Proteins--The basis of life.
The word 'protein' occurs regularly in everyday speech, but there is poor understanding about the nature of proteins ('Something that we eat?'). Furthermore, it is not realised that proteins are responsible for a wide diversity of substances and functions in our lives.
This teaching session has been used with Year 9, 10 and 11 students of three Sydney schools, with the learning goals of:
(1) providing knowledge about protein as one of the important three components of our diet (together with carbohydrates and fats).
(2) going beyond the collective word, 'protein', to open students' eyes to the surprising diversity of substances that are 'proteins' (plural).
(3) providing historical perspective of the difficulties experienced by early protein chemists,
(4) indicating the underlying similarity in chemistry (linear polymers of amino acids) of proteins (depending on the seniority of the students).
(5) reinforcing these goals by having students make a collage of proteins and perform the simple experiment of isolating a food protein (gluten from dough).
Oral teaching and questioning was backed up with a Power Point presentation that follows the description below. The file is available gratis from the author. The classroom/laboratory exercise is suited to the broad range of science-based subjects--biology, chemistry, agriculture, textiles, food science and nutrition. It is directed at high-school students, being adaptable to any grade level.
BEFOREHAND--IN SEARCH OF PROTEIN SAMPLES AND INFORMATION
A day or two beforehand, the class was invited to research the topic of proteins themselves and thus to select examples of proteins to bring to the class session, preferably something that is a 'pure protein'. Industrious students who searched the Internet were taken into some realms difficult to fully understand. Examples include:
* 'A protein is a macromolecule composed of amino acid residues joined by peptide bonds in a characteristic sequence.'
* 'Proteins have structural, enzymatic or regulatory functions in organisms.'
* 'Proteins determine what job a cell will do.'
* Proteins, called enzymes, help generate energy in your body.'
* 'Proteins called hormones act as internal project managers'.
* 'The name "Protein was adopted by a punk alternative, post-grunge metal band formed in 1994 in San Francisco, California.' (Not helpful, but funny.)
* 'Proteins ... any of a large group of nitrogenous organic compounds that are essential constituents of living cells; consist of polymers of amino acids; essential in the diet of animals for growth and for repair of tissues; can be obtained from meat, eggs, milk and legumes.' (At least this definition suggests some examples [maybe not 'pure proteins'] to take to school for the proteins session.)
THE WHOLE CLASS--IN SEARCH OF PROTEINS
Inevitably, the protein examples brought to the class related to food--a can of tuna and an egg (egg white is a good example of a 'pure' protein). A less obvious example was 'my blood'. After some discussion: 'my hair'. Most comments offered about 'what-is-a-protein?' failed to go much deeper than food sources.
To expand the discussion, a series of 'spot-the-proteins' slides were shown, starting with the breakfast table (Figure 1). The immediate answers still included food sources--milk (casein), egg (ovalbumin), cheese, butter, yoghurt, bread and biscuits (gluten protein). Discussion followed about the heat-induced change in egg white from clear to ... 'white'. A further breakfast slide went beyond food: a silk dressing gown, a woollen table mat, and a hand (skin, finger nails, hair and the pink colour of haemoglobin in the blood under the skin).
The food theme was expanded (via a photo of a pavlova) into the effects of processing on proteins. Beating and heat turns egg white into the meringue centre of the pavlova. The beating (aeration) of cream provides the semisolid coating of the pavlova. Both examples of processing, plus bread-baking, involve protein denaturation--loss of the 'native' conformation or structure of the protein.
How about the request for some pure forms of protein? As previously discussed, egg white qualifies, but most sources of food proteins contain significant proportions of nonprotein material (the high fat content in cheese and egg yolk, and milk sugar [lactose] of skim milk). Blood contains many proteins, especially the haemoglobin in the red blood cells and albumin in the serum, but also the blood carbohydrate, glycogen.
A few photos, relevant to the outdoors (Figure 2), revealed some pure forms of protein: spider web, bird beak and feathers, wool, cat claw, cow horn, cow hide and cow hair. These examples are quite pure forms of protein.
Yet an these visible proteins obscure the claim (the title of this lesson) that proteins are 'the basis of life'. A multitude of proteins are literally the basis of life in our bodies:
* The many enzymes responsible for breaking down our food;
* Enzyme systems catalysing the reactions that turn food energy into heat and movement;
* Hormones that help regulate the body processes; and
* Muscles that help us to walk, talk, digest and breathe.
To reinforce the diversity of substances (Figure 2) that are unified under the chemical title of protein, have the class individually prepare a collage of images of proteins. This exercise may lend itself to the artistically inclined students who claim a lack of interest in science. Such a collage might even find its way into the students' visual art subject, under the title of 'My Favourite Proteins', making a learning experience for the arts staff!
