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I have found graphs for the light absorption of water, haemoglobin, lipids, melanin, etc., but I am having a hard time finding graphs about enzymes.

Are there any websites or other reference sources for this type of information?

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It is not typical to investigate spectral absorption of compounds that are not useful for light absorption qualities, such as the compounds you've listed. Enzymes in particular will have less information because of the changes in binding partners our physical conformation to be enzymatically active. If you think there should be known info then I would try Google Scholar, Pubmed and a google search, in that order.

Edit: The intended meaning of the question is related to spectroscopy and quantification of protein concentration. There are two common ways to determine protein concentration using optical methods (spectroscopy). The methods use colorimetric assays (colour intensity is proportional to protein concentration) or light absorbance at a reference wavelength. Both methods rely on light absorbance at a specific wavelength, but the former relies on an reagent that changes colour in response to protein quantity, and the latter depends on direct UV light absorbance. Here's a quick breakdown of each.

For both of these assays, to determine the quantity of a specific protein, they require a source of purified protein.

Colorimetric Assays:

These are traditional biochemical assays, of which the most widely used are the: Bradford, BCA and Lowry protein assays. These assays rely on reduction-oxidation reactions between the key assay reagent (Bradford's reagent, bicinchonic acid, or Lowry's reagent) and protein. For example, the BCA reagent is reduced in the presence of protein, causing a chelated copper ion, Cu(II), to be reduced to Cu(I), and effecting a purple colour. This purple colour intensity, which read at a specific wavelength (~520 nm) will indicate the amount of protein.

Near-UV Absorbance:

This method relies on 280 nm (near-UV) absorbance. Usually absorbance at 260 nm is also used for a correction factor. Specifically, amino acids with aromatic rings (eg, tryptophan) cause absorption at 280 nm. Peptide bonds absorb at 200 nm, and various other bonds, pH, ionic conditions, all influence and absorb at various wavelengths.

These methods cannot be used with a protein sample alone, because there are a number of conditions that can affect readings. To correct for this, a protein standard is also used. A protein standard uses a series of known concentrations of a known protein. The common protein concentrations can range from 20-2000 µg/mL; the common protein standards are bovine serum albumin, chicken ovalbumin or purified IgG. The protein standard acts as a reference for protein concentrations that are valid for that experiment, under those specific conditions. A curve is fit to the readings of the protein standard, and the sample concentrations can be read off the standard curve by using the absorbance readings.

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I read that they usually use specterscopy to determine the concentration of the enzyme. They cant do it without knowing its absorbance,right? –  Abdelrahman Esmat Oct 16 '12 at 20:35
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I think the usual lab method is measuring the enzymes' activity by supplying the substrate and measuring how much product is created in what time. –  rwst Oct 17 '12 at 6:20
    
@AbdelrahmanEsmat - they don't need to know specific information about the enzyme (or any protein's) absorbance because it is inferred based on knowledge of how amino acids absorb light at specific wavelengths. –  leonardo Oct 17 '12 at 17:11
    
@rwst - This is true, and is only possible if an in vitro assay for enzyme activity exists. It also relies on the purified enzyme being active, which is not always the case. The question asked was more broadly stated. –  leonardo Oct 17 '12 at 17:13
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