Created by: CK-12/Adapted by Christine Miller

Protein Shake

Drinks like this shake contain a lot of protein. Muscle tissue consists mainly of protein, so such drinks are popular with people who want to build muscle. Making up muscles is just one of a plethora of functions of this amazingly diverse class of biochemicals.

Proteins are a major class of biochemical compounds made up of small monomer molecules called amino acids. More than 20 different amino acids are typically found in the proteins of living things. Small proteins may contain just a few hundred amino acids, while large proteins may contain thousands.

When amino acids bind together, they may form short chains of two or just a few amino acids. These short chains are called peptides. When amino acids form long chains, the chains are called polypeptides. A protein consists of one or more polypeptides.

Proteins may have up to four levels of structure, from primary to quaternary.  As a result, they can have tremendous diversity. Here are some additional details about the levels of protein structure:

Figure 3.6.2 Four protein structures.

The diversity of protein structures explains why this class of biochemical compounds can play so many important roles in living things. What are the roles of proteins?

• Some proteins have structural functions. They may help cells keep their shape or make up muscle tissues.
• Many proteins are enzymes that speed up chemical reactions in cells. Enzymes are usually highly specific and accelerate only one or a few chemical reactions. Thousands of different biochemical reactions are known to be catalyzed by enzymes, including most of the reactions involved in metabolism. A reaction without an enzyme might take millions of years to complete, whereas with the proper enzyme, it may take just a few milliseconds!
• Other proteins are antibodies, which bind to specific foreign substances, like proteins on the surface of bacterial cells. This process targets the cells for destruction.
• Still other proteins carry messages or materials. Myoglobin, for example, is an oxygen-binding protein found in the muscle tissues of most mammals (including humans).

The chief characteristic of proteins that allows their diverse set of functions is their ability to bind to other molecules so specifically and tightly. Myoglobin can bind specifically and tightly with oxygen. The region of a protein responsible for binding with another molecule is known as the binding site. This site is often a depression on the molecular surface, determined largely by the tertiary structure of the protein. Changes to this tertiary structure, called denaturation, will therefore affect the function of the protein. Denaturation is often caused by extremes of pH or temperature that would be outside the range of normal conditions in which the protein would function. This is why heating an egg white will reverse its structure permanently!

Proteins are necessary in the diets of humans and other animals. We cannot make all the different amino acids we need, so we must obtain some of them from the foods we consume. In the process of digestion, we break down the proteins in food into free amino acids that can then be used to synthesize our own proteins. Protein synthesis from amino acid monomers takes place in all cells and is controlled by genes. Once new proteins are synthesized, they generally do not last very long before they are degraded and their amino acids are recycled. A protein’s lifespan in mammalian cells is generally just a day or two.

Structures of Protein by OpenStax, on Wikimedia Commons, is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

References

Amoeba Sisters. (2018, September 24). Protein structure and folding. YouTube. https://www.youtube.com/watch?v=hok2hyED9go

OpenStax. (2012, Aug 22). Figure 9. The four levels of protein structure can be observed in these illustrations. (credit: modification of work by National Human Genome Research Institute). In Biology. OpenStax CNX. © Rice University. https://cnx.org/contents/GFy_h8cu@10.53:2zzm1QG9@7/Proteins (last revised May 27, 2016).