Macronutrients 101: Protein Structure

Now that I’ve discussed what proteins do, let’s ask the million dollar question! Where in the food chain do proteins come from?

If you think they come from animals than you’re absolutely wrong. Those animals actually got their proteins from plants. Yes plants

Proteins unlike other macronutrients contain nitrogen atoms. The issue is that nitrogen is naturally found as a dimer with an extremely stable triple bond. Neither plants nor animals are able to break this bond. However, nitrogen-fixing soil bacteria possess the specific enzymes needed to break it down. These commensal bacteria reside at the roots of legume plants where they exchange the accessible nitrogen for glucose. The plants than utilize the ammonia to form amino acids and in turn proteins.

So what are these amino acids? Well, amino acids are monomeric units that make up proteins. There’re 20 different amino acids found in human proteins and they all have the same generic structure. There is a central (alpha) carbon atom that has an amine group (base), a carboxyl group (acid), and a variable side chain group.

The amine and carboxyl groups of two separate amino acids can undergo an acid-base reaction to form a peptide bond. Through this simple reaction, amino acids can form long chains called polypeptides. Now if you think about carbohydrates, large dietary polysaccharides like starch are only made up of glucose. However, having 20 different amino acids allows polypeptides to be constructed in multitude of diverse patterns.

The polypeptide sequence is dictated by our genes. One gene is a DNA sequence that will code for an amino acid sequence of a polypeptide. Within the polypeptide, amino acids will interact with each other to give it some 3D “secondary” structure. The two most common structures are alpha-helices or beta-sheets. These form through interactions within the invariable regions of the amino acid.

The variable groups of amino acids will then interact with each other to give the protein additional “tertiary” structural features. For example, two cysteine amino acids can form a disulfide bridge between its two sulfur atoms. Hence, this feature will create a fold within the polypeptide. All of these structural features make the proteins unique and determine their physiological function.

Lastly, a protein can be made up of multiple polypeptide chains and can include prosthetic (non-amino acid) groups. A common example of such protein is hemoglobin in red blood cells. This protein is made up of four polypeptide chains and heme “iron” containing prosthetic groups. Below is a ribbon diagram of what hemoglobin looks like based on x-ray crystallography predictions.

Beautiful ain’t it? These gorgeous proteins can be produced by our bodies, all from tiny little amino acids. Proteins are the essence of our bodies and control most of our physiological functions. In fact, I’ve taken an undergraduate course that was purely on proteins. There’s so much to learn about them! One thing is for certain is that we really don’t need much dietary protein, especially animal protein. 😉

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