protien

• The main role of amino acids is in the synthesis of structural and functional proteins
• About 15-20% of energy needs are met by oxidation of carbon skeleton of amino acids
• Amino acids provide several important biologically active molecules like monoamines or contribute carbon and nitrogen atoms for the synthesis of several important compounds
• Non-essential amino acids


  Either derived from the diet or synthesized in the body
• Essential amino acids


  Obtained from the diet, if one is deficient, protein synthesis cannot take place
• The body amino acid pool is always in a dynamic steady state
• In an adult, the rate of synthesis of proteins balances the rate of degradation, so that nitrogen balance is maintained
• Amino acids


  Organic acids containing an amino group (NH2) and a carboxylic acid (COOH) group, with a side chain that can be hydrogen, aliphatic, aromatic or heterocyclic group, all in L-amino acids configuration
• Classification of amino acids

  • Neutral amino acids (aliphatic, aromatic, heterocyclic, sulphur containing)
  • Acidic amino acids
  • Basic amino acids
• Essential amino acids


  • Arg
  • His
  • lle
  • Cys
  • Met
  • Phe
  • Thr
  • Trp
  • Tyr
  • Leu
  • Lys
  • Val
• Non-essential amino acids


  • Ala
  • Asn
  • Asp
  • Gln
  • Glu
  • Gly
  • Pro
  • Ser
• Humans can synthesize only 10 of the 20 amino acids
• Essential amino acids


  Humans (mammals) cannot synthesize their carbon skeletons de novo, some are essential because we cannot synthesize enough, especially for growth (children)
• Non-essential amino acids


  Synthesized from intermediates of glucose/TCA cycle except tyrosine (cannot make aromatic ring)
• Sources of amino acids

  • Absorption in the intestine
  • Formation during protein decomposition
  • Synthesis from carbohydrates and lipids
• Protein turnover


  Simultaneous synthesis and degradation of protein molecules, in healthy, fed adults the total amount of protein in the body remains constant because the rate of protein synthesis is just sufficient to replace the protein that is degraded
• Dietary protein can vary from none (for example, fasting) to over 600 g/day (high protein diets), 100 g/day is typical of the U.S. diet
• The amino acids not used in biosynthetic reactions are burned as a fuel
• Functions or importance of amino acids

  • Converted to carbohydrate (glucogenic amino acids)
  • Converted to acetyl CoA (ketogenic amino acids)
  • Synthesis of nitrogen containing compounds (creatine, purine, choline, pyrimidine)
  • Source of energy (oxidation - deamination, transamination, decarboxylation)
  • For gluconeogenesis
  • Formation of biologically active compounds
• Classification of proteins


  • Simple proteins
  • Conjugated proteins
  • Derived proteins (primary and secondary)
• Functions or importance of proteins

  • Present in all living tissues as building or structural elements
  • Produce energy
  • Formation of enzymes
  • Some hormones are proteins
  • Structural proteins like actin and myosin are contractile proteins
  • Defense proteins like immunoglobulins (antibodies)
  • Molecular receptors that bind with specific molecules
  • Transport action, across the membrane
  • Storage protein like ferritin
  • Plasma proteins, albumin, globulin, clotting blood, thrombin, fibrinogen
• The intake of dietary protein is in the range of 50-100 g/day
• Dietary proteins are denatured on cooking & easily digested, About 30-100 g/day of endogenous protein is derived from the digestive enzymes and worn out cells of the digestive tract
• The digestion and absorption of proteins is very efficient in healthy humans, hence very little protein (about 5-10 g/day) is lost through feces
• Proteolytic enzymes are secreted as inactive zymogens, which are converted to their active form in the intestinal lumen
• Protein digestion


  1. Buccal cavity: No protein digestion

  2. Stomach: HCl and pepsin

  3. Small intestine: Trypsinogen, chymotrypsinogen, carboxypeptidase
• Stomach


  Hydrochloric acid makes the pH optimum for the action of pepsin and also activates pepsin, but HCl alone cannot break the peptide bonds, needs enzymes
• Pancreas


  The optimum pH for the activity of pancreatic enzymes (pH 8) is provided by the alkaline bile and pancreatic juice, peptide hormones cholecystokinin and pancreozymin stimulate the secretion of pancreatic juice
• Intestine


  The luminal surface contains aminopeptidases that repeatedly cleave the oligopeptides to produce free amino acids and small peptides
• Absorption of amino acids

  Occurs mainly in the small intestine, is an energy requiring process, carrier mediated systems, glutathione also plays an important role
• Free amino acids are taken into the intestinal cells by Na-linked secondary transport system, di-and tri peptides are taken up by H+- linked transport system
• Only free amino acids are found in the portal vein after meals containing proteins, these are either metabolized by the liver or released into the general circulation, branched-chain amino acids are not metabolized by the liver
• Unlike fat and carbohydrate, amino acids are not stored in the body, protein that exists is to maintain the supply of amino acid for future use
• Amino acid must be supplied from the diet (Exogenous) OR Catabolism of normal protein (Endogenous)
• In animals, amino acids undergo oxidative degradation in 3 different metabolic circumstances: during normal protein turnover, during starvation, and during consumption of a high-protein diet
• Digestion of proteins


  1. Pepsin (stomach glands) in the presence of HCl

  2. Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase)

  3. Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase)
• Absorption of proteins


  Amino acids enter the capillary blood in the villi and are transported to the liver via the hepatic portal vein
• Protein


  • Large polypeptides
  • Small polypeptides, small peptides
  • Amino acids (some dipeptides and tripeptides)
• Unlike fat and carbohydrate, Amino Acids are not stored in the body; Protein that exist is to maintain the supply of amino acid for future use. Amino acid must be supplied from the diet (Exogenous) OR Catabolism of normal protein (Endogenous)
• Amino acid metabolism


  1. Transamination & Oxidative deamination

  2. Ammonia & a Ketoacids (Carb. Skeleton)

  3. Portion of ammonia is excreted in the urine but most of it will be converted to the less toxic compound "Urea"

  4. a Ketoacids are common intermediate of energy producing metabolic pathways