6.1: Introduction to Proteins

Learning Objectives

• Describe the structure of amino acids.
• Differentiate between an essential amino acid and a non-essential amino acid.
• Describe protein synthesis.

What Is Protein?

Proteins are large, complex molecules that are essential components of all human tissues (e.g., bones, blood, hormones, enzymes, antibodies). Proteins contain the elements carbon, hydrogen, and oxygen just as carbohydrates and lipids do, but proteins are the only macronutrient that contains nitrogen. Proteins are macromolecules composed of amino acids. Amino acids are commonly called the "building blocks" of protein. Amino acids contain five elements (Figure \(\PageIndex{1}\)). Each amino acid consists of a central carbon atom (C) connected to a side chain (R), a hydrogen (H), a nitrogen-containing amino group (NH₂), and a carboxylic acid group (COOH) — hence the name “amino acid”. Amino acids differ from each other by which specific side chain (R) is bonded to the carbon center. The side chain of an amino acid can be as simple as one hydrogen bonded to the carbon center (glycine; Figure \(\PageIndex{2a}\)) or more complex like leucine (Figure \(\PageIndex{2b}\)).

Figure \(\PageIndex{1}\): Amino Acid Structure. (CC BY 3.0; by Tyagi.anuj via Wikimedia Commons)

Figure \(\PageIndex{2a}\): Chemical structure of glycine. (CC BY-SA 3.0; by Borb via Wikimedia Commons)

Figure \(\PageIndex{2b}\): Chemical structure of leucine. (CC0 1.0; by Hbf878 via Wikimedia Commons)

Essential and Nonessential Amino Acids

There are twenty different amino acids and we require all of them to make the many different proteins found throughout the body. Amino acids can be classified as essential or nonessential (Table \(\PageIndex{1}\)). Eleven of the amino acids are considered nonessential because the body can synthesize them through transamination (which is the process of transferring the amine group from one amino acid to another acid group and side chain). However, nine of the amino acids are called essential amino acids because we cannot synthesize them either at all or in sufficient amounts. These essential amino acids must be obtained from the diet. Sometimes during infancy, 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.

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 (Protein Synthesis)

A protein consists of multiple amino acids. Different proteins are produced because there are 20 amino acids that can be combined in unique sequences. Each amino acid is connected to the next amino acid by a special chemical bond called a peptide bond (Figure \(\PageIndex{3}\)). The peptide bond forms between the carboxylic acid group of one amino acid and the amino group of another, releasing a molecule of water. When amino acids join together, they form peptides and are named based on how many amino acids are in the peptide. For example, a dipeptide contains 2 amino acids, a tripeptide contains 3 amino acids, an oligopeptide has 4-9 amino acids, and a polypeptide contains 10 or more amino acids.

Figure \(\PageIndex{3}\): Connecting amino acids with peptide bonds builds proteins. (CC BY 3.0; by OpenStax College - Anatomy & Physiology)

Each protein in the human body differs in its amino acid sequence. Genes regulate amino acid bonding (a gene is a section of DNA that acts as a template for protein synthesis). Protein synthesis also involves transcription and translation.

• Transcription: messenger RNA (mRNA) copies (transcribes) information from DNA in the nucleus and carries that information to ribosomes in the cytoplasm
• Translation: information from the mRNA is converted (translated) into a growing strand of amino acids that bind together to form a specific protein

Existing proteins can be broken down to provide amino acids for new protein synthesis. The newly synthesized protein is structured to perform a particular function in a cell. The 3-D protein shape (tertiary structure) is critical to its function. If the protein loses its shape then it loses its function. Denaturation is the term used to describe the protein's loss of shape. Exposure to heat, acids, bases, heavy metals, alcohol can denature proteins.

Protein synthesis can be limited by missing amino acids. The amino acid that is missing or in the smallest supply is called the limiting amino acid. Without the proper amount of essential amino acids, proteins cannot be made which can impact the body's functions.

Key Takeaways

• Amino acids are the building blocks of proteins. The structure of amino acids include a central carbon atom, a hydrogen atom, an amino group, a carboxylic acid group, and a side chain.
• Some amino acids are nonessential in the diet because the body can synthesize them, and some are essential in the diet because the body cannot make them.
• Proteins are chains of amino acids held together by peptide bonds.

References

Chapter cover photo "Protein-rich Foods" by Smastronardo is licensed under CC BY-SA 4.0