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• - Biomolecules that contain many  amide bonds, formed by joining amino acids
• - Proteins account for 50% of the dry weight of the human body.
• - Unlike lipids and carbohydrates, proteins are not stored, so they must be consumed daily
• - Current recommended daily intake for adults is 0.8 grams of protein per kg of body weight (more is needed for children)
• - Contains two functional groups—an  Amino group (NH2) and a Carboxyl group (COOH)
• - The amino group is attached to the α-carbon, the C atom adjacent to the carbonyl group
• - The simplest amino acid is Glycine
• - The R group, called the side chain, determines the identity of the amino acid
• - If R = a basic N atom, it is a basic amino acid
• - If R = an additional COOH group, it is an acidic amino acid
• - Since amino acids contain a base (NH2) and an acid (COOH), a proton transfers from the acid to the base to form a Zwitterion
• Essential Amino Acid


  A standard amino acid needed for protein synthesis that must be obtained from dietary sources because the human body cannot synthesize it in adequate amounts from other substances
• Essential amino acids


  • There are nine essential amino acids for adults
  • A tenth one is needed for growth in children
• Complete Dietary Protein


  • A protein that contains all of the essential amino acids in the same relative amounts in which the body needs them
  • May or may not contain all of the nonessential amino acids
  • Protein from animal sources is usually a complete dietary protein
• Incomplete Dietary Protein


  • A protein that does not contain adequate amounts, relative to the body's needs, of one or more of the essential amino acids
  • Protein from plant sources tends to be incomplete dietary protein
• Limiting Amino Acid


  An essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein
• Complementary Dietary Proteins


  Two or more incomplete dietary proteins that, when combined, provide an adequate amount of all essential amino acids relative to the body's needs
• Gelatin


  • Common incomplete dietary protein that comes from animal sources
  • A protein in which tryptophan is the limiting amino acid
• Soy


  The only common plant protein that is a complete dietary protein
• Amino acids often limiting in plant proteins


  • Lysine (wheat, rice, oats, and corn)
  • Methionine (beans and peas)
  • Tryptophan (corn and beans)
• Genetic Modification


  Improve a plant's protein by causing it to produce increased amounts of amino acids that it normally has in short supply
• Stereochemistry of Amino Acids

  - All amino acids (except Glycine) have a chirality center on the α-carbon

   

   

   

   

   

   

   

   

  - L amino acids have the –NH3+ group on the left

  - D amino acids have the –NH3+ group on the right

   
• Peptides

  - Peptides and proteins are formed when amino acids are joined together by amide bonds

  - A dipeptide has two amino acids joined together by one amide bond

   

   

   

   

  - The amide bond is called a peptide bond

  - A tripeptide has three amino acids joined together by 2 amide bond

   

   

   

   

  - Polypeptides have many amino acids, while  proteins have more than 40 amino acids.
• Oxytocin and Vasopressin


  Cyclic nonapeptide hormones, which have identical sequences except for two amino acids
• Slightly different sequence


  Gives the two peptides vastly different effects on the body
• Oxytocin


  • Stimulates the contraction of uterine muscles, and signals for milk production; it is often used to induce labor
• Vasopressin


  • Antidiuretic Hormone (ADH)
  • Targets the kidneys and helps to limit urine production to keep body fluids up during dehydration
• Glutathione


  • The most abundant single tripeptide distributed in animals, plants and bacteria
  • a.k.a. Glutamyl Cysteinyl Glycine
• The Chemistry of Glutathione


  • The reduced species (GSH) contain free Sulfhydryl (-SH) and the oxidized form GSSG, contain an -S-S- or disulfide linkage forming an intra chain disulfide bond
  • Glutathione (GSH) contributes to the maintenance of the structural integrity of cellular protein from oxidation
  • GSH functions as a free radical scavenger, removing all the free radical formed from the oxidation reaction
• Glucagon


  • Glucagon is a peptide hormone which has a profound effect on glycogen metabolism
  • It is secreted in the pancreas during fasting state when blood glucose levels are decreasing
  • Glucagon stimulates Glycogenolysis and Gluconeogenesis that lead to the production of glucose in the liver , preventing the development of hypoglycemia
• Insulin


  • A peptide hormone in the pancreas
  • It facilitates the utilization of glucose and increases the uptake of blood glucose by muscle and tissues and liver
  • It enhances glycogenesis, the formation of glycogen from excess glucose in blood
• Enkephalins


  • Pentapeptides made in the brain, act as painkillers and sedatives by binding to pain receptors
  • Addictive drugs morphine and heroin bind to these same pain receptors, thus producing a similar physiological response, though longer lasting
  • Enkephalins belong to the family of polypeptides called endorphins, which are known for their pain reducing and mood enhancing effects
• Primary Structure


  • The sequence of amino acids joined together by peptide bonds
  • All bond angles are 120O, giving the protein a zigzag arrangement
• Secondary Structure


  • The 3D arrangement of localized regions of a protein
  • These regions arise due to hydrogen bonding between the N—H group of one amide with the C═O group of another
  • Two stable arrangements are the α-helix and the β-pleated sheet
• α-helix


  • Diagram
• β-pleated sheet


  • Diagram
• Most proteins have regions of α-helix and β-pleated sheets, and other regions that are random arrangements
• Tertiary and Quaternary Structure


  • The tertiary structure is the 3D shape adopted by the entire peptide chain
  • The quaternary structure of the protein is the shape adopted when two or more folded poly-peptide chains come together into one complex
• α-Keratins


  • Are the proteins found in hair, hooves, nails, skin, and wool
  • Made of two mainly α-helix chains coiled around each other in a superhelix
  • Collagen requires three chains in a superhelix
  • Vitamin C helps stabilize the chains, and, when missing, poorly formed collagen fibers result
• Hemoglobin and Myoglobin


  • Both hemoglobin and myoglobin are globular and conjugated proteins, meaning they contain both a protein and non-protein component
  • Their non-protein unit is a heme, an organic complex surrounding a Fe+2 ion
  • Myoglobin has 153 amino acids in 1 polypeptide
  • Hemoglobin has 4 polypeptide chains, each carrying a heme unit
  • Carbon monoxide (CO) is poisonous because it binds 200 times more strongly to the Fe+2 more than does O2
  • Sickle cell anemia is a disease where a single amino acid is different in two of the subunits of hemoglobin