PROTEIN ANALYSIS

Introduction Proteins are polymers of amino acids Major structural components of many

natural foods Food analysts are interested in

knowing the total concentration, type, molecular structure and functional properties of proteins in foods.

Determination of Protein Concentration

1. Kjeldahl Method A food digested with strong acid so that it

releases nitrogen which can be determined by a suitable titration techniques

Does not measure the protein content directly

Conversion factor is needed to convert the measured nitrogen concentration to a protein concentration

PrinciplesA. Digestion

Sample is heated in the presence of sulfuric acid (oxidizing agent), anhydrous sodium sulfate, and a catalyst.

Digestion converts any nitrogen in the food into ammonia and other organic matter to CO2 and H2O

Ammonia gas remains in acid solution because it is in the form of ammonium ion which binds to the sulfate ion to form ammonium sulfate

B. Neutralization The solution in the digestion flask is then

made alkaline by addition of sodium hydroxide which converts the ammonium sulfate into ammonia gas

The ammonia gas moves out into receiving flask which contains an excess of boric acid

In receiving flask, ammonia gas converts to ammonium ion and boric acid to borate ion.

C. Titration Titrate with standard sulfuric or hydrochloric

acid using a suitable indicator The concentration of hydrogen ions required

to reach the end-point is equivalent to the concentration of nitrogen

Once nitrogen content has been determined, it is converted to a protein content using appropriate conversion factor.

Advantages: universal high precision good

reproducibility

Disadvantages: does not give a measure

of true protein different proteins need

different correction factors

the use of concentrated sulfuric acid at high temperature

time consuming

2. Enhanced Dumas Method Principles: a sample of known mass is

combusted in a high temperature to release CO2, H2O and N2

Nitrogen content is measured by separating N2 from CO2 and H2O using a column

Also used conversion factor to determine protein content

Advantages Faster Doesn’t need toxic

chemicals or catalysts

Easy to use Samples can be

measured automatically

Disadvantages High initial cost Does not give a

measure of true protein

Small sample size make its difficult to obtain a representative sample

3. Methods using UV-Visible Spectroscopy Use either the natural ability of proteins to

absorb (or scatter) light or chemically or physically modify proteins to make them absorb (or scatter) light in the region.

Calibration curve of absorbance versus protein concentration must be built first

Main difference: the chemical groups which are responsible for the absorption or scattering of radiation

Direct Measurement at 280 nm Principle: tryptophan and tyrosine absorb

ultraviolet light strongly at 280 nm Use the same w/length to measure protein

concentration Advantages: simple to carry out, non-destructive

and no special reagents are required Disadvantages: nucleic acid absorb strongly at

280nm To overcome: measure the absorbance at two

different w/lengths

Biuret Method A violet-purplish color is produced when

cupric ions (Cu2+) interact with peptide bonds under alkaline conditions

Absorbance is read at 540 nm Advantages: no interference from

materials that absorb at lower w/lengths and less sensitive to protein type

Disadvantages: low sensitivity

Lowry Method Combines biuret reagent and Folin-

Ciocalteau phenol reagent which reacts with tyrosine and tryptophan residues in proteins

Gives a bluish color which can be read at w/length between 500 – 750 nm

More sensitive to low concentrations of protein than biuret method

Dye Binding Methods A known excess of a negatively charged

dye is added to a protein solution which are positively charged (by adjusting the pH)

Bound dye and protein form an insoluble complex while the unbound dye remains soluble

The amount of protein present proportional to the amount of dye that bound to it.

Turbimetric Method Proteins from precipitate by addition of

certain chemical e.g. trichloroacetic acid The concentration of protein is

determined by measuring the degree of turbidity

Advantages: Rapid Simple to carry out Sensitive to low

protein concentration

Disadvantages: Time consuming and

laborious Absorbance depends on

the type of protein analyzed

difficult to quantitatively extract proteins from certain types of food e.g. processed food

Other instrumental techniques Bulk physical properties

Density – protein has the greatest density. So increase in protein content will increase density of food

Refractive index – RI increases as protein concentration increases

Measurement of Adsorption of Radiation Infra-Red: protein absorb IR naturally

due to characteristic vibrations of certain chemical groups

NMR: measuring the area under a peak in an NMR chemical shift spectra that corresponds to the protein fraction

Measurement of Scattering of Radiation Light scattering: use degree of turbidity

to measure protein content Ultrasonic scattering: ultrasonic velocity

and absorption are related to concentration of protein aggregates present.

