Proteins

• The two main stages of protein synthesis are transcription (DNA to mRNA) and translation (mRNA to protein).
• An extraordinary number of different proteins, each with different functions, exist in the human body
• A typical human cell contains about 9,000 different kinds of proteins, and the human body contains about 100,000 different proteins
• Proteins are needed for the synthesis of


  • Enzymes
  • Certain hormones
  • Some blood components
  • New tissues
• Proteins are also needed for the maintenance and repair of existing tissues
• Proteins are also sometimes needed for energy
• Protein


  A naturally-occurring, unbranched polymer in which the monomer units are amino acids

  
• Proteins are most abundant molecules in the cells after water – account for about 15% of a cell's overall mass
• Elemental composition of proteins

  • Carbon (C)
  • Hydrogen (H)
  • Nitrogen (N)
  • Oxygen (O)
  • Sulfur (S)

  
• The average nitrogen content of proteins is 15.4% by mass
• Other elements present in some specialized proteins

  • Iron (Fe)
  • Phosphorus (P)
  • Magnesium (Mg)
• Amino acid


  An organic compound that contains both an amino (-NH2) and carboxyl (-COOH) groups attached to same carbon atom (α-amino acid)

  
• The position of carbon atom is Alpha (α)
• Over 700 naturally occurring amino acids are known but only 20 of them, called standard amino acids, are normally present in proteins

  
• Common names


  Currently used for naming amino acids
• Three letter abbreviations


  Widely used for naming amino acids
• One-letter symbols


  Commonly used for comparing amino acid sequences of proteins
• Standard amino acids are divided into four groups based on the properties of R-groups


  • Non-polar
  • Polar-neutral
  • Polar acidic
  • Polar basic

  
• Non-polar amino acids

  1. groups are non-polar, hydrophobic or "water fearing" (insoluble in water)
• Polar-neutral amino acids


  Contains one amino group, one carboxyl group and a polar but neutral side chain
• Polar acidic amino acids

  Contains one amino group, two carboxyl groups, the 2nd carboxyl group as part of the side chains

  
• Polar basic amino acids

  Contains two amino groups, one carboxyl group, the 2nd amino group as part of the side chain

  
• Essential amino acid


  An amino acid needed in the human body that must be obtained from dietary sources because it cannot be synthesized within the body from other substances in adequate amounts
• A complete dietary protein is a protein that contains all the essential amino acids
• Chirality


  Four different groups are attached to the α-carbon atom in all of the standard amino acids except glycine

  
• In glycine R-group is hydrogen
• 19 of the 20 standard amino acids contain a chiral center

  
• The amino acids found in nature as well as in proteins are L isomers

  
• Zwitterion


  An ion with + (positive) and – (Negative) charges on the same molecule with a net zero charge
• Isoelectric point (pI)


  pH at which the concentration of Zwitterion is maximum -- net charge is zero
• At isoelectric point - amino acids are not attracted towards an applied electric field because they net zero charge
• Electrophoresis


  The process of separating charged molecules on the basis of their migration toward charged electrodes associated with an electric field
• Cysteine


  The only standard amino acid with a sulfhydryl group ( — SH group)

  
• Cystine

  Two cysteine residues linked via a covalent disulfide bond

  
• Peptide


  An unbranched chain of amino acids
• Peptide bond


  A covalent bond between the carboxyl group of one amino acid and the amino group of another amino acid

  
• Dipeptide

  Bond between two amino acids
• Oligopeptide


  Bond between ~ 10 - 20 amino acids

  
• Polypeptide


  Bond between large number of amino acids

  
• 1. terminal end


  The end with the free NH3+