Skip to main content
Medicine LibreTexts

2.2: Proteins

Proteins are another major macronutrient that, like carbohydrates, are made up of small repeating units. But instead of sugars, proteins are made up of amino acids. In the following sections, you will learn more about how proteins are synthesized and why they are important in the body.

  • 2.2A: Amino Acids
    Similar to carbohydrates, proteins contain carbon (C), hydrogen (H), and oxygen (O). However, unlike carbohydrates (and lipids) proteins also contain nitrogen (N). Proteins are made up of smaller units called amino acids. This name, amino acid, signifies that each contains an amino and carboxylic acid groups. The only structural difference in the 20 amino acids is the side group represented by the R below.
  • 2.2B: Protein Synthesis
    The process of protein synthesis is not as simple as stringing together amino acids to form a polypeptide. As shown below, this is a fairly involved process. DNA contains the genetic code that is used as a template to create mRNA in a process known as transcription. The mRNA then moves out of the nucleus into the cytoplasm where it serves as the template for translation, where tRNAs bring in individual amino acids that are bonded together to form a polypeptide.
  • 2.2C: Protein Structure
    Primary structure is the linear polypeptide chain. Secondary structure occurs when hydrogen bonding between amino acids in the same polypeptide chain causes the formation of structures such as beta-pleated sheets and alpha-helices. Tertiary structure occurs as a result of an attraction between different amino acids of the polypeptide chain and interactions between the different secondary structures. Some proteins contain quaternary structure where multiple polypeptide chains are bonded together.
  • 2.2D: Protein Functions
    There are various functions of proteins in the body that are described below.
  • 2.2E: Types of Amino Acids
    There are 20 amino acids our body uses to synthesize proteins. These amino acids can be classified as essential, non-essential, or conditionally essential.
  • 2.2F: Amino Acid Structures
    It is a good idea to have a general idea of the structure of the different amino acids and to be able to recognize them as amino acids. You are not expected to memorize these structures.
  • 2.2G: Protein Quality
    Proteins can be classified as either complete or incomplete. Complete proteins provide adequate amounts of all nine essential amino acids. Animal proteins such as meat, fish, milk, and eggs are good examples of complete proteins. Incomplete proteins do not contain adequate amounts of one or more of the essential amino acids. For example, if a protein doesn't provide enough of the essential amino acid leucine it would be considered incomplete.
  • 2.2H: Protein-Energy Malnutrition
    Protein deficiency rarely occurs alone. Instead it is often coupled with insufficient energy intake. As a result, the condition is called protein-energy malnutrition (PEM). This condition is not common in the U.S., but is more prevalent in less developed countries. Kwashiorkor and marasmus are the two forms of protein energy malnutrition.