Proteins are central to the functions and structures of all living organisms. These complex macromolecules can carry out an array of tasks due to their diverse nature, based on their amino acid sequence. This set of notes will explore this diversity in detail.
Amino Acid Sequence and Protein Diversity
The sequence of amino acids within a protein is a defining characteristic that determines its specific function and structure.
Primary Structure
- Definition: The specific sequence of amino acids in a polypeptide chain.
- Significance: It lays the foundation for the structure and function of the protein. Even a minor change can alter the protein's properties, sometimes leading to diseases like sickle cell anaemia.
Secondary Structure
- Definition: Local folding of the polypeptide chain into alpha-helices and beta-sheets.
- Significance: These structures form the backbone for more complex folding, often stabilised by hydrogen bonds.
Tertiary Structure
- Definition: The three-dimensional shape created by the folding of the secondary structure.
- Significance: It allows proteins to achieve their functional conformation.
Quaternary Structure
- Definition: Arrangement of multiple polypeptide chains.
- Significance: Often allows the full functionality of the protein, like in haemoglobin.
Significance in Organism Structure
Proteins' roles in forming and maintaining the structure of organisms are multifaceted and critical.
Structural Proteins
- Collagen
- Structure: Triple helix of polypeptides.
- Function: Provides strength in skin, ligaments, and tendons.
- Keratin
- Structure: Helical and fibrous.
- Function: Key component in hair, nails, and skin's outer layer.
- Elastin
- Structure: Highly elastic.
- Function: Permits tissues to resume shape after stretching.
Motor Proteins
- Myosin
- Structure: Complex protein with several subunits.
- Function: Enables muscle contraction.
These structural and motor proteins underline the vast array of functions that proteins perform within the body.
Functional Diversity of Proteins
Proteins also operate as hormones, antibodies, receptors, and more, illustrating their functional diversity.
Hormones
- Insulin
- Function: Regulates glucose uptake in cells.
- Significance: Crucial in managing diabetes.
- Glucagon
- Function: Increases blood glucose levels.
- Significance: Works antagonistically with insulin to regulate blood sugar.
Antibodies
- Immunoglobulins
- Structure: Y-shaped proteins.
- Function: Identifies and neutralises pathogens.
- Significance: Central to immunity.
Receptors
- G-Protein-Coupled Receptors
- Structure: Seven-transmembrane helices.
- Function: Transmits extracellular signals into the cell.
- Significance: Involved in many diseases, target for many drugs.
Proteins as Enzymes
Proteins often act as enzymes, speeding up chemical reactions within the body.
- Examples: Lipase (fat digestion), DNA polymerase (DNA replication).
- Importance: Essential for metabolism and energy production.
Transport Proteins
- Haemoglobin
- Function: Oxygen transport in the blood.
- Structure: Four polypeptide chains with iron-containing heme groups.
- Transferrin
- Function: Iron transport in the blood.
FAQ
Protein domains are specific regions of a polypeptide chain that fold independently and often have a distinct function. They enable parts of a protein to evolve and function separately from other parts, increasing functional diversity. Domains often facilitate interactions with other molecules or parts of the same protein, making them essential for many biological processes.
Protein receptors are embedded in cell membranes and detect signals like hormones or neurotransmitters. They transmit these signals into the cell, initiating a cellular response. An example is the insulin receptor, which binds insulin and triggers glucose uptake. Receptors are essential in cellular signalling, allowing cells to respond to external cues and communicate with each other.
Yes, proteins can be used as drugs. Insulin, for instance, is a protein administered to diabetics to control blood sugar. Proteins are typically administered through injections, as oral consumption might lead to digestion, rendering the protein ineffective. They are utilised for various therapeutic purposes, including regulating immune responses, treating hormonal imbalances, and combating specific diseases, showcasing their versatility in medical applications.
Prions are infectious agents composed of misfolded proteins. They can induce normal proteins to misfold into the prion form. This alteration in folding leads to diseases such as Creutzfeldt-Jakob disease in humans and mad cow disease in cattle. The misfolded proteins accumulate in neural tissue, leading to tissue damage and characteristic symptoms of these neurodegenerative diseases.
X-ray crystallography, nuclear magnetic resonance (NMR) spectroscopy, and cryo-electron microscopy are common methods used to determine a protein's structure. X-ray crystallography provides a detailed view by examining diffraction patterns of X-rays through crystallised proteins. NMR spectroscopy uses magnetic fields to identify atomic structures, and cryo-electron microscopy freezes proteins to visualise them using electron beams. Each technique offers unique insights into protein structure.
Practice Questions
Proteins have a vast array of functions in living organisms. Structural proteins like collagen provide strength to skin, ligaments, and tendons. Keratin is another example, forming the key component in hair and nails. Hormones, such as insulin and glucagon, are proteins that play a critical role in regulating blood glucose levels. Insulin decreases blood sugar, while glucagon increases it. Antibodies or immunoglobulins are proteins that function in the immune system by identifying and neutralising pathogens. These examples underline the functional diversity of proteins in structure and function within living organisms.
The sequence of amino acids in a polypeptide chain determines the primary structure of a protein, which leads to the secondary (alpha-helices and beta-sheets), tertiary (three-dimensional shape), and sometimes quaternary (multiple polypeptide chains) structures. The correct sequence is vital for proper folding and function. For example, a single amino acid change in haemoglobin leads to sickle cell anaemia, where the altered structure affects oxygen transport. Similarly, the unique amino acid sequence of insulin dictates its specific function in regulating glucose uptake. Thus, the sequence of amino acids is crucial for the proper structure and specific functionality of proteins.
