peptide

Examples of peptide in the following topics:

• Antimicrobial Proteins

  • Antimicrobial peptides (also called host defense peptides) are an evolutionarily-conserved component of the innate immune response found among all known species.
  • Antimicrobial peptides are a unique and diverse group of molecules.
  • As peptides, they consist of chains of amino acids that determine their composition and structure.
  • This causes the peptide to bind to bacterial membranes instead of host cell membranes.
  • Describe the role of antimicrobial peptides in the innate immune system

• Chemistry of Hormones

  • There are three classes of hormones: peptide hormones, lipid hormones, and monoamine hormones.
  • Peptide hormones consist of short chains of amino acids, such as vasopressin, that are secreted by the pituitary gland and regulate osmotic balance; or long chains, such as insulin, that are secreted by the pancreas, which regulates glucose metabolism.
  • Some peptide hormones contain carbohydrate side chains  and are termed glyco-proteins, such as the follicle-stimulating hormone.
  • All peptide hormones are hydrophilic and are therefore unable to cross the plasma membrane alone.

• Adrenal Medulla

  • When stimulated, chromaffin cells secrete adrenaline and noradrenaline along with enkephalin and enkephalin-containing peptides into the bloodstream.
  • The enkephalins and enkephalin-containing peptides are related to, but also distinct from, the endogenous peptides named endorphins (secreted from the pituitary).
  • All of these peptides bind to opioid receptors and produce analgesic (and other) responses.

• Gastric Phase

  • The gastric phase is a period in which swallowed food and semidigested protein (peptides and amino acids) activate gastric activity.
  • As dietary protein is digested, it breaks down into smaller peptides and amino acids, which directly stimulate the G cells to secrete even more gastrin: a positive feedback loop that accelerates protein digestion.
  • Small peptides also buffer stomach acid so the pH does not fall excessively low.
  • But as digestion continues and these peptides are emptied from the stomach, the pH drops lower and lower.

• Gastrointestinal Reflex Pathways

  • The digestive system functions via a system of long reflexes, short reflexes, and extrinsic reflexes from gastrointestinal (GI) peptides working together.
  • Movement and secretion are regulated by both long reflexes from the central nervous system (CNS), short reflexes from the enteric nervous system (ENS), and reflexes from gastrointestinal system (GI) peptides working in harmony with each other.
  • GI peptides are signal molecules that are released into the blood by the GI cells themselves.

• Antigen-Presenting Cells

  • The host cell digests cytoplasmic proteins by a specialized enzyme complex, the proteasome, into small peptides.
  • A specialized carrier, the Transporter associated with Antigen Processing (TAP) complex moves the peptide into the endoplasmic reticulum, allowing the antigenic peptide to be coupled to an MHC Class I molecule and transported to the cell surface.
  • Recognition of antigenic peptides through Class I by CTLs leads to the killing of the target cell, which is infected by virus, intracytoplasmic bacterium, or are otherwise damaged or dysfunctional.
  • The DC uses lysosome-associated enzymes to digest pathogen-associated proteins into smaller peptides.
  • In the lymph node, the DC will display these antigenic peptides on its surface by coupling them to MHC Class II molecules.

• Intestinal Phase

  • Stretching of the duodenum accentuates vagal reflexes that stimulate the stomach, and peptides and amino acids in the chyme stimulate G cells of the duodenum to secrete more gastrin, which further stimulates the stomach.
  • The enteroendocrine cells also secrete glucose dependent insulinotropic peptide.
  • Originally called gastric-inhibitory peptide, it is no longer thought to have a significant effect on the stomach.

• Insulin Secretion and Regulation of Glucagon

  • Glucagon is a peptide hormone that works in conjunction with insulin to maintain a stable blood glucose level.
  • Glucagon and insulin are peptide hormones secreted by the pancreas that play a key role in maintaining a stable blood glucose level.

• Antigenic Determinants and Processing Pathways

  • Free peptides flowing through the body are not recognized by T cells, rather the peptides associate with molecules coded for by the major histocompatibility complex (MHC) and it is the complex of MHC molecules + peptide that is recognized by T cells.
  • Antigen processing occurs within an APC that phagocytizes an antigen and then digests it through fragmentation (proteolysis) of the antigen protein, association of the fragments with MHC molecules, and expression of the peptide-MHC molecules at the cell surface where they can be recognized by the T cell receptor on a T cell during antigen presentation.

• Major Histocompatibility Complex Antigens (Self-Antigens)

  • During antigen processing prior to presentation, protein peptides from pathogens and MHC (I or II) travel through the cytoplasm and then travel to the cell membrane surface.
  • After processing, the MHC can present the peptide antigen bound to it to naive T cells.
  • MHC class I pathway: proteins in the cytosol are degraded by the proteasome, liberating peptides internalized by TAP channel in the endoplasmic reticulum, there associating with MHC-I molecules freshly synthesized.
  • MHC-I/peptide complexes enter Golgi apparatus, are glycosylated, enter secratory vesicles, fuse with the cell membrane, and externalize on the cell membrane interacting with T lymphocytes.