Introduction to Proteins

What Are Proteins?

Proteins are organic macromolecules that play essential roles in the human body. While proteins provide energy (4 Calories per gram), their primary function is structural and functional rather than energetic. Proteins are responsible for building and maintaining muscle, tissues, and bones, and they serve as the main components of enzymes, hormones, antibodies, and transport and storage systems in the body.

Common food sources of protein include meat, poultry, eggs, dairy products, legumes, nuts, and seeds.

Amino Acids: The Building Blocks of Proteins

The smallest unit of a protein is an amino acid. The human body uses 20 different amino acids to build the thousands of proteins it needs. Of these, nine are classified as essential amino acids, meaning they cannot be synthesized by the body and must be obtained through the diet.

The nine essential amino acids are:

• Phenylalanine
• Valine
• Tryptophan
• Threonine
• Isoleucine
• Methionine
• Histidine
• Leucine
• Lysine

(Some students remember these using the mnemonic PVT-TIM-HLL.)

Some amino acids are considered conditionally essential. For example, tyrosine is normally synthesized from phenylalanine. However, in individuals with phenylketonuria (PKU), phenylalanine cannot be properly metabolized, making tyrosine an essential dietary amino acid for those individuals.

Structure of an Amino Acid

All amino acids share a common basic structure consisting of:

• An amino group (–NH₂)
• A carboxyl group (–COOH)
• A hydrogen atom
• A variable side chain (R group)

The R group is what distinguishes one amino acid from another. Differences in side chains determine whether an amino acid is polar or non-polar, positively or negatively charged, or hydrophobic or hydrophilic. These properties ultimately influence how proteins behave in food systems and in the body.

How Proteins Are Made in the Body

Proteins are synthesized according to genetic instructions encoded in DNA. This process occurs in two main steps: transcription and translation.

During transcription, a specific segment of DNA unwinds and is copied to form messenger RNA (mRNA). The mRNA carries the instructions for building a particular protein. During translation, the mRNA travels to the cytoplasm of the cell and attaches to a ribosome. The ribosome functions as a molecular assembly site, linking amino acids together in the correct order to form a protein.

Levels of Protein Structure

Proteins can be organized into four levels of structure:

1. Primary structure – a linear chain of amino acids linked by peptide bonds.
2. Secondary structure – folding of the chain into alpha helices and beta sheets due to hydrogen bonding.
3. Tertiary structure – further folding into a three-dimensional shape.
4. Quaternary structure – interaction of two or more protein subunits.

Protein Denaturation

Proteins can lose their native structure when the bonds holding them together are disrupted. This process is called denaturation. Factors that can cause denaturation include heat, mechanical shear, acids, alcohols, and salts.

Common food examples of denaturation include the firming of egg whites during cooking and the curdling of milk when acid is added. Understanding protein structure helps food scientists predict and control protein functionality during food processing.

Can We Get Enough Protein Without Meat?

Some foods lack one or more essential amino acids or contain them in limited amounts. These are known as limiting amino acids. Foods that do not provide all essential amino acids in sufficient quantities are called incomplete proteins.

Animal-based foods such as eggs, milk, fish, meat, and poultry are considered complete proteins. Plant-based proteins can also meet nutritional needs when properly combined. For example, beans are rich in lysine but low in methionine, while grains such as rice are low in lysine but high in methionine. When eaten together, they provide all essential amino acids.

Protein Quality

Protein quality refers to how well a protein source meets human amino acid requirements and how easily it can be digested. Two commonly used measures of protein quality are the chemical score and the Protein Digestibility Corrected Amino Acid Score (PDCAAS).

The chemical score compares the amount of a limiting amino acid in a food to that of a reference protein. For example, if a food contains 45 mg of lysine and the reference protein contains 100 mg, the chemical score is:

(45 ÷ 100) × 100 = 45%

The PDCAAS adjusts this value based on digestibility. If digestibility is 87%, then:

PDCAAS = 0.45 × 0.87 = 0.39

Reference: Potter, N. N. & Hotchkiss, J. H. (1998). Food Science, 5th edition. New York, NY: Springer Science+Business Media LLC.