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 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 three main parts:

• Amine group (NH2)

• Acid group (COOH)

• Side chain (R group, varies for each amino acid)

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 joined together

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

Genetic Control of Protein Synthesis

Protein synthesis is regulated by genetic information:

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

• Translation: mRNA is used to assemble amino acids in the correct sequence to synthesize 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

Protein function 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 and Function

Protein shape is crucial for its function. Denaturation occurs when proteins lose their shape due to heat, acids, bases, heavy metals, or alcohol, resulting in loss of function.

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 and Complementary Proteins

Combining two incomplete proteins to make a complete protein is called mutual supplementation. 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:

• 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 Balance

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

Role of Proteins in Electrolyte Balance

Proteins regulate electrolyte balance by transporting ions (e.g., sodium, 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 break polypeptides into single amino acids for absorption.

Protein Digestibility

Protein quality is affected by digestibility. 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 state:

• 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

Nitrogen Balance

Nitrogen balance describes the relationship between nitrogen intake and excretion:

• Positive nitrogen balance: Growth, pregnancy, recovery

• Negative nitrogen balance: Starvation, illness, injury

• Equilibrium: Healthy adults

Protein Sources

Protein can be obtained from a variety of sources:

• Meat, poultry, seafood, pork

• Dairy products

• Legumes, nuts, grains

• Soy products

Protein Content of Common Foods

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:

• Lower intake of fat and total energy

• Lower blood pressure

• Reduced risk of heart disease and some cancers

• Fewer digestive problems

• Reduced risk for kidney stones and gallstones

Challenges:

• Potential deficiencies in iron, calcium, zinc, vitamins D and B12

• Need for careful planning to ensure adequate protein and micronutrient intake

• Use of complementary proteins is important

Nutrients of Concern in Vegan Diets

Consequences of High Protein Intake and Protein Deficiency

Excessive protein intake can be harmful:

• Kidney disease: High-protein diets may increase risk in susceptible individuals

• Heart disease: Animal protein sources often high in saturated fat

• Bone loss: Too little protein causes bone loss; higher intake may benefit bone health in healthy populations

Protein-Energy Malnutrition

Protein-energy malnutrition results from inadequate intake of protein and energy. Two common forms:

• Marasmus: Severe deficiency of protein, energy, and nutrients; characterized by extreme tissue wasting and stunted growth

• Kwashiorkor: Extremely low protein intake; symptoms include weight loss, muscle wasting, edema (swollen belly), retarded growth