protein

• Proteins


  naturally occurring, unbranched polymer in which the monomer units are amino acid.
• casein


  main protein of milk; contains phosphorus.
• hemoglobin


  oxygen-transporting protein of blood; contains iron.
• Amino Acid


  organic compound that contains both an amino group or nitrogen group ( − NH2), and a carboxyl group (−COOH).
• backbone – peptide bond
• 𝛂-amino acid

  amino acid in which the amino group and the carboxyl group are attached to the αcarbon atom.
• R


  is called the amino acid side chain
• standard amino acid

  – one the 20 α-amino acids normally found in proteins.
• Types of Amino Acids

  four types: nonpolar, polar acidic, polar neutral, and polar basic.
• Nonpolar Amino Acid


  contains one amino group, one carboxyl group, and a nonpolar side chain
• hydrophobic


  insoluble in water.
• tryptophan


  weakly interact through hydrogen bonding with the NH ring location.
• Polar Acidic Amino Acid


  amino acid that contains one amino group and two carboxyl groups, the second carboxyl group being part of the side chain.
• Polar Neutral Amino Group


  amino acid that contains one amino group (amide – NH), one carboxyl group (alcohol – OH), and a side chain that is polar but neutral.
• amino acid that contains two amino groups (NH2 or N2) and one carboxyl group, the second amino group being part of the side chain.

  Polar Basic Amino Acid
• essential amino acid


  standard amino acid needed for protein synthesis that must be obtained from dietary source because the human body cannot synthesize it in adequate amount from other substances
• arginine


  – only needed for growth in children
• complete dietary protein


  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.
• incompletely 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
• animal protein


  complete dietary protein; meat, fish, and eggs.
• plant protein


  – incomplete dietary protein
• ; limiting amino acids

  – lysine (wheat, rice, oats, and corn), methionine (beans and peas), and tryptophan (corn and beans).
• soy


  complete dietary protein
• complementary dietary proteins


  two or more incomplete dietary proteins that, when combines, provide an adequate amount of all essential amino acids relative to the body’s needs.
• Chirality and Amino Acids


  → 19 out of the 20 standard amino acids possess a chiral center.

  → each of the 19 amino acids exist in left and righthanded forms
• D – NH2 on the right.
• L – NH2 on the left.
• Acid-Base Properties of Amino Acids


  in pure form amino acids are white crystalline solids; most amino acids decompose before they melt.

  → not very soluble in water due to strong intermolecular forces within their crystal structures.
• −COOH → −COO − + H +


  in neutral solution, carboxyl groups have a tendency to lose protons (H+ ): produces negative charge.
• −NH2 + H + → N +H3

  → in neutral solution, amino groups have a tendency to accept protons (H+ ); produces a positive charge.
• zwitterion


  “double ion”; molecule that has a positive charge on one atom and a negative charge on another atom, but which has no net charge:
• basic solution


  the N +H3 of the zwitterion loses a proton, and a negatively charge species is formed.
• the zwitterion accepts a proton to form a positively charged ion.

  acidic solution
• neutral form

  no net charge; exists primarily in zwitterion form; important pH value, relative to the various form an amino acid can have in a solution
• 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
• cystine


  – two cysteine residues linked via a covalent disulfide bond.
• Cysteine


  the only standard amino acid with a sulfhydryl group (— SH group).
• peptide – unbranched chain of amino acids
• tripeptide – bond between three amino acid