# 2.16 Role of Proteins Proteins are fundamental macromolecules involved in nearly all cellular functions. They are categorized based on their primary roles: structural (providing support and composition) and functional (participating in physiological activities). ## Structural Proteins These proteins are involved in the support, shape, and composition of body parts. | Protein | Role | | :--- | :--- | | **Collagen** | Forms the matrix of bone and cartilage; a key component of ligaments and tendons. | | **Elastin** | Provides support and elasticity for connective tissues like tendons and ligaments. | | **Keratin** | A fibrous protein that strengthens protective coverings such as hair, nails, quills, feathers, horns, and beaks. | | **Histone** | A protein complex that helps organize and compact DNA into chromosomes within the nucleus. | ## Functional Proteins These proteins are involved in various physiological and metabolic activities within the body. | Protein | Role | | :--- | :--- | | **Enzymes** | Act as biological catalysts to speed up biochemical reactions, thereby controlling the body's metabolism. | | **Hormones** | Some hormones (e.g., insulin) are proteins that regulate physiological activities like blood glucose levels, digestion, and blood pressure. | | **Antibodies** | Produced by white blood cells (WBCs) in response to foreign particles (antigens) to provide immunity. | | **Haemoglobin** | A protein found in red blood cells (RBCs) responsible for transporting oxygen and, to a lesser extent, carbon dioxide. | | **Fibrinogen** | A blood plasma protein that is converted to fibrin during the blood clotting process. | | **Ovalbumin & Casein** | Storage proteins; Ovalbumin is found in egg whites and Casein is found in milk. They serve as a reserve of amino acids. | ### Movement and Transport Proteins like **Actin and Myosin** are essential for muscle contraction and cellular movement. Additionally, proteins in the act as carriers or channels for . ## Biochemical Tests for Proteins Specific chemical tests can be used to confirm the presence of proteins or amino acids in a sample. | Test | Procedure | Positive Observation | Inference | | :--- | :--- | :--- | :--- | | **Biuret Test** | Add 2 ml NaOH and a few drops of 1% copper sulphate solution to 2 ml of the test solution. Shake well. | The solution turns **pink or violet**. | Protein is present. | | **Millon's Test** | Add 1 ml of Millon's reagent to 1 ml of the test solution. Shake and let it settle. Heat if necessary. | An initial **white precipitate** forms, which turns **red upon heating**. | Protein is present. | | **Ninhydrin Test** | Add 3-4 drops of aqueous ninhydrin reagent to 2 ml of the test solution. Boil the mixture and let it cool. | A **purple or violet color** appears. | Amino acids or protein is present. |