PROTEINS

• Carbon Atom


  Center of amino acid structure
• Side Chain


  Part of amino acid structure attached to the carbon atom
• Carboxyl Group


  • COOH group in amino acid structure
• Amino Group


  • NH2 group in amino acid structure
• Proteins


  • Naturally occurring, unbranched polymer in which the monomer units are amino acids
  • Account for about 15% of the cell's overall mass
  • Contain the elements: carbon, hydrogen, oxygen, nitrogen, and some also contain sulfur
• Amino Acid


  • Organic compound that contains both an amino group or nitrogen group (-NH2), and a carboxyl group (-COOH)
  • Amino acids in proteins are always α-amino acids
• Backbone

  Peptide bond
• R

  Amino acid side chain
• Types of Amino Acids

  • Nonpolar
  • Polar Acidic
  • Polar Neutral
  • Polar Basic
• Nonpolar Amino Acids


  • Contain one amino group, one carboxyl group, and a nonpolar side chain
  • Hydrophobic, insoluble in water
• Polar Acidic Amino Acids

  • Contain one amino group and two carboxyl groups, the second carboxyl group being part of the side chain
• Polar Neutral Amino Acids

  • Contain one amino group (amide - NH), one carboxyl group (alcohol - OH), and a side chain that is polar but neutral
• Polar Basic Amino Acids

  • Contain two amino groups (NH2 or N2) and one carboxyl group, the second amino group being part of the side chain
• Essential Amino Acids

  • Arginine
  • Histidine
  • Isoleucine
  • Leucine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Valine

  
• Complete Dietary Protein


  Contains all of the essential amino acids in the same relative amounts in which the body needs them
• Incomplete Dietary Protein


  Does not contain adequate amounts, relative to the body's needs, of one or more of the essential amino acids
• Limiting Amino Acid

  Essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein
• Limiting Amino Acids in Plant Proteins

  • Lysine (wheat, rice, oats, and corn)
  • Methionine (beans and peas)
  • Tryptophan (corn and beans)
• 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
• Chirality of Amino Acids

  • 19 out of the 20 standard amino acids possess a chiral center
  • Each of the 19 amino acids exist in left and right-handed forms

  
• L

  NH2 group on the left
• R

  NH2 group on the right
• Acid-Base Properties of Amino Acids

  • In neutral solution, carboxyl groups have a tendency to lose protons (H+), producing negative charge
  • In neutral solution, amino groups have a tendency to accept protons (H+), producing positive charge
  • Amino acids in solution exist in three different species (zwitterions, positive ion, and negative ion); equilibrium shifts with change in pH
• Zwitterion

  Molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge
• Isoelectric Point


  pH at which an amino acid exists primarily in a zwitterion form; pH at which the concentration of Zwitterion is maximum; net charge is zero
• Isoelectric Points of Amino Acids

  • alanine (6.01)
  • arginine (10.76)
  • asparagine (5.41)
  • aspartic acid (2.77)
  • cysteine (5.07)
  • glutamic acid (3.22)
  • glutamine (5.65)
  • glycine (5.97)
  • histidine (7.59)
  • isoleucine (6.02)
  • leucine (5.98)
  • lysine (9.74)
  • methionine (5.74)
  • phenylalanine (5.48)
  • proline (6.48)
  • serine (5.68)
  • threonine (5.87)
  • tryptophan (5.88)
  • tyrosine (5.66)
  • valine (5.97)

  
• Cysteine

  • The only standard amino acid with a sulfhydryl group (-SH group)
  • Cystine - two cysteine residues linked via a covalent disulfide bond
• Peptide


  Unbranched chain of amino acids
• Types of Peptides

  • Dipeptide (two amino acids)
  • Tripeptide (three amino acids)
  • Oligopeptide (~ 10 - 20 amino acids)
  • Polypeptide (large numbers of amino acids)
• Peptide Bond


  • Covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid
  • Water is a byproduct of peptide bond formation
• Amino Acid Residue


  Portion of an amino acid structure that remains after the release of H2O, when an amino acid participates in peptide bond formation
• Peptide Nomenclature

  1. terminal amino acid residue keeps its full name, other residues have names ending in -yl, sequence begins at N-terminal

  
• Isomeric peptides

  Peptides that contain the same amino acids but in different order are different molecules (constitutional isomers) with different properties
• Biochemically Important Small Peptides

  • Hormones
  • Neurotransmitters
  • Antioxidants
• Small Peptide Hormones

  • Oxytocin and vasopressin (antidiuretic hormone) - best-known peptide hormones; produced by the hypothalamus stored in the posterior pituitary gland.
  • nonapeptides (9 amino acid residues) with a disulfide bond

  
• Small Peptide Neurotransmitters

  • Enkephalins - reduce pain
  • (met-enkephalin and leu-enkephalin) - best known enkephalins
• Small Peptide Antioxidants

  • Glutathione (glu-cys-gly) - a tripeptide that regulates oxidation-reduction reactions and protects cells from oxidizing agents
• General Structural Characteristics of Proteins

  • Proteins contain at least 40 amino acid residues
  • May have one or more peptide chains (monomeric or multimeric)
  • Simple proteins contain only amino acid residues, conjugated proteins contain additional non-amino acid entities (prosthetic groups)
• Primary Structure of Proteins

  Order in which amino acids are linked together in a protein
• Peptide chain


  A chain of amino acids linked by peptide bonds