Advantages: Non-destructive Little or no sample

preparation Rapid and precise

Disadvantages: Calibration curve must

be prepared Can only be used to

analyze foods with relatively simple compositions

Difficult to disentangle the contribution that the protein makes to the overall measurement from that of the other component

Protein Separation and Characterization Why need to have a knowledge of the

effects of environmental conditions on protein structure and interaction?

1. It helps to determine the most suitable conditions to use to isolate particular protein from mixture of protein

2. Not select conditions which will not adversely affect the molecular structure of the protein

A. Methods Based on Different Solubility Characteristics

Protein can be selectively precipitated or solubilized by altering pH, ionic strength, dielectric constant or temperature of a solution

The most simple to use when large quantities of sample are involved

1. Salting Out Proteins precipitate from aqueous solutions

when the salt concentration exceeds a critical level

Salt commonly used: Ammonium sulfate A two-steps procedure:

Salt is added just below that necessary to precipitate out interest protein.

Salt concentration is increased to point just above that necessary to cause precipitation

2. Isoelectric Precipitation Isoelectric point (pI): pH where the net

charge on the protein is zero Proteins tend to precipitate at their pI

because there is no electrostatic repulsion keeping them apart

Different amino acid has different pI.

3. Solvent Fractionation ↓ in dielectric constant will ↑ magnitude of

the electrostatic interaction between charged species

Therefore, decrease the solubility of protein in solution

Dielectric constant can be lowered by adding water-soluble organic solvents such as ethanol or acetone

4. Denaturation of Contaminating Proteins To isolate protein that stable at high

temperature or at extremes of pH Proteins are denatured and precipitated

when heated at high temperature or in the high acid solution

B. Methods based on adsorption characteristics Involves the separation of compounds

by selective adsorption-desorption at a solid matrix that is contained within a column

Separation can be carried out using either an open column or high-pressure liquid chromatography

1. Ion Exchange Chromatography Relies on the reversible adsorption-

desorption of ions in solution to a charged solid matrix or polymer network

Positively charged is called anion – exchanger while negatively charged is called cation – exchanger

Two types of column are used1. To bind protein of interest to the ion-

exchange column2. To favor adsorption from the column

2. Affinity Chromatography Uses stationary phase that consists of a

ligand covalently bound to a solid support

The protein of interest binds to the ligand in the column, and eluted using another buffer solution

C. Methods Based on Size Differences Depends on the Stokes radius of a

protein Stokes radius is the average radius that

a protein has in solution Stokes radius increases in the following

order: compact globular protein < flexible random-coil < rod-like protein

1. Dialysis Use semi-permeable membranes that permit

the passage of molecules smaller than a certain size through

A protein solution is placed in dialysis tubing which is sealed and placed into a large volume of water or buffer which is slowly stirred

LMW solutes flow through the bag while HMW remains.

2. Ultrafiltration A protein solution is placed in a cell

containing a semi-permeable membranes and pressure is applied to speed up the separation

LMW protein pass through the membrane whereas the LMW molecules remains in the solution

Use to concentrate a protein solution, remove salt, exchange buffers or fractionate protein on the basis of their size

3. Size Exclusion Chromatography Also known as gel filtration Column which is packed with porous beads

made of a cross-linked polymeric material Molecules larger than the pores in the

beads are excluded and move quickly through the column whereas the movement of molecules which enter the pores is retarded

D. Separation by Electrophoresis Relies on differences in the migration of

charged molecules in a solution when an electrical field is applied across it

It can be used to separate protein on the basis of size, shape or charge

1. Non-denaturing Electrophoresis A buffered solution of native proteins

is poured onto a porous gel (usually polyacrylamide, starch or agarose) and a voltage is applied across the gel

Gels may be contained between two parallel plates or in cylindrical tubes

2. Denaturing Electrophoresis Separate protein based on their molecular weight Proteins are denatured prior to analysis by mixing

them with mercatoethanol and sodium dodecyl sulfate (SDS)

Smaller protein moving more rapidly through the matrix than larger molecules

Type of electrophoresis commonly used is sodium dodecyl sulfate – polyacrylamide gel electrophoresis (SDS-PAGE)

3.Isoelectric Focusing Electrophoresis

Protein are separated by charge on a gel matrix which has a pH gradient across it

Has the highest resolution selection of suitable gel is important

4. Two Dimensional Electrophoresis Isoelectric focusing and SDS-PAGE are

used together to improve resolution of complex protein mixture

Proteins are separated in one direction on the basis of charge using isoelectric focusing and then in a perpendicular direction on the basis of size using SDS-PAGE

E. Amino Acid Analysis To determine amino acid composition of

proteins A protein sample is first hydrolyzed to

release amino acids which are then separated using chromatography