Protein

• elemental composition

  carbon hydrogen oxygen nitrogen
• Amino acid structure


  • Contains an amino group (NH2), a carboxyl group (COOH), a central carbon (C), and a variable group (R) that changes with each amino acid
• Amino acids


  • When the variable group (R) is hydrogen (H) the amino acid is glycine
  • When the variable group (R) is HS-CH₂ the amino acid is cysteine
• Elements found in proteins

  • Carbon (C)
  • Hydrogen (H)
  • Oxygen (O)
  • Nitrogen (N)
  • Sulfur (S)
  • Iron (Fe)
  • Phosphorus (P)
• Essential amino acids


  Cannot be manufactured by the body, therefore must be obtained from food
• Essential amino acids


  • Valine
  • Lysine
  • Leucine
  • Isoleucine
  • Phenylalanine
  • Methionine
  • Threonine
  • Tryptophan
  • Histidine (children)
  • Arginine (children)
• Non-essential amino acids


  Can be manufactured by the body, therefore do not need to be obtained from food
• Non-essential amino acids


  • Alanine
  • Aspartic acid
  • Cysteine
  • Ornithine
  • Serine
  • Asparagine
  • Proline
  • Tyrosine
  • Glycine
  • Glutamic acid
• Proteins are one of the macronutrients. Proteins form the main part of all cells and tissues, making them essential for growth and repair.
• Proteins are large molecules composed of amino acids. Amino acids are joined together by peptide links to form long polypeptide chains that make up proteins.
• The human body uses 20 different amino acids to make all the proteins it needs to function. These are all found in food.
• Peptide links


  Formed when two amino acids join together, resulting in the loss of a water (H2O) molecule in a condensation reaction
• Stages in the formation of peptide links


  1. The COOH (acidic) group of one amino acid reacts with the NH₂ (basic) group of another

  2. The COOH (acidic) group loses an OH group, the NH₂ (basic) group loses a hydrogen (H) atom, and a water (H₂O) molecule is lost

  3. The result is a CO-NH bond, forming a dipeptide (two amino acids joined together)
• When more than 20 amino acids join together, a polypeptide is formed. When more than 50 amino acids join together, a protein is formed. Each protein consists of one or more polypeptide chains.
• Hydrolysis


  The reverse of the condensation reaction, involving the addition of water and enzyme action, occurring during digestion when proteins are broken down into individual amino acids
• Primary structure of proteins


  • The order or sequence of amino acids in protein chains.
  • insulin is one of the simplest proteins containing 51 amino acids
• Secondary structure of proteins


  • The folding of the primary structure of proteins into definite shapes, with polypeptide chains either folding in on themselves or cross-linking with another polypeptide chain
• Disulfide bonds


  • Formed when two sulfurs from two amino acids (cysteine) join together from either a single polypeptide chain or two different polypeptide chains
• Hydrogen bonds


  • Formed when a hydrogen (H) from the N-H group of one amino acid and an oxygen (O) from the C=O group of another amino acid join together from either a single polypeptide chain or two different polypeptide chains
• Tertiary structure


  • Involves the folding of the secondary structure of proteins into three-dimensional shapes
  • Further cross-linking between amino acids forms definite shapes, which may be fibrous (elongated) or globular (folded over itself to form a compressed unit)
• Fibrous


  Polypeptide chains are arranged in straight, spiral or zigzag shapes
• Globular


  Polypeptide chains are arranged in a globular shape
• Fibrous protein shapes


  • Straight
  • Spiral
  • Zigzag
• Fibrous proteins


  • Insoluble in water
  • Not easily denatured
• Globular proteins


  • Soluble in water
  • Easily denatured
• Fibrous proteins


  • Gluten (wheat)
  • Elastin and collagen (meat connective tissue)
• Globular proteins


  • Ovalbumin (egg white)
  • Lactalbumin (milk)
• Conjugated proteins


  Proteins that combine with a non-protein molecule
• Conjugated proteins


  • Lipoproteins (lipid and protein)
  • Phosphoproteins (phosphate and protein)
• Conjugated protein examples


  • Lecithin
  • Caseinogen
• Animal protein sources


  • Meat
  • Fish
  • Eggs
  • Milk
  • Cheese
• Plant protein sources


  • Beans
  • Nuts
  • Lentils
  • Peas
  • Cereals
• Biological value


  A measure of the protein quality in a food, displayed as a percentage. It is determined by the number of essential amino acids a food contains in proportion to the needs of the body.
• High biological value (HBV) proteins


  Contain all essential amino acids, mostly found in animal sources with the exception of soya beans
• Low biological value (LBV) proteins


  Lack one or more of the essential amino acids, mostly found in plant sources with the exception of gelatine
• LBV protein sources


  • Wheat
  • Maize
  • Meat bones
• Simple protein groups


  • Animal fibrous
  • Animal globular
  • Plant
• Animal fibrous proteins


  • Elastin
  • Collagen
• Animal globular proteins


  • Ovalbumin
  • Lactalbumin
• Plant proteins


  • Glutenins- Glutenin, oryzenin

  
  

  • Prolamines- gliadin, zein