THE DISCOVERY, NAMING AND UNIFORMITY OF PROTEINS
If this great diversity of substances goes under the single chemical class known as proteins, how are they similar? How could early chemists work out that all these substances represent a single class of chemical compounds?
We now know that proteins are simply linear polymers of amino adds. That is the chemical basis of their uniformity. A wide range of properties can be produced from this basic protein structure, because these polymers may be of almost any length, and because there are some twenty aminoacid building units with a wide diversity of side-group chemistries. Three amino acids are shown in Figure 3. The adjoining -OH and [H.sub.2]N- groups at the top of Figure 4 split out [H.sub.2]O and join to form a peptide bond (-O-HN-).
How did chemists of recent centuries come to classify proteins as having this chemical uniformity? The first proteins to be isolated in reasonably pure form were eye-lens crystallin and gluten (from wheat flour). The latter was first isolated by an Italian chemist named Beccari in 1728. He made a flour-and-water dough and kneaded it under running water. The starch particles washed out and he was left with a small rubbery ball of wet gluten. Gluten washing is offered below as a class experiment.
During the next hundred years, chemists' tests on various substances found some uniformity in the ratios of carbon, nitrogen, oxygen, sulphur and hydrogen in substances for which the name 'protein' was suggested. On the 10th July, 1838, the Swedish chemist Berzelius wrote (in French) to Mulder in Germany, suggesting the name 'proteinel, derived from the Greek proteos, which means 'primary substance'. However, it was still many years before there was a full realisation of the reasons for protein diversity and before the structure of the diverse amino acids was determined.
DEMONSTRATION OF GLUTEN WASHING
At this point, a demonstration was provided showing how the gluten-washing exercise would be performed later in the class. Initially, wheat grains were passed out for students to see that one end contains the germ, from which a shoot emerges on germination. Cutting or biting the grain reveals the white endosperm, which becomes white flour after milling. Students are able to watch the flour being mixed with a suitable amount of water.
PROTEINS AND OUR BODIES
Our body is able to produce 14 of the 20 amino acids. These include glycine, alanine, glutamine and tyrosine. We cannot make the remaining six 'essential amino acids', so we must get them from the foods we eat. These essential amino acids are methionine, cysteine, lysine, valine, leucine and isoleucine.
Protein is thus an essential part of our diet. Proteins form the major components of our muscles, skin, tendons, blood vessels, hair, and the cores of our bones and teeth. They help us grow and heal wounds. Proteins contribute the collagen--the connective tissue that gives your body its shape. Possibly most important are the enzymes (also proteins) that catalyse all the complex biochemical reactions of our digestion, energy use and movement.
MANY PROTEIN COMPONENTS ... AND LARGE MOLECULES
As protein research progressed throughout the twentieth century, the early chemists began to realise that the many sources of apparently pure proteins were actually complex mixtures of many different proteins. For example, cow's milk contains many proteins: in particular casein (in a few isoforms), lactalbumin and lactoglobulin. Egg white also contains many different proteins--ovalbumin (about half the protein content), ovotransferrin, ovomucoid, lysozyme and more minor proteins. These specific proteins serve distinct functions. For example, lysozyme provides a natural form of protection from pathogens. Lysozyme also provides protection for us by being present in human milk, our tears, our saliva and mucus.
Proteins are very large, compared to simple organic molecules such as common sugar. For example, egg albumin (ovalbumin) has a molecular weight of 45,000; that is 45,000 times the atomic mass of hydrogen. Lysozyme has a molecular weight of 17,000. Some of the protein components of wheat gluten have molecular weights of over ten million.
How CAN WE DEMONSTRATE THE PRESENCE OF SO MANY COMPONENTS?
Casein, a major component of milk proteins, can be easily separated from the other milk proteins by adding some acid (e.g., lemon juice). A white precipitate of casein appears when acidity falls below about pH 5 and most of the other proteins stay in solution (called the 'whey' ... remember Miss Muffet?). Other methods of demonstrating that apparently pure proteins have many components, involve taking advantage of their differences in solubility. However, gel electrophoresis is one of the various more effective methods of fractionating complex protein mixtures.
FRACTIONATING PROTEINS BY GEL ELECTROPHORESIS
Proteins differ in molecular size and electrical-charge properties at any given pH. Thus, they can be separated from one another by using their charge properties to move them through a gel medium that acts like a sieve, separating the protein molecules according to differences in size (and also in charge). The gel medium (polyacrylamide) used for protein electrophoresis is made with a range of pore-sizes that are similar to the sizes of protein macromolecules.
An example of the 'banding patterns' resulting from gel electrophoresis is shown in the electrophoretogram of Figure 4. A series of extracts of wheat flour proteins have been placed in the 12 sample positions at the top of the gel medium. These gluten proteins are all positively charged at the pH used (pH 3), so they have moved down into the gel towards the negative electrode. When the fastest components reached the bottom of the gel, the direct-current power was turned off and the proteins (normally colourless) were stained with a blue dye.
