Proteins: Structure, Function, and Nutrition

What Are Proteins?

Proteins are large, complex molecules essential for the structure and function of all living cells. They play critical roles in metabolism, immunity, fluid balance, and nutrient transport, and can serve as an energy source under certain conditions. Proteins are unique among macronutrients because they contain nitrogen in a form readily usable by the human body.

Amino Acids: Building Blocks of Proteins

Amino acids are nitrogen-containing molecules that combine to form proteins. There are 20 amino acids in the human body, classified as essential or nonessential:

• Essential amino acids: Cannot be synthesized by the body and must be obtained from food (9 out of 20).

• Nonessential amino acids: Can be synthesized by the body.

Each amino acid consists of a central carbon atom, an amine group, an acid group, a hydrogen atom, and a unique side chain (R group) that determines its properties.

Transamination: Amino Acid Modification

Transamination is the process by which the amine group from one amino acid is transferred to a different acid group and side chain, allowing the body to synthesize nonessential amino acids.

Protein Synthesis: Formation of Peptide Bonds

Proteins are formed by linking amino acids together via peptide bonds:

• Dipeptide: Two amino acids joined together.

• Polypeptide: Ten or more amino acids bonded together.

The formation of a peptide bond releases a molecule of water (condensation reaction).

Protein Synthesis: Transcription and Translation

Protein synthesis involves two main steps:

• Transcription: Genetic information in DNA is used to make messenger RNA (mRNA), which carries the code to the ribosome.

• Translation: The ribosome reads the mRNA and assembles amino acids in the correct sequence to form a protein.

Protein Turnover: Synthesis and Breakdown

Protein turnover refers to the continuous process of protein synthesis and degradation in the body. Amino acids from food and cell breakdown enter the amino acid pool, which is used for various functions:

• Synthesis of nonprotein compounds (e.g., creatine, serotonin)

• Synthesis of body proteins (e.g., enzymes, antibodies)

• Synthesis of fat or glucose from amino acid carbon skeletons

• Energy production

• Excess nitrogen is converted to urea and excreted

Levels of Protein Structure

The function of a protein is determined by its structure, which has four levels:

• Primary structure: Sequence of amino acids

• Secondary structure: Spiral shape due to chemical bonding

• Tertiary structure: Further folding into a unique three-dimensional shape

• Quaternary structure: Association of multiple polypeptide chains

Protein Shape Determines Function

Proteins lose their shape (denaturation) when exposed to heat, acids, bases, heavy metals, or alcohol, resulting in loss of function. For example, sickle cell anemia is caused by a change in protein structure.

Protein Quality: Complete and Incomplete Proteins

Protein synthesis can be limited by the availability of essential amino acids:

• Incomplete protein: Lacks one or more essential amino acids (low-quality protein)

• Complete protein: Contains all essential amino acids in sufficient amounts (high-quality protein; e.g., animal proteins, quinoa, soy)

Mutual supplementation involves combining two incomplete proteins to make a complete protein. Complementary proteins are two sources that together supply all essential amino acids (e.g., beans and rice).

Functions of Proteins in the Body

Proteins serve many functions, including:

• Cell growth, repair, and maintenance

• Enzymes and hormones

• Fluid and electrolyte balance

• pH balance

• Antibodies for immunity

• Energy source

• Transport and storage of nutrients

• Formation of neurotransmitters, fibrin, collagen

Role of Proteins in Fluid and Electrolyte Balance

Proteins help maintain fluid balance by attracting water and keeping it within blood vessels. Insufficient protein can lead to edema, a condition characterized by swelling due to fluid accumulation.

Proteins also regulate electrolyte balance by transporting sodium and potassium across cell membranes.

Protein Digestion and Absorption

Protein digestion begins in the stomach, where acids and enzymes break proteins into polypeptides. Digestion continues in the small intestine, where proteases from the pancreas break polypeptides into single amino acids for absorption.

Protein digestibility affects protein quality. Animal proteins, soy, and legumes are highly digestible, while plant proteins (grains, vegetables) are less digestible due to fiber content.

Recommended Protein Intake

Protein needs vary by age, activity, and physiological status. Nitrogen balance describes the relationship between protein intake and excretion:

• Positive nitrogen balance: Needed for growth, pregnancy, recovery

• Negative nitrogen balance: Results from illness, starvation, or inadequate protein intake

• Recommended Dietary Allowance (RDA): 0.8 grams per kilogram of body weight per day

• Acceptable Macronutrient Distribution Range (AMDR): 10–35% of total energy intake

Protein Sources and Content

Protein is found in a variety of foods, including meat, poultry, seafood, dairy, soy products, beans, and nuts. Protein supplementation may benefit certain athletes, depending on training intensity and type.

Vegetarian Diets and Protein

Vegetarianism involves restricting the diet to foods of plant origin. There are several types of vegetarian diets, each with varying restrictions:

Health Benefits and Challenges of Vegetarianism

Benefits include lower fat and energy intake, reduced risk of heart disease, lower blood pressure, fewer digestive problems, and reduced risk of certain cancers, kidney stones, and gallstones. Challenges include potential deficiencies in iron, calcium, zinc, vitamins D and B12, and the need for careful dietary planning.

Consequences of High Protein Intake and Protein Deficiency

Excessive protein intake can increase risk of kidney disease (especially in susceptible individuals), heart disease (due to saturated fat in animal proteins), and bone loss (though adequate protein is beneficial for bone health). Protein-energy malnutrition includes two serious disorders:

• Marasmus: Severe deficiency of protein and energy, causing extreme tissue wasting and stunted growth.

• Kwashiorkor: Extremely low protein intake, leading to edema, muscle wasting, and retarded growth.

Additional info: Protein requirements may be higher for children, adolescents, pregnant/lactating women, athletes, older adults, and vegetarians. Complementary protein combinations are essential for those consuming plant-based diets to ensure all essential amino acids are obtained.