proteins

proteins

1. essential constituents of all organisms

2. occur in great varieties

3. they are the first recognizable and distinctive expression of genetic information

4. functions

   • structural
     • structural proteins provide mechanical support and shape
       • collagen and elastin form the matrix for bones and ligaments, which provides structural strength of the organs and elasticity to the vascular system
   • dynamic
     • many proteins function as enzymes
       • for reactions to occur at a rate compatible with life they require a specific biochemical enzyme catalyst, which is capable of enhancing reaction rates a million time
       • thus the characteristics and functions of a cell are based on the specific enzyme make of the cell
     • some proteins bind other molecules for transport and storage
       • hemoglobin transports O2 and CO2 in blood erythrocytes
       • iron is stored as a complex with the protein ferritin in the liver
     • assemblies of proteins can do mechanical work such as coordinated motion
       • actin and myosin are proteins, which are major components of muscle, and they function in muscle contraction
       • contractile assemblies (spindle fibers) consisting of proteins produce movement/separation of chromosomes during mitosis
     • some proteins serve other specialised functions such as immune protection and defense proteins
       • immunoglobulins (antibodies) act against bacterial infections
       • interferon combats cell infected with viruses
     • many proteins are responsible for generation and transmission of nerve impulses
       • receptor proteins mediate the response of nerve cells to specific stimuli (hormones)
       • some proteins are hormones, which regulate biochemical activity in target cells or tissues
     • control of growth and differentiation
       • proteins are responsible for the controlled sequential expression of genetic information, which is essential for the orderly growth, and differentiation of cells

5. proteins are the workhorses of the cell, influencing almost every facet of the cells activity

   • therefore to understand the normal functioning of an organism, it requires a clear insight into the structure and properties of proteins

6. the chains comprising proteins unlike those of carbohydrates and fats are unbranched

7. the monomeric unit of a polypeptide chain is an amino acid

8. protein synthesis

   • DNA
     • transcription
       • mRNA
         • translation
           • protein

9. physical properties

   • ultraviolet absorption
     • absorbance range = 190 - 300 nm wavelength
       • far UV = 190 - 240 nm
         • the amide bond contributes to absorbance in this range and has a maximum absorption at 214 nm
       • near UV = 250 - 300 nm
         • this range includes absorption principally from the aromatic amino acids and cystine
       • measurement of light absorbance is an extremely rapid and convenient means of estimating the protein concentration of a solution
   • solubility
     • soluble in water
     • insoluble or slightly soluble in alcohol
     • insoluble in ether
     • exceptions
       • tyrosine - is more soluble in hot water
       • cystine - is difficult to solubilise in both hot and cold water
       • proline and hydroxyproline - is soluble in alcohol and ether
     • generally soluble in dilute acids and bases, in which they form their corresponding salts

10. chemical properties

    • electrolyte behaviour
      • amino acids in aqueous solutions are ionized and can act as acids or bases
        • knowledge of the acid-base properties of amino acids is extremely important in understanding the physical and biological properties of proteins
          • moreover the technology of separating, identifying and qualifying the different amino acids, which are necessary in determining the amino acid composition and sequence of protein molecules, is based largely on their characteristic electrolyte behaviour
      • predominant ionic forms
        • in the cationic form (acidic solution) the a-COOH group is protonated (-COOH) and the a-amino group is ionized (NH+3)
        • amino acids in solution at neutral pH are predominantly dipolar (or zwitterions - german for hybrid ions). in the dipolar form the a-amino group is protonated (NH+3) and the a-carboxylase group is dissociated (COO-).
        • in the anionic form (basic solution) the a-COOH group is dissociated (COO-) and the a-amino group is unionized (NH2)
        • depends on the pH of the solution and the nature of the R-group of an amino acid
        • isoelectric point = (pKa1 +pKa2)/2
      • henderson hasselbach equation
        • pH=pKa + log[A-/HA]