There are many protein bands in each vertical lane. Some proteins, in the same positions right across all lanes, are the same for all the wheat-flour samples. However, there are many places where the speed of travel has been different, giving different end positions, indicating differences in the protein compositions of the 12 different wheat varieties used as samples.
Wheat gluten contains two groups of proteins. The gliadin proteins have a molecular weight range under 100,000. They are the protein components fractionated in Figure 5. The gliadin proteins can be extracted from flour (or from a gluten ball) with 6% urea solution. The other half of gluten (glutenin) consists of much larger molecules with molecular weights ranging up into the millions. These glutenin polymers provide the dough with 'strength' (resistance to extension).
The same electrophoresis technique can be used to demonstrate the range of different protein components in examples such as raw egg white, in milk whey and in blood serum. In these cases, the raw egg white, whey or serum can be applied directly to the top of the gel.
PRACTICAL APPLICATIONS OF PROTEIN ELECTROPHORESIS
Proteins are synthesised (made) under the genetic control of the genes (DNA) of an organism, although growth conditions may also contribute to the composition of the resulting proteins. Thus, analysis of protein composition by gel electrophoresis can be used to determine genetic identity.
For example, Figure 5 shows that wheat varieties can be identified by their gel electrophoresis patterns. This identification capability has valuable applications in agriculture. Take the case of a farmer who has stored seed of a few different varieties for sowing next year in separate bins. He chalked the variety names on the storage bins, but the rain washed the names off. His problem of identification can be solved by gel electrophoresis of the proteins from the farmer's grain samples beside authentic samples of those varieties.
In a similar way, the thaw fluid from frozen fish fillets can be used to fractionate the many fish-muscle proteins in the fluid. The resulting electrophoretic patterns are different and distinct for each fish species, whereas identification is difficult by visual examination of the fillets. This process was used forensically, some years ago, to identify fish species at the Sydney Fish Markets when a cheap fish (ling) was claimed to be sold by some traders as the more expensive and popular barramundi. The claims were substantiated by gel electrophoresis.
EXPEMMENT: ISOLATION OF GLUTEN PROTEIN FROM WHEAT FLOUR
By washing gluten proteins from dough made from wheat flour, we are repeating the historic achievement of the Italian chemist, Beccari, to obtain a protein in reasonably pure form. That was nearly 300 years ago. Today, the isolation of gluten from dough is performed on an industrial scale to produce millions of tonnes of dry gluten for use in food manufacture.
* A 'strong' bread flour, about 100 grams per student.
* Sink with gentle stream of water from tap, or a bucket or tub of water in which the lump of dough can be squeezed.
* Container (plastic cup) and stick to mix the dough.
* Container (plastic cup) under stream of tap water in which to catch the starch that is washed out.
* Iodine reagent (in potassium iodide), to test for the presence of starch.
Add water slowly, while stirring, to about 100 grams of flour in a cup or beaker. Add no more water than is needed to form a stiff dough. Take the dough piece in the fingers and knead it under a very gentle stream of water from the tap. A cup placed under the stream of water catches the washed out starch, seen as white turbidity that settles as a white sediment. As the starch washes out, a lump of sticky yellowish gluten is left in the fingers.
Evaluation of success in purifying gluten:
Test the starch sediment with iodine reagent to demonstrate the dark blue colour reaction. Test the gluten ball at various stages of washing to demonstrate the gradual disappearance of starch.
Variations in the procedure:
If there are insufficient taps and sinks available, additional students can be assigned to a tub of water. The dough is kneaded under the water, and starch sediment appears at the bottom of the tub.
If a 'strong' bread flour is used, the gluten will hold together more easily during washing. For any flour sample, the strength of the gluten can be improved by adding some common salt to the water used for dough making. The flour used must be from wheat. The proteins of barley, rice or corn flour do not form into a coherent dough. That is why only wheat is suited for making leavened bread of good quality.
Evaluation of learning experience
Questions such as the following help to evaluate how well students have assimilated the information, either asked and discussed at the end of the lesson, or set as homework to be completed.
What is your favourite protein, or are there several?
Make an interesting collage of pictures illustrating the diversity of protein types.
How can there be such a great diversity of protein types?
What building blocks of proteins are essential for us?
Can you imagine life without proteins?
Was Berzelius justified in using a name meaning 'primary substance'?
ABOUT THE AUTHOR:
Dr Colin Wrigley, AM, has been involved in CSIRO research in Sydney for fifty years. In recent years, he has participated in CSIRO's Scientists in Schools program. He is an Adjunct Professor with the University of Queensland. (firstname.lastname@example.org)
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|Title Annotation:||Hands On|
|Date:||Jun 1, 2012|
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