Amino Acids & Structural Proteins

Chemical change with or without heat in  
albumin: the major protein in eggs
 
 

 

Adapted from Wikipedia and the ChemWiki / Libretext

Eggs (albumin)

Egg white is the clear, sticky, viscous liquid within an egg. In chickens, it is formed from secretions in the anterior section of the hen's oviduct during the passage of the egg. Fortunately, for eggs to be widely useful as a food, it forms around unfertilized egg yolks as well as fertilized. Food grade eggs are inspected to be sure the eggs are unfertilized. The egg white acts as a fluid cushion that protects the yolk from shock and rupturing if the egg is shaken, and it provides additional nutrition for the growth of the chicken embryo (when fertilized) as the chick developes.

Egg whites contain a little more than 50% of the total protein in the egg. The remainder is found in the yolk.

Egg white consists of ~90% water and ~10% dissolved proteins (including albumins, mucoproteins, and globulins). Ovalbumin (abbreviated OVA) is the main protein found in egg white, which makes up 60-65% of the total protein.

A sequence of 385 amino acids are combined through peptide bonds to form hen ovalbumin. The molecular weight (molar mass) of the polypeptide chain of ovalbumin is 42,699 daltons (amu), having a molar mass = 42,699 g/mol.. The amino acid sequence of ovalbumin deduced from experimentation using amino acid sequencing agrees with the experimental sequence of mRNA that codes for the ovalbumin. A portion of the amino acid sequence, which is referred to as the primary structure follows. (Refer to the amino acid table for the a list of letter designations for each of the 20 amino acids, which is referred to as a "residue" in the protein.)

GSIGAASMEFCFDVFKELKVHHANENIFYCPIAIMSALAMVYLG
AKDSTRTQINKVVRFDKLPGFGDSIEAQCGTSVNVHSSLRDILNQ
ITKPNDVYSFSLASRLYAEERYPILPEYLQCVKELYRGGLEPINF
QTAADQARELINSWVESQTNGIIRNVLQPSSVDSQTAMVLVNAIV

The carbohydrate content of egg white is less than 1%. It contains almost no fat, unlike the yolk, which is very high in fats and other lipids.

Egg white has many uses in food and drink: mayonnaise, custards, ice cream bases, lemon curd, mousse, macaroons, Pavlova, meringues, fritatas, and the white froth in cocktails such as a Ramos Gin Fizz. It is also used as a medium to produce seasonal vaccines that protect against 3 or 4 different types of viruses (trivalent or quadrivalent vaccines).

The albumin in eggs " denatures" when heated, that is, it takes on a different shape and structure. The albumin solidifies rather than turning more liquid as in some other types of proteins such as collagen. This is because of the difference in shapes that the two types of proteins take. One is more organized than the other. This organization involves higher, more complex levels of structure that proteins have in comparison to the primary structure, which is only based on the amino acid sequence. Refer to Amino Acids - Proteins Presentation .html , ppt, pdf

The four structural levels of proteins are:

• 1o (primary): The sequence of amino acids, eg. ARDV: Ala.Arg.Asp.Val.
H2N- on the left and on the right -COOH

• 2o (secondary): Structures which include, folds, turns, alpha-helices, and beta-sheets held in place by hydrogen bonds.

• 3o (tertiary): 3-d arrangement of all atoms in a single polypeptide chain.

• 4o (quaternary): Arrangement of all polypeptide chains into a functional protein, eg. hemoglobin with 4 strands: 2 pairs of 2 chains, one "alpha" and one "beta"

This experiment examines a number of factors that affect the overall protein structure of chicken egg albumin.

Experimental Procedure

    1. Add about 75 cm3 of water to a 150 cm3 beaker and heat it to at least 80.oC. Carefully break one fresh egg into a second 150 cm3 beaker. Using an eye dropper, separate the egg white from the yolk and place it in a small beaker. Discard the yolk. Add a few drops of the egg white to the hot water after the temperature of the water has reached 80.o C

    2. Place one or two drops of egg white into a test tube or small beaker which contains about 3 cm3 of 6 M HCI.

    3. Place one or two drops of egg white into a test tube or small beaker which contains about 3 cm3 of vinegar.

    4. Place one or two drops of egg white into a test tube or small beaker which contains a few grams of NaHCO3 (baking soda) dissolved in 3-4 cm3 of water.

    5. Place one or two drops of egg white into a test tube or small beaker which contains a few grams of NaCl (table salt) dissolved in 3-4 cm3 of water.

    6. Record your observations.

Report Form

Post Lab Questions

    1. How many different amino acids are there in the following peptide segment that is found in albumin: GSIGAASMEF CFDVFKELKV ?

    2. Name the amino acid residue that appears most frequently in the peptide segment.

    3. What do you think happens in the reaction of egg white with the hydrochloric acid and with the hot water? Is there a difference between them? What might account for this?

    4. Provide a reasonable explanation for the difference observed in treating the egg white with sodium chloride and with sodium bicarbonate.

    5. Provide a plausible reason that would explain albumin reacting with hydrochloric acid but not vinegar (acetic acid)?

    6. What is the mass of 0.10 moles of albumin? (Show your calculation.)

    7. How many moles of egg white are there in one large AA grade egg?

      [Egg Numbers, Sizes & Quality: hens lays about 320 eggs during a laying cycle ... Medium eggs are at least 49 grams; Large eggs are at least 56 grams; Extra Large eggs are at least 64 grams; Jumbo eggs are 70 grams or more. A 60 gram large egg has ~30 grams of white and ~20 grams of yolk. Grade AA means the egg has a firm yolk and a thick white, which does not spread. Grade A eggs have round yolks, thick whites, but have more thin whites than Grade AA. Eggs usually keep one month if refrigerated. Egg shells are porous and permeable. Therefore, eggs absorb odors very easily.]

    8. Briefly describe how to prepare a perfect soft boiled egg. Refer to class slide and note and the Exploratorium: https://www.exploratorium.edu/cooking/eggs/eggscience.html

    9. Provide a clear explanation in your own words of why copper and not stainless steel bowls have been used in recipes and in practice for over 200 years to froth egg whites.

      A scientific explanation was published in Nature 308, 667 - 668 (12 April 1984)

    10. Consider the nursery rhyme Humpty Dumpty. Which of the experimental steps #1, #2, #3, #4, if done on a whole Humpty egg before Humpty's fall, would have allowed all of the king's men to put Humpty back together? Briefly explain why.

    11. There are many pharmaceutical drugs that are closely related to proteins or are proteins themselves. Based on what you explored in this activity and what you know of stomach acidity, do you think that it would be possible to take these pharmaceuticals orally? Briefly explain your answer. (The drugs must enter your blood stream chemically intact in order to work pharmacologically.)

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