Proteins Summary

• Standard amino acids


  • Alanine (Ala, A)
  • Valine (Val, V)
  • Leucine (Leu, L)
  • Isoleucine (Ile, I)
  • Proline (Pro, P)
  • Methionine (Met, M)
  • Phenylalanine (Phe, F)
  • Tryptophan (Trp, W)
  • Glycine (Gly, G)
  • Serine (Ser, S)
  • Threonine (Thr, T)
  • Tyrosine (Tyr, Y)
  • Cysteine (Cys, C)
  • Glutamine (Gln, Q)
  • Asparagine (Asn, N)
  • Aspartic acid (Asp, D)
  • Glutamic acid (Glu, E)
  • Histidine (His, H)
  • Lysine (Lys, K)
  • Arginine (Arg, R)

  
• Standard amino acid


  • One of the 20 α-amino acids normally found in proteins
  • General structure includes an amino group and a carboxyl group, both bonded to the α-carbon, which is also bonded to a hydrogen and to the side chain group (R group)

  
• R group


  Determines the identity of the particular amino acid

  
• Categories of amino acids

  • Nonpolar (hydrophobic)
  • Polar (uncharged)
  • Polar Acidic
  • Polar Basic

  
• Nonpolar amino acid


  • Contains one amino group, one carboxyl group, and a nonpolar side chain
  • Hydrophobic, generally found in the interior of proteins

  
• Polar neutral amino acid


  • Contains one amino group, one carboxyl group, and a side chain that is polar but neutral
  • More soluble in water than nonpolar amino acids

  
• Polar acidic amino acid


  • Contains one amino group and two carboxyl groups, the second carboxyl group being part of the side chain
  • Side chain bears a negative charge at physiological pH

  
• Polar basic amino acid

  • Contains two amino groups and one carboxyl group, the second amino group being part of the side chain
  • Side chain bears a positive charge at physiological pH

  
• All of the standard amino acids are necessary constituents of human proteins
• Essential amino acids


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

  
• Arginine is required for growth in children but is not an essential amino acid for adults
• 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
• Limiting amino acid


  An essential amino acid that is missing, or present in inadequate amounts, in an incomplete dietary protein
• Protein from animal sources is usually complete dietary protein
• Protein from plant sources tends to be 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
• Chiral


  • Molecules with a chiral center that has a carbon atom with four different groups bonded to it

  
• 1. amino acids


  The amino acids found in proteins, not superimposable on their mirror images (D-amino acids)

  
• Rules for drawing Fischer projection formulas for amino acid structures

  1. The -COOH group is put at the top, the R group at the bottom

  2. The -NH2 group is in a horizontal position, on the left for L-isomer, on the right for D-isomer

  
• Amino acids in pure form

  • White crystalline solids
  • Relatively high decomposition points
  • Not very soluble in water due to strong intermolecular forces

  
• 1. isomer


  Preferred for monosaccharides
• Drawing Fischer projection formulas for amino acid structures

  1. The -COOH group is put at the top, the R group at the bottom

  2. The -NH2 group is in a horizontal position, left for L-isomer, right for D-isomer

  
• Amino acids in pure form

  • White crystalline solids
  • Relatively high decomposition points
  • Not very soluble in water due to strong intermolecular forces

  
• Amino acids


  Have both an acidic group (-COOH) and a basic group (-NH2) on the same carbon
• Amino acid behavior in neutral solution

  1. Carboxyl groups tend to lose protons, producing a negatively charged species

  2. Amino groups tend to accept protons, producing a positively charged species

  3. The -COOH group donates a proton to the -NH2 of the same amino acid, resulting in a zwitterion

  
• Zwitterion


  A molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge

  
• Amino acid form changes with pH

  1. In acidic solution, positively charged species predominates

  2. In neutral solution, zwitterion predominates

  3. In basic solution, negatively charged species predominates
• Amino acid form at different pH levels:

  
• Isoelectric point


  The pH at which an amino acid exists primarily in its zwitterion form

  
• Most amino acids have isoelectric points between 4.8-6.3, basic amino acids have higher, acidic amino acids have lower

  
• Peptide


  An unbranched chain of covalently linked amino acids
• Peptide bond


  A covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid
• The reaction between a carboxylic acid and an amine produces an amide

  
• Peptide chain direction


  1. terminal end to C-terminal end, N-terminal on left, C-terminal on right
• Amino acid residue


  The portion of an amino acid structure that remains after release of H2O when it becomes part of a peptide chain
• A peptide has a regularly repeating backbone and a variable sequence of R groups
• Peptide nomenclature rules


  1. terminal amino acid keeps full name, other amino acids end in -yl, naming sequence from N-terminal

  
• Isomeric peptides with same amino acids in different order have different properties
• Small peptide hormones (oxytocin, vasopressin)

  • Nonapeptides with disulfide bond forming a loop
  • Oxytocin regulates uterine contractions and lactation
  • Vasopressin regulates water excretion and blood pressure
• Small peptide neurotransmitters (enkephalins)

  • Pentapeptides that bind to receptors in brain to reduce pain
  • Met-enkephalin and Leu-enkephalin differ only in C-terminal amino acid