Chemistry and Function of Proteins
1. Introduction and Historical Background
Proteins are indispensable macromolecules that serve as the foundational components of all living organisms. They participate in nearly every cellular process, from providing structural integrity to facilitating metabolic reactions. The term "protein" was introduced in 1838 by the Dutch chemist Gerardus Johannes Mulder, derived from the Greek word proteios, meaning "primary" or "of first rank," emphasizing the essential role of proteins in life.
Historically, the study of proteins began with analyses of food composition and extended to understanding physiological impacts. With the discovery of amino acids and the elucidation of peptide bonds in the early 20th century, scientists began to unlock the structural and functional diversity of proteins. Milestones such as the sequencing of insulin (the first protein to be sequenced) and advances in technologies like X-ray crystallography and NMR spectroscopy laid the groundwork for structural biology and modern proteomics.
2. Chemical Structure of Proteins
Proteins are polymers composed of amino acid monomers linked by peptide bonds. Each amino acid features a central (alpha) carbon bonded to a hydrogen atom, an amino group (-NH₂), a carboxyl group (-COOH), and a unique R group (side chain) that determines its properties.
- Primary Structure: Linear sequence of amino acids; dictates the protein's identity and function.
- Secondary Structure: Regular folding patterns such as α-helices and β-sheets, stabilized by hydrogen bonds.
- Tertiary Structure: The three-dimensional conformation formed by interactions among side chains, including disulfide bridges, ionic bonds, and hydrophobic interactions.
- Quaternary Structure: The spatial arrangement of multiple polypeptide subunits in multi-subunit proteins (e.g., hemoglobin).
3. Classification of Proteins
By Composition
- Simple Proteins: Yield only amino acids upon hydrolysis (e.g., albumins, globulins).
- Conjugated Proteins: Comprise a protein plus a non-protein prosthetic group:
- Glycoproteins: Carbohydrate group (e.g., mucins)
- Lipoproteins: Lipid group (e.g., chylomicrons)
- Metalloproteins: Metal ions (e.g., ferritin)
- Hemoproteins: Heme group (e.g., hemoglobin)
- Derived Proteins: Result from partial hydrolysis or denaturation (e.g., peptones, proteoses).
By Shape
- Fibrous Proteins: Elongated, structural proteins, insoluble in water (e.g., collagen, keratin).
- Globular Proteins: Compact, spherical proteins involved in dynamic metabolic functions (e.g., enzymes, antibodies).
By Function
- Structural Proteins: Provide mechanical support (e.g., actin, tubulin)
- Catalytic Proteins: Act as enzymes to accelerate reactions (e.g., amylase, trypsin)
- Transport Proteins: Move molecules across membranes or within the bloodstream (e.g., hemoglobin)
- Hormonal Proteins: Regulate physiological processes (e.g., insulin)
- Defensive Proteins: Involved in immune responses (e.g., immunoglobulins)
- Storage Proteins: Reserve amino acids or ions (e.g., casein, ferritin)
- Contractile Proteins: Enable motion (e.g., actin, myosin)
- Receptor Proteins: Mediate cellular signaling (e.g., hormone receptors)
4. Biological Functions of Proteins
- Structural: Integral to cellular architecture and extracellular matrix (e.g., collagen, elastin).
- Enzymatic: Nearly all enzymes are proteins, catalyzing reactions essential for metabolism.
- Transport and Storage: Carry and store molecules like oxygen and iron (e.g., myoglobin, transferrin).
- Immune Defense: Protect the body through antibodies and complement proteins.
- Signaling: Involved in hormone activity and receptor-mediated signal transduction.
- Motility: Facilitate movement in muscles and cells.
- Gene Regulation: Transcription factors and histones influence gene expression.
5. Dietary Protein Requirements
Protein is an essential nutrient required daily, as the body lacks a dedicated storage mechanism for it. Requirements vary based on life stage, health status, and activity level:
- Adults: ~0.8 g/kg body weight/day
- Children/Adolescents: Increased needs due to growth
- Pregnancy/Lactation: Higher intake for fetal development and milk production
- Athletes/Elderly: Increased intake for muscle maintenance and repair
Proteins vary in quality:
- Complete Proteins: Contain all essential amino acids (e.g., eggs, milk, meat, soy)
- Incomplete Proteins: Lacking in one or more essential amino acids (e.g., cereals, legumes), but can be complemented through dietary combinations
6. Protein Bioavailability
Protein bioavailability refers to the proportion of dietary protein that is digested, absorbed, and utilized by the body. It is influenced by:
- Source: Animal proteins generally have higher digestibility than plant-based proteins.
- Amino Acid Profile: A balanced composition of essential amino acids enhances utilization.
- Processing and Preparation: Cooking and fermentation can increase digestibility.
- Anti-nutritional Factors: Compounds such as tannins and phytates can impair absorption.
- Scoring Systems: PDCAAS and DIAAS are used to evaluate protein quality and digestibility.
- Nutrient Interactions: Nutrients like vitamin B6 and zinc play roles in protein metabolism.
7. Protein Deficiency and Related Disorders
Inadequate protein intake can lead to a range of health complications:
- Kwashiorkor: Protein deficiency with adequate caloric intake; marked by edema, fatty liver, and dermatitis.
- Marasmus: Severe deficiency of both protein and calories; characterized by muscle wasting, emaciation, and weakened immunity.
- Protein-Energy Malnutrition (PEM): Encompasses both kwashiorkor and marasmus; prevalent in regions with food insecurity.
- Chronic Deficiency: In adults, may result in delayed wound healing, muscle loss, and reduced immunity.
Ensuring adequate intake of high-quality proteins from diverse sources is vital for maintaining physiological homeostasis, supporting growth, and preventing disease. Public health initiatives, education, and improved access to protein-rich foods are essential strategies for mitigating protein malnutrition globally.