Proteins are large molecules made up of many smaller building blocks that are called amino acids. There are 20 basic amino acids whose names are shown in the box to the left.
The amino acids shown in pink are called "essential". This is because the human body cannot make them and therefore depends entirely upon food for their acquisition. If we become deficient in even one of these essential amino acids, we cannot build proteins and eventually we die. Essential amino acids REALLY are essential.
The term "protein quality" refers to a protein's essential amino acid composition. The more of all of the essential amino acids a protein provides, the higher is considered its quality. Milk and egg proteins are examples of high quality proteins. Collagen (gelatin) is a "low-quality" protein because it totally lacks several important amino acids.
Think of protein as a long chain made of twenty different kinds of links. Now imagine that the chain floats in, say, water. Then imagine that each link (each amino acid) has many little magnets built into it in a very particular way that is unique to each amino acid. Now imagine that long chain of magnets floating... What happens? The magnets (at least in theory) begin to attract and repel each other so that the chain begins to fold up into some sort of ball (or other shape). That is what proteins do except that the "magnets" are actually chemical charges. But hopefully you get the picture. The point is that the way proteins fold up is determined by the sequence of their amino acids but is also very complex. To the right is a molecular model of a protein called "C-Reactive Protein". It's too small to actually see but the this model is based on some very clever science. That's the way it "would" look if we could see it. Again, the point is that it is VERY complex.
Proteins are more than just pretty. Special proteins called "enzymes" are the workhorses of the cell. They are basically little machines that run nearly all the chemical reactions in the cell and the body. They do this by acting as "catalysts" which make otherwise hard-to-happen chemical reactions easy. The magic of all this lies in the folding. The "magnets" I talked about above don't just determine folding in enzymes, rather, folding, in turn also determines the arrangement of the "magnets" and that arrangement is what causes enzymes to function as catalysts.
Other proteins have structural roles in the cell and body. They don't catalyze chemical reactions like enzymes but instead impart physical properties. Muscle is a perfect example of a structural (but also a functional) protein.
Still other proteins (like the C-Reactive Protein shown above) function as pores (holes) that span across the outer layer (membrane) of cells. But these aren't just ordinary holes. They are machines that open and close to allow or disallow the flow of chemicals like sodium and potassium in and out of the cell. Receptor proteins are similar but often also catalyze chemical reactions on the inside of the cell.
Smaller proteins (called "peptides") often control brain and hormone functions in the body. Many of these play important roles in weight regulation.
Moving from the science and beauty of proteins to the practical nutritional issues, things become a bit easier and simpler. Let's look at where we get proteins, which ones are "high quality" and how much we need.
To be continued. 6-22-2013