Protein Analysis

Once a protein has been isolated, the next step is figuring out what it is and how it behaves. Protein analysis covers four goals: determining structure, amino acid composition, functional activity, and concentration.

Key Takeaways

• Structure is determined with X-ray crystallography (high resolution, crystallized) or NMR (smaller proteins, in solution).

• Simple hydrolysis reveals amino acid composition only; Edman degradation reveals sequence, one N-terminal residue at a time, with a practical limit well below its ~50–60 residue outer bound.

• Enzyme activity is measured by tracking a color-change reaction the enzyme catalyzes over time.

• UV spectroscopy (280 nm, tryptophan/tyrosine) estimates concentration but is contaminant-sensitive; colorimetric assays (BCA, Lowry, Bradford) offer an alternative, all based on color change proportional to protein amount.

Determining Protein Structure

To find out what a protein looks like in three dimensions, scientists typically use one of two methods.

X-ray crystallography involves crystallizing the protein and analyzing the diffraction pattern produced as X-rays scatter through the crystal lattice, giving a high-resolution picture of the protein's structure. NMR (nuclear magnetic resonance) spectroscopy relies instead on the magnetic behavior of certain atomic nuclei, and is generally used for smaller proteins since it reveals structural details while the protein remains in solution.

Determining Amino Acid Composition and Sequence

If the goal is just to know which amino acids are present and in what amounts, simple hydrolysis breaks the protein down into its individual amino acids for identification and quantification — but this only reveals composition, not order.

To determine the exact sequence of amino acids, a more precise method is needed: Edman degradation. This technique removes and identifies one amino acid at a time from the protein's N-terminus, determining the sequence step by step.

MCAT Callout — Edman Degradation Length Limit: Edman degradation has practical limits on peptide length. It can sequence peptides up to roughly 50–60 amino acids at the outer bound, but reliable accuracy in practice is often much lower — commonly cited around 30 residues — because incomplete cleavage accumulates error with each cycle, causing the reaction to lose synchrony across the peptide population.

Measuring Enzyme Activity

If the isolated protein is catalytic, its activity can be assessed by following the progress of a known reaction it catalyzes. This reaction often produces a color change that can be tracked over time — the rate of that color change reflects how active the enzyme is.

Determining Protein Concentration

Figuring out how much protein is actually present in a sample is usually done through spectroscopic methods.

UV spectroscopy takes advantage of the fact that most proteins contain aromatic amino acids — primarily tryptophan and tyrosine (with phenylalanine contributing only minimally) — that absorb UV light around 280 nm. This method is fast and simple, but it can be thrown off by contaminants that also absorb UV light at that wavelength.

To avoid that problem, researchers often turn to colorimetric assays instead. These rely on a specific chemical reaction between the protein and a reagent that produces a visible color change. The three most common are the BCA assay, the Lowry assay, and the Bradford assay.

The Bradford assay uses a dye called Brilliant Blue, which is normally a brownish color when protonated. When the dye binds certain amino acid groups on the protein, it loses protons and shifts to a blue form — so as protein concentration rises, more dye binds and the solution turns more intensely blue. The assay's main limitation is that detergents or high buffer concentrations can interfere with the dye's ability to bind properly.

All three colorimetric assays share the same basic principle: more protein produces more color change. Researchers use this relationship to build a standard curve and determine an unknown sample's protein concentration from how strong its color is — a measurement essential for getting the right amount of protein into downstream experiments.

Common MCAT Mistakes

• Confusing simple hydrolysis with Edman degradation. Hydrolysis only tells you which amino acids are present and in what amounts, not their order; only Edman degradation reveals the actual sequence.

• Overstating Edman degradation's practical range. The outer bound is roughly 50–60 residues, but reliable accuracy in practice is commonly cited around 30 residues — incomplete cleavage error accumulates with each cycle.

• Mixing up X-ray crystallography and NMR spectroscopy. X-ray crystallography requires the protein to be crystallized and gives high-resolution detail; NMR works in solution and is generally reserved for smaller proteins.

• Misreading the Bradford assay's color shift. The dye starts brownish (protonated) and turns blue (deprotonated) as it binds more protein — a more intense blue means more protein, not less.

MCAT-Style Concept Check

Question: A researcher has already determined a 25-residue peptide's amino acid composition using simple hydrolysis. She now wants to determine the exact order of those residues. Which technique should she use, and why is it appropriate here?

• A) Simple hydrolysis again, repeated until the sequence emerges

• B) Edman degradation, since it sequentially removes and identifies residues from the N-terminus and 25 residues falls within its practical accuracy range

• C) UV spectroscopy, since aromatic residues at 280 nm reveal the order of amino acids

• D) The Bradford assay, since dye binding intensity correlates with residue position

Answer: B

Explanation: Simple hydrolysis (Option A) only reveals composition, never order, no matter how many times it's repeated. Edman degradation removes and identifies one amino acid at a time starting from the N-terminus, directly determining sequence; a 25-residue peptide is within its commonly cited ~30-residue practical accuracy range, well under its ~50–60 residue outer bound. UV spectroscopy (Option C) and the Bradford assay (Option D) both measure protein concentration, not sequence — they say nothing about residue order.

FAQ

What is the difference between X-ray crystallography and NMR spectroscopy?

X-ray crystallography crystallizes the protein and analyzes how X-rays diffract through the crystal lattice, giving high-resolution structural detail. NMR spectroscopy uses the magnetic properties of certain atomic nuclei to reveal structure while the protein stays in solution, and is generally used for smaller proteins.

What is the difference between simple hydrolysis and Edman degradation?

Simple hydrolysis breaks a protein down into its individual amino acids, revealing which ones are present and in what amounts, but not their order. Edman degradation removes and identifies one amino acid at a time from the N-terminus, revealing the actual sequence.

Why does Edman degradation have a practical limit on peptide length?

Each cycle of Edman degradation has some incomplete cleavage, and that error accumulates across the peptide population with every additional cycle. The outer bound is roughly 50–60 residues, but reliable accuracy in practice is commonly cited around 30 residues.

What is the difference between UV spectroscopy and colorimetric assays for measuring protein concentration?

UV spectroscopy measures absorbance from aromatic amino acids (mainly tryptophan and tyrosine) at 280 nm, but contaminants that also absorb at that wavelength can skew results. Colorimetric assays like BCA, Lowry, and Bradford instead use a chemical reaction that produces a visible color change proportional to protein amount, avoiding that UV interference.