7.2: Types of Protein

Protein makes up approximately 20% of the human body and is present in every single cell. The word "protein" is a Greek word meaning “of utmost importance.” Proteins are called the workhorses of life, as they provide the body with structure and perform a vast array of functions. You can stand, walk, run, skate, swim, and more because of your protein-rich muscles. Protein is necessary for proper immune system function, digestion, and hair and nail growth and is involved in numerous other body functions. More than 100,000 different proteins are estimated to exist within the human body. In this chapter, you will learn about the components of protein, the important roles that protein serves within the body, how the body uses protein, the risks and consequences associated with too much or too little protein, and where to find healthy sources of it in your diet.

Figure \(\PageIndex{1}\): Proteins in action. Proteins allow us to move, think, process energy, and perform a vast array of functions. Source: "Baseball at Dodgers Stadium" by Rafael Amado Deras is licensed under CC BY 2.0.

What Is Protein?

There are over one hundred thousand different proteins in the human body. Different proteins are produced because twenty types of naturally occurring amino acids are combined in unique sequences. Additionally, proteins come in many different sizes. The hormone insulin, which regulates blood glucose, is composed of only fifty-one amino acids, whereas collagen, a protein that acts like glue between cells, consists of more than one thousand amino acids. Titin is the largest known protein. It accounts for the elasticity of muscles and consists of more than twenty-five thousand amino acids! The abundant variations of proteins are due to the unending number of amino acid sequences that can be formed.

To compare how so many different proteins can be designed from only twenty amino acids, think about music. All of the music that exists in the world has been derived from a basic set of seven notes: C, D, E, F, G, A, B and variations thereof. As a result, there is a vast array of music and songs, all composed of specific sequences from these basic musical notes. Similarly, the twenty amino acids can be linked together in an extraordinary number of sequences, much more than is possible for the seven musical notes to create songs. As a result, enormous numbers of variations and potential amino acid sequences can be created. For example, if an amino acid sequence for a protein is 104 amino acids long, the possible combinations of amino acid sequences is equal to 20¹⁰⁴, which is 2 followed by 135 zeros!

Amino Acids: The Building Blocks of Proteins

Proteins, simply put, are macromolecules composed of amino acids. Amino acids are commonly called protein’s building blocks (Figure \(\PageIndex{2}\)). Proteins are crucial for the nourishment, renewal, and continuance of life. Proteins contain the elements carbon, hydrogen, and oxygen, just as carbohydrates and lipids do, but proteins are the only macronutrient that contains nitrogen. In each amino acid, the elements are arranged into a specific conformation around a carbon center. Each amino acid consists of a central carbon atom connected to a side chain, a hydrogen, a nitrogen-containing amino group, a carboxylic acid group—hence the name “amino acid.” Amino acids differ from one another based on which specific side chain is bonded to the carbon center.

Figure \(\PageIndex{2}\): Amino acid structure. Amino acids contain four elements. The arrangement of elements around the carbon center is the same for all amino acids. Only the side chain (R) differs. Source: “Amino acid structure” from An Introduction to Nutrition (v. 1.0) is licensed under CC BY-NC-SA 3.0.

 

It’s All in the Side Chain (or R group)

The side chain of an amino acid, sometimes called the “R” group, can be as simple as one hydrogen bonded to the carbon center or as complex as a six-carbon ring bonded to the carbon center. Although each side chain of the twenty amino acids is unique, there are some chemical likenesses among them. Therefore, they can be classified into different different groups based on their chemical properties (Figure \(\PageIndex{3}\)). Two important chemical properties are affected by the type of R group on an amino acid. First, R groups can determine if an amino acid can interact with water. Nonpolar amino acids do not interact well with water because the R groups are bulky. These amino acids are also called hydrophobic, which means "water-fearing." Nonpolar amino acids include alanine (Ala), leucine (Leu), isoleucine (Ile), proline (Pro), tryptophan (Trp), valine (Val), phenylalanine (Phe), and methionine (Met). Polar amino acids interact well with water. They are also called hydrophilic, which means "water-loving". Polar amino acids include serine (Ser), threonine (Thr), cysteine (Cys), tyrosine (Tyr), asparagine (Asn), and glutamine (Gln). The acidity or charge of the amino acid is also affected by the R group. Some amino acids are acidic or negatively charged. These amino acids are hydrophilic and include aspartic acid (Asp) and glutamic acid (Glu). Basic amino acids are positively charged. These amino acids are also hydrophilic and include lysine (Lys), arginine (Arg), and histidine (His).

