Protein (Amino Acids)
Description
Greek: πρωτεῖος (proteîos) - primary, of first importance
Proteins are large, complex biological molecules made up of amino acid chains linked by peptide bonds. They are composed primarily of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. Proteins perform a vast array of functions in living organisms, including catalyzing biochemical reactions as enzymes, providing structural support in cells and tissues, facilitating transport of molecules, regulating gene expression, and acting as signaling molecules such as hormones and receptors. Their unique three-dimensional structures determine their specific functions.
Summary
Proteins, derived from the Greek "proteios" meaning "primary" or "of first importance," are large, complex biomolecules made up of amino acids linked by peptide bonds. Chemically, proteins consist of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur. The sequence of amino acids determines a protein’s unique three-dimensional structure and function. Proteins perform a wide range of vital roles in the human body, including acting as enzymes to catalyze biochemical reactions, forming structural components of cells and tissues, transporting molecules, regulating gene expression, and serving as signaling molecules such as hormones and receptors.
Absorption of Protein
Protein digestion begins in the stomach where gastric acid denatures proteins, and the enzyme pepsin breaks them into smaller peptides. In the small intestine, pancreatic enzymes like trypsin and chymotrypsin further break peptides into even smaller peptides and amino acids. These amino acids and small peptides are absorbed through the intestinal lining (enterocytes) via active transport mechanisms. Inside the cells, peptides are broken down into amino acids, which enter the bloodstream and are transported to tissues for protein synthesis or energy production.
Functions
Enzymatic catalysis: Proteins speed up biochemical reactions as enzymes.
Structural support: Proteins like collagen provide strength and support to tissues.
Transport: Hemoglobin and membrane proteins transport molecules across membranes.
Regulation: Hormonal proteins regulate physiological processes (e.g., insulin).
Immune defense: Antibodies are proteins that identify and neutralize pathogens.
Cell signaling: Receptor proteins mediate communication between cells.
Movement: Contractile proteins such as actin and myosin facilitate muscle contraction.
Animal vs Plant proteins
Animal proteins possess a complete amino acid profile, containing all nine essential amino acids required for human nutrition, whereas most plant proteins are incomplete, lacking one or more essential amino acids.
This makes animal-derived proteins superior in supporting optimal protein synthesis and physiological function. Furthermore, certain plant proteins have been associated with pro-inflammatory and allergenic responses, due to anti-nutritional factors and specific protein structures that trigger immune reactivity.
In contrast, animal proteins exhibit higher digestibility and lower allergenic potential, reinforcing their advantage as a more bioavailable and physiologically favorable protein source.
Food Sources
Meat, organs, fish, eggs, milk, cheese, yoghurt,
Chemical Structure
Molecular formula: Varies widely depending on amino acid sequence (general formula: CₓHᵧN𝓏OₐS_b)
Molecular mass: Typically ranges from several thousand to millions of atomic mass units (amu), depending on protein size
Atomic composition: Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sulfur (S) (in some amino acids)
Bond types:
Peptide bonds: Covalent amide bonds (–CONH–) linking amino acids
C–C, C–H, N–H bonds: Single covalent (sigma, σ) bonds
C=O bonds: Double covalent bonds (carbonyl group in peptide bond)
Disulfide bonds (–S–S–): Covalent bonds between cysteine residues (in some proteins)
Functional groups:
Amino group (–NH₂) at N-terminus of amino acids
Carboxyl group (–COOH) at C-terminus of amino acids
Variable R-groups (side chains) that define amino acid properties (polar, nonpolar, acidic, basic)
Bond order:
Single bonds in backbone and side chains (bond order 1)
Double bonds in peptide bond carbonyl groups (bond order 2)
Bond length:
C–N peptide bond length ~132 pm
C=O bond length in peptide bond ~120 pm
C–C bond length in amino acid backbone ~154 pm
Electron configuration (typical carbon atom): 1s² 2s² 2p²
Molecular polarity:
Polar backbone due to peptide bonds (amide groups)
Variable polarity from side chains, enabling hydrophilic and hydrophobic regions
Solubility:
Generally soluble in water depending on protein structure and environment
Amphipathic nature allows interaction with both polar and nonpolar environments
Functionality:
Catalysis (enzymes), structural support, transport, signaling, immune response, movement, regulation of gene expression
Physiological Functions
Muscle Structure and Movement
Proteins such as actin and myosin are essential components of muscle fibers, enabling contraction and movement. These contractile proteins convert chemical energy (ATP) into mechanical force, facilitating voluntary and involuntary motions.
Enzymatic Catalysis
Proteins act as enzymes that accelerate biochemical reactions necessary for digestion, metabolism, DNA replication, and cellular repair, ensuring efficient physiological function.
Structural Support
Proteins like collagen, elastin, and keratin provide tensile strength and elasticity to connective tissues, skin, hair, and nails, maintaining the body’s structural integrity.
Transport and Storage
Hemoglobin transports oxygen in the blood, while myoglobin stores oxygen in muscle tissue. Other proteins facilitate transport of ions and molecules across cell membranes and store essential nutrients.
Regulation of Metabolism and Gene Expression
Hormonal proteins such as insulin regulate glucose metabolism. Transcription factors and other regulatory proteins control gene expression, impacting growth, development, and cellular function.
Immune Defense
Antibodies and cytokines mediate immune responses by recognizing pathogens and facilitating communication between immune cells, protecting the body against infections.
Cell Signaling and Communication
Receptor proteins detect extracellular signals like hormones and neurotransmitters, triggering intracellular pathways that regulate cellular activity and maintain homeostasis.
References
Alberts, B., Johnson, A., Lewis, J., Morgan, D., Raff, M., Roberts, K. and Walter, P., 2015. Molecular biology of the cell. Garland Science.
Nelson, D.L. and Cox, M.M., 2017. Lehninger principles of biochemistry. W. H. Freeman.
Kapp, K., Rechsteiner, M.C., Gläser, M., Dembski, S., Richter, M., Biniossek, M.L. and Huber, R., 2021. Proteins as enzymes: Mechanisms and regulation. Annual Review of Biochemistry, 90, pp.345-372.
Schnell, J.R. and Chou, J.J., 2008. Structure and mechanism of membrane proteins. Current Opinion in Structural Biology, 18(4), pp.470-479.
Janeway, C.A., Travers, P., Walport, M. and Shlomchik, M.J., 2001. Immunobiology: The immune system in health and disease. Garland Science.
Touw, W.G., Baakman, C., Black, J., te Beek, T.A., Krieger, E. and Vriend, G., 2015. A series of PDB-related databanks for everyday needs. Nucleic Acids Research, 43(D1), pp.D364-D368.
Janin, J., 1995. Protein–protein interactions and their role in molecular recognition. Proteins: Structure, Function, and Genetics, 23(3), pp.245-251.