Proteins: Structure, Function, Digestion, and Dietary Importance

Proteins

Defining Protein

Proteins are essential macromolecules composed of amino acid subunits. They account for approximately 20% of the human body and are crucial for numerous structural and functional roles. Over 100,000 different proteins exist in the human body, each with unique functions determined by their amino acid sequence and structure.

• Proteins: Polymers made from amino acids, containing carbon, hydrogen, oxygen, and nitrogen.

• Amino acids: The building blocks of proteins, each with a central carbon, an amino group, a carboxyl group, and a variable side chain (R group).

Amino acids are classified into four groups based on their side chains:

• Nonpolar amino acids: Hydrophobic, with long or bulky side groups.

• Polar amino acids: Hydrophilic, not charged.

• Acidic amino acids: Hydrophilic, negatively charged.

• Basic amino acids: Hydrophilic, positively charged.

Amino acids are also categorized by nutritional aspects:

• Nonessential amino acids: Synthesized by the body in sufficient amounts.

• Essential amino acids: Must be obtained from the diet.

• Conditionally essential amino acids: Required from the diet only during certain life stages or conditions (e.g., growth, illness).

Building Proteins with Amino Acids

Proteins are synthesized through a series of steps:

1. Transcription: DNA is copied into messenger RNA (mRNA).

2. Translation: mRNA is decoded to synthesize a protein chain.

3. Protein folding: The amino acid chain folds into its functional three-dimensional shape.

Amino acids are linked by peptide bonds to form proteins. The formation of a peptide bond releases water (a condensation reaction).

Protein Organization

Proteins have four levels of structural organization, each contributing to their complexity and function:

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding into alpha helices or beta sheets.

• Tertiary structure: Overall three-dimensional shape of a single polypeptide chain.

• Quaternary structure: Assembly of multiple polypeptide chains into a functional protein.

The Role of Proteins in Foods: Cooking and Denaturation

Protein Denaturation

Denaturation is the process by which a protein loses its native structure due to external stress, such as heat, acid, high salt concentrations, alcohol, or mechanical agitation. This process unravels the protein, affecting its function and often its texture in foods.

• Denaturation: Physical change in protein structure, resulting in loss of function.

• Examples: Cooking an egg (the egg white solidifies due to protein denaturation).

Protein Digestion and Absorption

From the Mouth to the Stomach

Protein digestion begins with mechanical breakdown in the mouth and continues in the stomach, where gastric juices containing hydrochloric acid and the enzyme pepsin initiate chemical digestion. Pepsin breaks proteins into smaller fragments.

From the Stomach to the Small Intestine

In the small intestine, pancreatic enzymes (chymotrypsin and trypsin) and enzymes from intestinal cells further break down protein fragments into individual amino acids, which are then absorbed into the bloodstream.

• Protein turnover: The continual breakdown and synthesis of proteins in the body. Over 250 grams of protein are recycled daily.

• Amino acids are also used to build other nitrogen-containing molecules (e.g., DNA, RNA) and for energy production.

Protein’s Functions in the Body

Structure and Motion

Proteins provide structural support and enable movement. For example, collagen is a strong, fibrous protein that makes up a significant portion of bone, tendons, ligaments, cartilage, skin, and muscle.

• Collagen: Provides strength and flexibility to bones and connective tissues.

Enzymes

Enzymes are proteins that catalyze biochemical reactions, lowering the activation energy required and increasing reaction speed. Each enzyme is specific to its substrate.

Hormones

Many hormones are proteins that act as chemical messengers, regulating physiological processes by turning enzymes on or off.

Fluid and Acid-Base Balance

Proteins help maintain fluid distribution and pH balance in the body. Albumin, the most abundant protein in blood, plays a key role in these processes.

Protection

The immune system relies on proteins such as enzymes and antibodies to defend against pathogens.

• Lysozyme: An enzyme that destroys bacteria.

• Antibodies: Proteins that recognize and neutralize foreign invaders.

Wound Healing and Tissue Regeneration

Proteins are essential for wound healing and the regeneration of tissues, involving phases of inflammation, proliferation, and remodeling.

Energy Production

Some amino acids can be used for energy, especially when dietary intake is insufficient. The liver breaks down amino acids, feeding their carbon skeletons into the citric acid cycle.

Diseases Involving Proteins

Health Consequences of Protein Deficiency

Severe protein deficiency can lead to two main syndromes:

• Kwashiorkor: Characterized by swelling, poor skin health, growth retardation, low muscle mass, and liver malfunction.

• Marasmus: Characterized by severe emaciation, poor skin health, and growth retardation.

Protein deficiency is a leading cause of death in children under five in developing countries.

Health Consequences of Excess Protein

Consuming more than 30% of calories from protein may increase the risk of cardiovascular disease, diabetes, and compromise intake of other essential nutrients. Excess protein can also stress the kidneys, bones, and liver.

Proteins, Diet, and Personal Choices

Recommended Dietary Allowance (RDA) for Protein

The RDA for protein for adults is 0.8 grams per kilogram of body weight. This amount maintains nitrogen balance, where protein intake equals protein use and excretion.

Equation for RDA:

$(\text{Weight in lbs.} \div 2.2) \times 0.8 = \text{grams of protein per day}$

Dietary Sources of Protein

Protein-rich foods include meat, seafood, poultry, eggs, soy, beans, peas, and seeds. These sources differ in their amino acid profiles and digestibility.

• Complete protein sources: Contain all nine essential amino acids (e.g., animal proteins, soy).

• Incomplete protein sources: Lack one or more essential amino acids (e.g., most plant proteins).

• Complementary foods: Combining different plant foods to provide all essential amino acids (e.g., rice and beans).

The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is used to assess protein quality, considering both amino acid content and digestibility. Milk protein, egg whites, whey, and soy have the highest PDCAAS (score of 1).

Vegetarians, Vegans, and Protein

Vegetarians and vegans can meet protein needs with high-quality plant-based sources such as tofu, soy, quinoa, and pistachios. Lacto-ovo vegetarians can also include eggs and dairy. Fortified foods may be necessary for nutrients like vitamin B12, D, calcium, omega-3 fatty acids, and choline.

Vegetarian diets are associated with lower risks of diabetes, heart disease, and cancer. Older adults may require more protein to prevent muscle loss (sarcopenia). Protein or amino acid supplements are generally unnecessary for most people, including athletes.

Summary Table: Types of Amino Acids

Type	Definition	Examples
Nonpolar	Hydrophobic, long/bulky side chains	Glycine, Leucine, Tryptophan
Polar	Hydrophilic, not charged	Serine, Threonine, Cysteine
Acidic	Hydrophilic, negatively charged	Aspartic acid, Glutamic acid
Basic	Hydrophilic, positively charged	Lysine, Arginine, Histidine

Summary Table: Protein Quality

Protein Source	Complete/Incomplete	PDCAAS Score
Egg white	Complete	1
Milk protein	Complete	1
Soy	Complete	1
Beans	Incomplete	0.6–0.7
Wheat	Incomplete	0.4–0.5

Key Takeaways:

• Proteins are vital for structure, function, and regulation in the body.

• Amino acids are classified by chemical and nutritional properties.

• Protein digestion involves both mechanical and chemical processes, with absorption in the small intestine.

• Protein deficiency and excess both have significant health consequences.

• Dietary protein should be balanced, with attention to quality and variety, especially for vegetarians and older adults.