I should be grateful if anyone would send me a link to an article or an encyclopedia/handbook contaning an explanation of the concept of enzymatic activity. Surprisingly, I did not manage to find anything else but a brief definition.
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Excerpted from a very popular Worthington publication which was originally published in 1972 as the Manual of Clinical Enzyme Measurements. While some of the presentation may seem somewhat dated, the basic concepts are still helpful for researchers who must use enzymes but who have little background in enzymology.
Source: Worthington Biochemical Corporation.
The quantity or concentration of an enzyme can be expressed in terms of activity in enzyme units. See Wikipedia:
Enzyme activity = moles of substrate converted per unit time = rate × reaction volume. Enzyme activity is a measure of the quantity of active enzyme present and is thus dependent on conditions, which should be specified. The SI unit is the katal, 1 katal = 1 mol s−1, but this is an excessively large unit. A more practical and commonly used value is enzyme unit (U) = 1 μmol min−1. 1 U corresponds to 16.67 nanokatals.
Here are some additional useful, simple videos explaining enzyme activity:
The entry for Enzymes in Wikipedia contains 28 instances of the word ‘activity’. I can only assume, therefore, that it is the fact that this term is not explicitly defined that is the problem — one for which a novice to the field need make no apology. I shall answer it in elementary — but not self-evident — terms.
Enzymes are biological catalysts, so catalysis is their fundamental property or quality. In this context my definition is:
Enzyme activity is the manifestation of the catalytic ability of an enzyme
My use of the word ‘manifestation’ is deliberate and important: the essence of enzyme activity is the action or functioning of the enzyme, because this is the way that we are aware of its presence and, hence, how we draw quantitative or qualitative conclusions about its behaviour or function.
Detection of enzyme activity
Enzyme activity is detected by observing a reaction which it catalyses, i.e. by observing — and usually measuring — the conversion of some chemical reactant (termed the substrate of the enzyme) to a product. This allows certain questions to be answered:
1. Is the enzyme present or not, or, if present, is it able to manifest its potential?
The most basic question that measurement of enzyme activity is used to answer is whether or not there is enzyme present, e.g. in a tissue. In fact, the absence of enzyme activity can have other interpretations besides the absence of enzyme protein: the enzyme could be present but denatured or poisoned, or it could be present in a inactive form because of the absence of some secondary agent (cofactor, regulatory molecule) required for it to function.
2. How much enzyme is there?
In experimental work where one wishes to compare amounts of an enzyme in different tissues, monitor its purification from other proteins, or study the effects of agents and conditions upon its functioning, one needs not just to detect it (e.g. by observing that a coloured substance is formed) but to measure how much enzyme or functioning enzyme there is. To quantify enzyme activity we measure the amount of substrate converted to product in a given time under suitable conditions.
Explanation of the units used and the suitable conditions are available in standard texts and on the web. My own self-teaching material on this topic can be found found here.
3. What is the nature of the reaction catalysed or is there more than one enzyme there?
These are other questions that measurement of the activity of an enzyme or impure preparation can answer. What is important here is what precise reaction is being catalysed. If an enzyme can catalyse the phosphorylation of D-glucose to glucose 6-phosphate, can it also catalyse the phosphorylation of L-glucose — i.e. is it specific for one rotamer? If the preparation can also phosphorylate fructose (for example), are there actually two different enzymes present in the preparation? If it phosphorylates some other sugar 50 times better than glucose, perhaps its function within the cell is different from that previously assumed.
Activity as a general concept in science and biology
The measurement of activity rather than quantity is not restricted to enzymes. One is also concerned with the activity of hormones, drugs, neurotransmitters etc. (e.g. to determine what tissues they affect and what their effects are). And more generally in science, we deduce the existence of things by their ability to do something — gravity is an example from physics.
Enzyme activity refers to the dynamic process of an enzyme catalyzing a reaction and is a measure of the quantity of catalytically competent enzyme present in a preparation.
We measure enzyme activity by making a trial of the enzyme reaction, that is we assay the enzyme. The trial, or assay, is done first and foremost under conditions where the enzyme works. The pH, for example, may be critically important, as may the presence or absence of inhibitors or activators. In addition, the assay is conducted under conditions where the experimenter can directly or indirectly detect the catalytic transformation, perhaps by monitoring the production of a product or the consumption of a substrate.
For example, if I want to measure the activity of a dehydrogenase, I could measure the decrease in absorbance at 340nm, where NADH but not NAD(+) absorbs strongly. In a spectrophotometer, this may be done continuously with time and is an example of a continuous assay.
However, there may be no convenient change in absorbance but we might have a radioactive substrate. In this case we might conduct our trial under conditions where we would expect the enzyme to work for, say, 30 min, stop the reaction (by denaturing the enzyme with acid, for example) and isolate a radioactive product. This would be an example of a discontinuous assay.
In both cases we are observing a change due to the enzyme catalyzing a specific reaction, that is we are measuring enzyme activity.
We now come to an important distinction. An enzyme assay is a measure of catalytic activity, which may or may not be related to the total amount of enzyme protein present. Consider a genetic disease that results in a particular enzyme activity being totally absent from an organ. We assay a liver sample for lactate dehydrogenase, for example, and find none present (to take a trivial example). This could mean that the enzyme is not being produced: there is no enzyme there. But is also could mean that the enzyme is present but is being produced in an inactive state. Maybe the 'normal' enzyme has an essential tyrosine at the active site, but a genetic mutation has changed this to a phenylalanine in the case under study.
If we have an antibody directed against the enzyme under study, we could develop an enzyme-linked immunoassay (ELISA) which will (if it is any good) specifically measure the total amount of protein present whether active or not, even in a crude liver extract. (A Western blot, of course, might also detect the presence of inactive protein). That is, an enzyme assay gives an estimate of the amount of enzyme present based on enzyme activity, whereas an ELISA gives an estimate of the total amount of enzyme protein present.
Enzyme activity is usually reported in terms of amount of substrate or product transformed per minute or per second: in micromoles/min, for example. It is also usual to measure the total amount of protein present and report the enzyme activity in terms of micromoles/min per milligram of (total) protein. This is the specific activity and allows for ease of comparison. Almost invariably, the 'per milligram' part of a specific activity measurement refers to the total amount of protein present (measured with the Lowry protein assay or the like).
(If, for example, Lab A reports that 20 microlitres of their liver extract has a-dehydrogenase activity of 5 micromoles/min and Lab B reports that 20 microlitres of their liver extract has a-dehydrogenase activity of 50 micromoles/min, these measurements may well be identical when compared on a protein basis (micromol/min per mg) and that the only difference is that liver A was extracted in 100 volumes of buffer but that liver B was extracted in 10 volumes of buffer).
Units of enzyme activity invariably refer to specific assay conditions, and this may be a very important consideration when designing an experiment. In addition, enzyme are labile and the activity may decrease with time (on prolonged storage for example). If, for example, a protocol requires you to add 10mg of of an enzyme preparation with a specific activity of 100 Units/mg (100 micromoles/min per mg), but freezing the enzyme preparation has resulted in denaturation, you are adding 10mg of enzyme protein but no enzyme activity. This may also be important.
Finally, a great reference: