Protein

• The living matter is composed of mainly carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. These elements together constitute about 90% of the dry weight of the human body
• Other functionally important elements found in cells

  • Ca
  • K
  • Na
  • Cl
  • Mg
  • Fe
  • Cu
  • Co
  • I
  • Zn
  • F
  • Mo
  • Se
• Chemical composition of the human body

  Water is the solvent of life and contributes to more than 60% of the weight, followed by protein (mostly in muscle) and lipid (mostly in adipose tissue), carbohydrate content is rather low in the form of glycogen
• Water


  • The most abundant chemical compound in living human cells, accounting for 65 percent to 90 percent of each cell, also present between cells e.g. in blood and cerebrospinal fluid
• Fat


  • The percentage varies from person to person, but even an obese person has more water than fat
• Protein


  • In a lean male, the percentages of protein and water are comparable, about 16 percent by mass, found in muscles, hair, fingernails, and skin
• Minerals


  • Account for about 6 percent of the body, include salts and metals like sodium, chlorine, calcium, potassium, and iron
• Carbohydrates


  • Although used as an energy source, only account for about 1% of body mass
• Carbon is the most predominant and versatile element of life, possessing the ability to form stable covalent bonds and C-C chains of unlimited length, about 90% of compounds found in living systems contain carbon
• Most elements are found within compounds, water and minerals are inorganic compounds, organic compounds include fat, protein, carbohydrates, and nucleic acids
• Proteins


  The most versatile macromolecules in living systems, serving crucial functions in essentially all biological processes including as catalysts, transport, structural support, immune protection, movement, nerve impulses, growth and differentiation
• Origin of the word 'protein'

  Derived from the Greek word proteios, meaning holding the first place, suggested by Berzelius and Mulder to refer to the high molecular weight nitrogen-rich and most abundant substances present in animals and plants
• Functions of proteins


  • Structural (e.g. collagen, elastin, keratin)
  • Dynamic (e.g. enzymes, hormones, blood clotting factors, immunoglobulins, membrane receptors, storage proteins, genetic control, muscle contraction, respiration)
• Proteins may also contain other elements such as P, Fe, Cu, I, Mg, Mn, Zn
• Proteins


  Polymers of amino acids, formed by the incorporation of 20 standard amino acids during synthesis, some of which undergo post-translational modifications
• Central dogma of molecular biology

  The flow of genetic information from DNA to RNA to proteins, first proposed by Francis Crick
• Amino acids


  Organic compounds containing amino and carboxyl functional groups, exist in ionized form in biological systems
• Optical isomerism of amino acids

  • Amino acids (except glycine) have four distinct groups attached to the alpha carbon, resulting in left-handed (L) and right-handed (D) optical isomers, proteins are composed of L-alpha amino acids
• The preference for L-amino acids in proteins is likely due to their slightly higher solubility compared to racemic mixtures
• Classification of amino acids based on polarity

  • Non-polar (hydrophobic)
  • Polar with no charge
  • Polar with positive charge
  • Polar with negative charge
• Nutritional classification of amino acids

  • Essential (indispensable)
  • Non-essential (dispensable)
  • Semi-essential
• Physical properties of amino acids

  • Soluble in water, insoluble in organic solvents, high melting points, varying tastes (sweet, tasteless, bitter)
• Aspartame


  An artificial sweetener 200 times sweeter than sucrose, a methyl ester of the aspartic acid/phenylalanine dipeptide, most stable at pH 4.3, deemed safe for human consumption by regulatory agencies
• Chemical properties of amino acids

  Undergo decarboxylation to form amines, dicarboxylic amino acids react with ammonia to form amides, act as ampholytes (can donate or accept protons)
• Aspartame


  Most stable at pH 4.3, where its half-life is nearly 300 days
• Aspartame has been deemed safe for human consumption by over 100 regulatory agencies in their respective countries, including the FDA
• Amino acids


  General reactions are mostly due to the presence of two functional groups: carboxyl (COOH) group and amino (NH2) group
• Decarboxylation


  Amino acids undergo decarboxylation to produce corresponding amines
• Reaction with ammonia


  The carboxyl group of dicarboxylic amino acids reacts with NH3 to form amide
• Amino acids as ampholytes

  Amino acids contain both acidic (COOH) and basic (NH2) groups, they can donate a proton or accept a proton
• Zwitterion or dipolar ion

  A hybrid molecule containing positive and negative ionic groups
• Amino acids rarely exist in a neutral form with free carboxylic (COOH) and free amino (NH2) groups
• Isoelectric point (pI)


  The pH at which a molecule exists as a zwitterion or dipolar ion and carries no net charge
• Isoelectric point (pI)


  • The negatively and positively charged molecular species are present in equal concentrations
  • The molecule is electrically neutral
• Neutral side chain amino acids

  • Characterized by two pKas: pKa1 for the carboxylic acid and pKa2 for the amine
  • The isoelectric point is the average of these two pKas
• Acidic side chain amino acids


  The isoelectric point is at a lower pH because the acidic side chain introduces an "extra" negative charge
• Basic side chain amino acids

  The isoelectric point is at a higher pH because the basic side chain introduces an "extra" positive charge
• Electrophoresis

  Used to show the distribution of charged species in a sample by observing the movement of solute molecules in an electric field
• Peptide bond


  Formed when the amino group of an amino acid combines with the carboxyl group of another amino acid
• Peptides containing more than 10 amino acids are referred to as polypeptides