History and Discovery of Functional Proteins
The concept of proteins having important biological functions emerged in the 19th century. In 1838, French chemist Anselme Payen isolated the first enzyme, diastase, which is now known as amylase, from malt. Further research revealed that enzymes were in fact proteins that could catalyze biochemical reactions without being used up in the process. By the early 1900s, scientists knew proteins played critical roles as enzymes, hormones, antibodies and structural components of cells and tissues. However, it was not until the middle of the 20th century that our understanding of proteins and their functions really took off.
Functional Protein development of new techniques like X-ray crystallography and the emergence of molecular biology in the 1950s and 1960s, scientists were able to determine the exact structure and sequences of different proteins. This opened doors to unravel how proteins folded into their three-dimensional shapes and how even small changes in their amino acid sequences could result in large differences in structure and function. Landmark discoveries in these years included the cracking of the structure of myoglobin by John Kendrew in 1958 and the determination of the α-helical structure of keratin by Linus Pauling and Robert Corey in 1951.
Key Classes of Functional Proteins
Proteins can generally be categorized into four main types based on their functions in the body:
Enzymes: Enzymes are catalytic proteins that speed up biochemical reactions without being used up in the process. They act as catalysts for reactions involved in digestion, cellular respiration, photosynthesis, nutrient assimilation, signal transduction and more. Important enzymes include proteases, lipases, amylases, DNA and RNA polymerases.
Hormones: Hormones are regulatory molecules secreted by endocrine glands to trigger or regulate specific physiological processes. Examples are insulin, glucagon, thyroid hormones, estrogens, testosterone and melatonin. They transmit signals from one cell or tissue to another cell to regulate processes like metabolism, growth, tissue function, sleep and reproduction.
Structural Proteins: These proteins serve structural or mechanical functions by providing structure, shape and integrity to cells and tissues. Key examples are keratin, elastin, collagen and microtubules. Keratin gives strength and flexibility to hair, skin, nails and feathers while collagen forms connective tissues.
Antibodies: Antibodies, also known as immunoglobulins, are Y-shaped glycoproteins produced by B-cells of the immune system in response to antigens like viruses or bacteria. They recognize and latch onto foreign substances to neutralize and eliminate pathogens from the body. Different classes of antibodies include IgA, IgD, IgE, IgG and IgM.
Protein Structure and Functions
The exact structure of a protein, from its primary amino acid sequence to its complex three-dimensional folding and shape, determines its specific function. Let's briefly examine some key aspects:
- Primary structure: This refers to the linear sequence of amino acids linked together by peptide bonds. Even small changes can impact function.
- Secondary structure: Parts of the chain fold into regular structures like alpha helices and beta pleated sheets held by hydrogen bonds. This gives proteins their initial shape.
- Tertiary structure: The complete folding of the polypeptide chain into a compact 3D structure. Interactions between R groups of amino acids stabilize the structure.
- Quaternary structure: Some functional proteins are made of multiple polypeptide subunits that assemble into an active protein complex. Hemoglobin and DNA polymerase are examples.
- Active site: This is the specific region of an enzyme where its substrate binds and the catalytic reaction occurs. The protein folds in a way to bring specific amino acids and functional groups together at this site.
- Signal peptides: These short sequences at the N-terminal aid in transporting proteins to correct organelles or targeting them to cell membranes.
Role of Proteins in Key Biological Processes
Without proteins, life itself would not be possible. Let's examine some examples:
- Metabolism: Enzymes catalyze thousands of biochemical reactions related to digestion, cellular respiration, nutrient assimilation, etc.
- Genetics: DNA/RNA polymerases, histones and associated proteins package DNA and transcribe genes. Microtubules help transport chromosomes.
- Signaling: Receptor and transporter proteins mediate signal transduction at cell surfaces. Hormones trigger physiological responses.
- Structure: Keratin, collagen, actin, tubulin and other filaments provide structure to cells, tissues and organ systems. Microtubules aid cell division, movement.
- Immunity: Antibodies recognize pathogens and mount immune defenses. Antigens induce production of immunoglobulins.
- Transport: Hemoglobin (oxygen), myoglobin (oxygen in muscle), transferrin (iron), etc. aid movements of gases and ions.
- Coagulation: Fibrinogen and thrombin clot blood and stop bleeding at wound sites. Clotting factors assist this process.
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