under the conditions of the respective laboratory may be chosen. Considering these arguments, in principle any method can be taken that is appropriate to analyze the compound to be observed, but one crucial aspect must be regarded. Reactions are time dependent and an appropriate detection method should be used to observe the complete reaction course continuously (continuous assay). This is possible with various methods, but if none of them can be applied for a special enzyme system, the reaction must be performed unobserved and stopped after a distinct time period. Thereafter, the amount of the substrate remaining or of the product formed during this period can be examined in the assay mixture by a suitable analysis method, such as a color-developing detection reaction, thin layer chromatography, high performance liquid chromatography (HPLC), or radioactive labeling (stopped assay). This procedure yields instead of a continuous progress curve only one single measure point. The complete reaction course can be simulated by combining several measure points, obtained by variation of the reaction time (Box 1.1).

Box 1.1 Major Methods to Determine the Enzyme Activity

ORD, optical rotatory dispersion; CD, circular dichroism; and FPLC, fast protein liquid chromatography.

A further crucial aspect to be considered with enzyme assays is the dimension of the reaction batch, which will be a compromise between two opposite arguments. Larger volumes guarantee better detection and higher confidence, but require more of the valuable reagents, above all, the enzyme, especially if many assays need to be performed within a short time. Such considerations call for assay volumes as small as possible. Often an accurate result is less important than the general information whether the reaction proceeds at all, i.e. whether the respective enzyme is present or not. In such cases, the assay procedure may be performed as a microassay in 96 well plates, and analyzed with a microplate reader. In the following chapters several microassays are described, but also many other assays can be modified in this sense. Otherwise, the procedures for the enzyme assays described in the following chapters are adapted to a moderate reaction volume of 1?ml, which gives sufficient accuracy for most detection methods.

Literature

Standard Books, Series

1. Advances in Enzymology and Related Areas of Molecular Biology. New York: Wiley.
2. Barmann, T.E. and Schomburg, D. (eds.). Enzyme Handbook. Berlin: Springer-Verlag.
3. Bergmeyer, H.U. (ed.) (1983). Methods of Enzymatic...