Proteins can be classified based on various criteria, including their structure, function, and evolutionary relationships. Here are some common ways to classify proteins:

1. Structural Classification: Proteins can be classified into different structural classes based on their overall three-dimensional structures. The major structural classes include:
   • Fibrous Proteins: These proteins have long, filamentous structures and are often involved in providing structural support and strength. Examples include collagen and keratin.
   • Globular Proteins: These proteins have compact, spherical structures and are involved in a wide range of functions such as enzymes, transporters, and antibodies. Examples include hemoglobin and enzymes like lysozyme.
2. Functional Classification: Proteins can be classified based on their biological functions and roles. Some common functional classes include:
   • Enzymes: These proteins catalyze chemical reactions in the body and play a vital role in metabolism and other cellular processes.
   • Transport Proteins: These proteins facilitate the movement of molecules across cell membranes or within the bloodstream.
   • Structural Proteins: These proteins provide support and structure to cells and tissues.
   • Hormones: These proteins act as chemical messengers, regulating various physiological processes.
   • Antibodies: These proteins are part of the immune system and help defend against foreign substances.

Functions of Proteins

Proteins play a diverse range of essential functions in biological systems. Here are some of the key functions of proteins:

1. Enzymatic Catalysis: Many proteins act as enzymes, facilitating and accelerating biochemical reactions in the body. Enzymes are involved in processes such as metabolism, digestion, DNA replication, and cellular signaling. They help convert substrates into products and regulate the speed of chemical reactions.
2. Structural Support: Proteins provide structural support to cells, tissues, and organs. They form the framework of various structures in the body, such as the cytoskeleton, extracellular matrix, and connective tissues. Examples of structural proteins include collagen, keratin, and actin.
3. Transport and Storage: Some proteins are involved in the transport of molecules and ions across cell membranes or within the bloodstream. Hemoglobin, for instance, transports oxygen in red blood cells, while membrane transporters facilitate the movement of nutrients, ions, and other molecules across cell membranes. Additionally, proteins like ferritin are involved in the storage of essential molecules, such as iron.
4. Hormones and Signaling: Certain proteins act as hormones or are involved in cellular signaling processes. Hormones are chemical messengers that regulate physiological processes and help maintain homeostasis. Examples include insulin, growth hormone, and adrenaline. Signaling proteins transmit signals within cells or between cells, enabling communication and coordination of various cellular activities.
5. Immune Response and Defense: Proteins play a vital role in the immune system’s defense mechanisms. Antibodies (immunoglobulins) are proteins produced by immune cells and are involved in recognizing and neutralizing foreign substances (antigens) in the body. Other proteins, such as complement proteins and cytokines, contribute to immune responses and inflammation.
6. Muscle Contraction: Proteins are crucial for muscle contraction. Actin and myosin, two proteins found in muscle cells, interact to generate the force required for muscle movement. The sliding filament model explains how these proteins work together to produce muscle contraction.
7. Catalysis of Cellular Processes: Proteins participate in various cellular processes beyond enzymatic reactions. They are involved in DNA replication, transcription, and translation, ensuring the accurate copying and expression of genetic information. Proteins called chaperones assist in proper folding and assembly of other proteins.
8. Cell Adhesion and Communication: Proteins are involved in cell adhesion, allowing cells to attach to each other and form tissues. They are also responsible for cell-to-cell communication through interactions with receptors and ligands, transmitting signals that regulate cell growth, differentiation, and development.
9. Regulation and Control: Proteins are involved in the regulation and control of various biological processes. Transcription factors bind to DNA, controlling gene expression. Protein kinases and phosphatases regulate the activity of other proteins by adding or removing phosphate groups. G-proteins are involved in signal transduction pathways, relaying signals from cell surface receptors to cellular responses.

These are just a few examples of the many functions proteins serve in living organisms. Their versatility and diverse roles make proteins indispensable for the structure, function, and regulation of biological systems.

Biologically important peptides

Peptides are short chains of amino acids linked by peptide bonds. They play important biological roles in various physiological processes. Here are some examples of biologically important peptides:

1. Antimicrobial Peptides (AMPs): These peptides are part of the innate immune system and help defend against microbial infections. AMPs exhibit broad-spectrum antimicrobial activity by disrupting the integrity of microbial cell membranes or interfering with intracellular processes.
2. Hormones and Neuropeptides: Many hormones and neuropeptides act as signaling molecules in the body, regulating various physiological processes. Examples include insulin (regulates glucose metabolism), oxytocin (involved in social bonding and childbirth), and endorphins (associated with pain relief and pleasure).
3. Enzyme Inhibitors: Some peptides act as enzyme inhibitors, regulating the activity of specific enzymes. For example, protease inhibitors bind to proteases and prevent their enzymatic activity. These inhibitors are important for maintaining balance in enzymatic processes.
4. Opioid Peptides: Opioid peptides are involved in pain regulation and reward pathways. They interact with opioid receptors in the central nervous system, producing analgesic (pain-relieving) effects and feelings of euphoria. Examples include endorphins, enkephalins, and dynorphins.
5. Cell-Penetrating Peptides (CPPs): These peptides have the ability to cross cell membranes and deliver cargo molecules, such as drugs or biomolecules, into cells. CPPs have applications in drug delivery, gene therapy, and imaging techniques.
6. Signal Peptides: Signal peptides are short sequences present at the N-terminus of proteins. They play a crucial role in targeting and directing proteins to their correct cellular location. Signal peptides guide protein translocation across membranes, such as the endoplasmic reticulum or mitochondria.
7. Immunomodulatory Peptides: These peptides modulate the immune response by influencing the activity of immune cells or regulating inflammatory processes. Examples include cytokines, which are small peptides involved in cell signaling and immune regulation.
8. Vasodilator Peptides: Vasodilator peptides promote the relaxation and widening of blood vessels, leading to increased blood flow. One well-known example is bradykinin, which is involved in regulating blood pressure and inflammation.
9. Growth Factors: Growth factors are peptides that regulate cell growth, differentiation, and tissue repair. They play critical roles in processes such as wound healing and embryonic development. Examples include epidermal growth factor (EGF) and transforming growth factor-beta (TGF-beta).

These are just a few examples of the biologically important peptides. Peptides have diverse functions in the body and continue to be an area of active research for their potential therapeutic applications in medicine, including drug development and targeted therapies.