Figure \(\PageIndex{3}\): Amino acids are classified into four groups. These are nonpolar, polar, acidic, and basic. “Amino acids diagram” from Biology, 2e by OpenStax is licensed under CC BY 4.0.

Essential and Nonessential Amino Acids

Amino acids are further classified based on nutritional aspects. Recall that there are twenty different amino acids, and we require all of them to make the many different proteins found throughout the body (Table \(\PageIndex{1}\)). Eleven are called nonessential amino acids because the body can synthesize them. However, nine amino acids are called essential amino acids because we cannot synthesize them at all or in sufficient amounts. These must be obtained from the diet. Sometimes, during infancy and growth, and in diseased states, the body cannot synthesize enough of some of the nonessential amino acids, and more of them are required in the diet. These types of amino acids are called conditionally essential amino acids. The nutritional value of a protein is dependent on what amino acids it contains and in what quantities.

Table \(\PageIndex{1}\): Essential and Nonessential Amino Acids

Essential	Nonessential
Histidine	Alanine
Isoleucine	Arginine*
Leucine	Asparagine
Lysine	Aspartic acid
Methionine	Cysteine*
Phenylalanine	Glutamic acid
Threonine	Glutamine*
Tryptophan	Glycine*
Valine	Proline*
	Serine
	Tyrosine*
*Conditionally essential

Building Proteins with Amino Acids

The building of a protein consists of a complex series of chemical reactions that can be summarized into three basic steps: transcription, translation, and protein folding. The first step in constructing a protein is the transcription (copying) of the genetic information in double-stranded deoxyribonucleic acid (DNA) into the messenger macromolecule ribonucleic acid (RNA). RNA is chemically similar to DNA, but it can travel throughout the cell to carry instructions for making a specific protein (Figure \(\PageIndex{4}\)).

The RNA that is created, or transcribed, using information from a given piece of DNA instructs the cells to gather all the necessary amino acids and add them to the growing protein chain in a very specific order. This is the second step in building a protein, and this process is referred to as translation. During translation, each amino acid is connected to the next amino acid by a special chemical bond called a peptide bond. The peptide bond forms between the carboxylic acid group of one amino acid and the amino group of another, releasing a molecule of water.

Figure \(\PageIndex{4}\): Building a protein involves three steps: transcription, translation, and folding. "Transcription and Translation" by Christinelmiller is licensed under CC BY-SA 4.0.

The third step in protein production involves folding it into its correct shape (Figure \(\PageIndex{5}\)). Specific amino acid sequences contain all the information necessary to fold into a particular shape spontaneously. A change in the amino acid sequence will cause a change in protein shape. Each protein in the human body differs in its amino acid sequence and, consequently, its shape. The newly synthesized protein is structured to perform a particular function in a cell. A protein made with an incorrectly placed amino acid may not function properly, which can sometimes cause disease.

Figure \(\PageIndex{5}\): Protein folding. Proteins are a series of amino acids connected by peptide bonds. After transcription, the string of amino acids is folded to give a protein a specific shape. "Protein Folding Schematic" by Tomixdf is in the Public Domain.

Video \(\PageIndex{1}\): From DNA to Protein

See the process of building a protein in real time in this animation.¹ (https://youtu.be/gG7uCskUOrA?si=arCZt8gWSz4wYE01).

Attributions

• Zimmerman, "An Introduction to Nutrition (Zimmerman)", CC BY-NC-SA 3.0. Figures were updated and the flow of information was slightly changed.

References

1. yourgenome. From DNA to Protein - 3D. [Video]. YouTube. https://youtu.be/gG7uCskUOrA?si=dbtXnga5sthnVpaR. Published January 7, 2015. Accessed August 3, 2023.