LSM2106 Chap.4 Proteins and Quantitative Protein Estimation
Proteins and Quantitative Protein Estimation
Four levels, and only the first is covalent
The fourth lecture covers proteins, which turn up in every tissue and do almost every kind of job a cell needs done, from holding its shape and generating movement to catalysing its reactions, carrying its signals and defending it. That breadth comes from shape, and shape is organised in four levels. The sequence of residues is held by peptide bonds.
Local repeating patterns are held by hydrogen bonds along the backbone. The fold of the whole chain is held by interactions between side chains, dominated by the exclusion of non-polar groups from water.
Assemblies of several chains are held by the same forces acting between chains rather than within one.
Because only the first level is covalent, heat, extremes of pH and competing hydrogen-bond donors destroy the upper three and leave the sequence intact. That is what denaturation is, and it is why purification is carried out in a buffer at a controlled pH and often in the cold.
It also connects directly to the previous chapter: the ionic pairs that stabilise a fold depend on which side of its pK each participating side chain sits, so a change in pH is a structural intervention rather than a background detail.
Two ways to put a number on a protein solution
The second practical estimates protein concentration twice, by different principles.
The direct method reads the protein's own ultraviolet absorbance, which peaks near 280 nanometres and comes primarily from tyrosine and tryptophan. It is quick, uses no reagent and is non-destructive, so the sample is recovered. Against it, the absorption depends on the aromatic content of the particular protein, and anything else absorbing in the ultraviolet contributes to the reading.
Nucleic acid is the classic contaminant, absorbing strongly near 260 nanometres, which is why the practical records a whole spectrum rather than a single point.
The dye-binding method works differently.
Coomassie dye binds non-covalently to protein, and binding shifts its absorption maximum from 465 to 595 nanometres, so the assay measures the increase at 595. Because the response depends on how a given protein binds the dye, the method needs standards, and it is linear only over a stated range.
A sample read above that range returns a concentration that is too low, because each additional milligram produces less additional absorbance as the dye is used up. Preparing two dilutions of each unknown is the standard guard: concordant results after correction mean both readings were inside the range.
What this chapter covers
- 01
Primary, secondary, tertiary and quaternary organisation and the forces holding each
- 02
Denaturation as loss of everything above the sequence
- 03
Ultraviolet absorbance at 280 nanometres and its origin in aromatic residues
- 04
The one per cent absorption figure and why it is protein-specific
- 05
Dye binding and the shift in the absorption maximum on binding protein
- 06
Standard curves, linear range and the direction of the error outside it
- 07
Choosing between the two methods, and saying why
Reconciling two estimates of the same unknown
- 2Correct the dye-method result for the dilution before comparing anything.
- 3Compute the ultraviolet estimate twice, with a generic and with a protein-specific coefficient.
- 2State which figure to report and what must accompany it.
Key terms
- Denaturation
- Loss of secondary, tertiary and quaternary organisation while the sequence of residues survives, caused by heat, extreme pH or agents that compete for hydrogen bonds.
- Quaternary structure
- The arrangement of two or more folded chains into one assembly, held by the same non-covalent interactions that stabilise a single fold.
- One per cent absorption figure
- The absorbance a ten milligram per millilitre solution of a particular protein gives in a one centimetre cell, used to convert an ultraviolet reading into a concentration.
- Standard curve
- A calibration line built from samples of known concentration under the same assay conditions, against which an unknown reading is converted into a concentration.
- Linear range
- The span of concentration over which the assay response rises in proportion to concentration. Outside it a reading converts to a value that is systematically too low.
- Dye binding assay
- A protein estimate based on the non-covalent binding of a dye whose absorption maximum shifts on binding, so the increase at the shifted wavelength reports the amount of protein.
Proteins and Quantitative Protein Estimation FAQ
Why does the practical run the ultraviolet method before the dye method?
Because the dye assay is only valid inside a stated concentration range, and the ultraviolet reading provides the rough estimate needed to choose a sensible dilution. Running them in the other order means guessing the dilution, which usually puts at least one unknown outside the range and wastes the standards already prepared.
What does a peak near 260 nanometres in my protein spectrum mean?
It means the dominant absorber in the cuvette is not protein. Nucleic acids absorb strongly near 260 while protein peaks near 280, so a maximum that has moved towards the shorter wavelength indicates contamination. Reporting a protein concentration from the 280 reading in that situation attributes another molecule's absorbance to protein.
Is a reading above the linear range too high or too low?
Too low. Beyond the top of the range each further increment of protein produces less additional absorbance, so projecting the reading back onto the straight part of the calibration understates the concentration. The remedy is to dilute into the range and multiply back, not to extrapolate the curve.
Which of the two methods should I report if they disagree?
First check that the dilution correction has been applied to both, since that accounts for most disagreements. If they still differ, report the dye result when the protein is unknown, because it was read against defined standards, and report the ultraviolet result when the protein is known and free of contamination, because it needs no reagent and leaves the sample intact.
Exam move
Draw the standard curve from memory with the linear range marked, then practise converting in both directions: from a concentration to the absorbance you would expect, and from an absorbance back to a concentration with a dilution factor in the way. Finish by writing, in two sentences, the condition under which each of the two methods fails. That pair of sentences answers most of the questions this topic generates.
Working through Proteins and Quantitative Protein Estimation in LSM2106? Sia is AskSia’s AI Biology tutor — ask any LSM2106 Proteins and Quantitative Protein Estimation question and get a clear, step-by-step explanation grounded in how LSM2106 is taught and assessed. Read this chapter free, then take your hardest questions to Sia.
