What is protein?

Protein is composed of carbon, hydrogen, and oxygen - similar to carbs and fat - but it stands out with nitrogen molecules. Some proteins contain sulfur. These molecules form amino acids, the building blocks of proteins. Amino acids’ basic structure includes a carbon with three functional groups: an acidic carboxyl (COOH), a side chain group, and an amino group (-NH2).¹ A dehydration synthesis reaction joins amino acids through peptide bonds by covalently linking the carboxyl and amino groups.¹

Two amino acids joined by a peptide bond is a dipeptide, and three amino acids joined is a tripeptide.¹ A peptide consists of 4-9 amino acids, and a polypeptide consists of anywhere between 10 to 2000 amino acids.¹ A polypeptide that has greater than 100 amino acids are known as proteins.² There a nine amino acids that are considered essential - lysine, tryptophan, methionine, valine, phenylalanine, isoleucine, leucine, threonine, and histidine (< 1 year old).² A limiting amino acid refers to a missing or inadequate amount of an essential amino acid within food.¹ All amino acids must be present for protein synthesis. 

Proteins can be classified in a variety of ways structurally, specifically primary, secondary, tertiary, and quaternary structures.¹ The following are descriptions of each structure:

Primary: polypeptide chain¹

Secondary: number and sequence of amino acids forming as B pleated sheet, a-helix, or a random coil¹

Tertiary: bonding and coiling of secondary structures to shape proteins, determining the structure of proteins such as globular or fibrous¹

Quaternary: two or more polypeptide chains folded, forming functional proteins such as insulin and hemoglobin¹

What are the types and functions of protein?

Several types of protein function in unique, specialized ways throughout the body. Here are some general functions of proteins within the body:

Structural: protein can be used to form the structure of certain components within the body.¹

Example: muscle/organ tissue, collagen (within hair and skin), and keratin (skin, hair, nails)

Regulatory: proteins that function to regulate body processes, including regulation of growth, blood sugar, and pain.¹

Example: insulin (blood glucose regulator hormone)

Contractile: proteins allow contractions of skeletal muscle by shortening of muscle fibers.¹

Example: myosin and actin

Immunological: proteins function as antibodies to help fight pathogens.¹

Example: antibody T cell

Transportation: carrier proteins transporting several substances.¹

Example: hemoglobin, insulin 

Catalytic: proteins function as enzymes to conduct reactions. Enzymes typically consist of an apoenzyme portion (protein portion), a cofactor (ions from minerals), or a coenzyme (derived from vitamins). Enzymes end in the suffix - ase and are grouped by reaction type.¹ For an enzymic reaction to occur, a substrate (ion, molecule, etc.) binds to the activation site.¹ Within the activation site, the substrate is transformed through synthesis, separation, or rearrangement.¹ The product is formed and the enzyme is able to continue.

Example: sucrase 

As previously described, proteins can be categorized based on shape: globular or fibrous. 

Globular: proteins that are mostly insoluble in water with spherical polypeptide chains.¹

Examples: enzymes, antibodies, complement proteins, hemoglobin, lipoprotein, albumin, membrane proteins, some hormones

Fibrous: proteins that are insoluble in water with parallel polypeptide chains¹

Examples: collagen, elastin, keratin, fibrin, actin, myosin 

What is the recommended amount of protein?

The AMDR is reflective of dietary patterns of nutrients while the RDA focus is on meeting the requirement for daily functioning. The AMDR for protein is as follows³:

Children:

1-3 years old: 5-20%

Children/Teens:

4-18 years old: 10-30%

Adults:

18+ years old: 10-35%

Proteins, like carbs, provide 4 kcals per gram. The RDA for protein/day is as follows³: