Proteins essential to life

Food.

It is fair to say we all think about food sometime during the day. It is essential for life - although not as essential as water or air. You can last a few weeks without food but only a few days without water and only a few minutes without air.

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Nutritionally, food consists of five major components - fats, proteins, carbohydrates, vitamins, and minerals.

Carbohydrates include sugars, starch, and cellulose or fibre which are all essential for life. Carbohydrates provide the energy upon which our cells run.

Fats come in several forms and are an abundant source of calories - more than twice as many calories per gram than either carbohydrates or proteins. This is why we and most other organisms are genetically programmed to seek out fatty food. We can get more life sustaining energy out of a single serving.

Vitamins are a very specific and select set of molecules which we need to obtain from our diet. They are vital for good health and prevent diseases such as scurvy, pellagra, and beri-beri. Minerals are found in our bones but also in our nerves, cells and intercellular fluid. Minerals include the iron found in hemoglobin which transports oxygen and allows us to exist.

But if you have watched any cooking show over the past few years, you would think food is all about "the proteins." Gordon Ramsay or one of the other judges on Masterchef will frequently ask "what is the protein?" or make a comment about the protein being the star of the dish.

We do have a bit of an obsession with protein. As a vegetarian, I am frequently asked how I get enough protein in my diet. It is easy as protein is in every living cell. Pretty much every food contains protein - even a banana. Protein is not restricted to meat.

The word protein derives from the Greek word "protos" which means first and "proteios" which means primary. Proteins were considered the essential or primary substance in food during the mid-nineteenth century during the development of nutritional science.

Proteins represent a class of organic polymeric compounds built from a series of amino acids. Depending upon where you look in the tree of life, there are 20, 21, or 22 amino acids in play. In our bodies, we generally recognize 20 amino acids of which nine are essential. (Essential means we must obtain them from our diet as our bodies can't synthesize them.)

Protein molecules represent one of the most abundant molecules in living systems. Everything from hormones to neurotransmitters to muscle fibres to molecular manufacturing facilities are built from complex arrangements of proteins.

A key feature of proteins is their shape.

It is not so much which amino acids are linked together to form a peptide chain which determines what a protein does as the shape or structure the peptide chain adopts. Of course, the shape is determined by the amino acids in the protein chain so it really is the combination which gives rise to function.

The last few weeks, we have been talking about cellular communication and the role chemical compounds play in conveying information between cells. While some of the neurotransmitters are small protein chains, all of the receptor sites on the receiving neuron are proteins and must have a shape complimentary to the neurotransmitter or hormone to which they bind.

Proteins represent the "lock" in the "lock-and-key" mechanism operation in cell membranes throughout the body but particularly in neurons. And the reason some substances, such as illicit drugs, have such a powerful effect is because their shape almost exactly matches the receptor site's structure. In effect, drugs are skeleton keys able to activate a receptor.

With 20 amino acids and typically 200 to 300 amino acids in a protein, any protein chain encodes a lot of information. In the early part of the 20th century, many biologists believed proteins must contain the hereditary factors. It was proteins and not DNA providing the blueprint for cells.

The reasoning went a little further as DNA had only four letters with which to spell instructions (A, C, G, T). How could something so simple spell out the instructions for something so complex as a protein chain?

The answer lay in codons consisting of not one but three DNA nucleotides. With four nucleotides and three in each codon, there are total of 64 (4 x 4 x 4) possibilities. A few codons signal stop but there are 61 which signal for one of the 20 amino acids leading to redundancy. For example, alanine is indicated by the codons GCA, GCC, GCG, and GCT. The important portion is the leading GC.

Essentially, DNA codes for proteins.

And proteins make all of the other components essential for life with the exception of minerals. It is no wonder they are considered to be of prime importance.

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