Protein Structure & Function


In principle: Proteins are polymers of amino acids

Amino Acid structure

          [sometimes NH3+ & COO- : depends on pH] [iG1 7.02]

R = radical group:
    asymmetric (Homework); levo (L)-rotatory  [cf. dextro (D)-rotatory]

    determines biological properties: 20 types (note 1- & 3-letter codes) [iG1 7.Table 1] [iG1 7.03]
 

Group properties

Three- & Single-letter codes

 neutral, non-polar  (hydrophobic

gly,ala, val, leu, ile, pro, met, phe, trp

 

 G    A    V    L    I    P    M     F    W

 neutral, polar (hydrophilic)

gly, ser, thr, cys, tyr, asn, gln

 

S    T    C    Y    N    Q

polar basic  (positive charge)

lys, arg, his

 

 K    R    H

polar acidic (negative charge)

asp, glu

 

D    E

[Memorization of the abbreviations is not required for exams, but will make your lives as biologists easier!]


Dehydration of carboxy & amino termini forms peptide bond [iG1 7.05]

Dehydration

Peptidyl Transferase catalyzes analogous condensation reaction
      carboxyl (C) terminus of growing polypeptide in P site
            cleaved from the tRNA &

           
joined to amino (N) terminus of new amino acid in A site

                => amino end unchanged, carboxyl end "grows": in the last example,

            NH2 - fmet - phe - gly - pro - COOH   +   NH2 - lys - COOH

            NH2 - fmet - phe - gly - pro -  lys - COOH

    Recall that amino acid in A site is linked to  tRNA through COOH terminus
    Repeating, remnant backbone subunit [N - C(R) - C ] is an amino acid residue


Proteins have four levels of structure

     Primary Structure - order of amino acid residues in polypeptide
                20N possible, where N is number of residues
                Potential for enormous variety:
                    e.g., 205 = 3.2 x 106  possible pentapeptides

     Secondary Structure - configuration of [-N-C(R)-C-] backbone [iG1 6.06]
        alpha helix: a right-handed helix
        beta-pleated-sheet: parallel / antiparallel chains
             both stabilized by H-bonds

     Tertiary Structure - 3-Dimensional folding of backbone [iG1 7.07]
        cys + cys pairs form disulfide bridges ( - S - S -) [iG1 7.08]
        pro residues form hydrophobic "corners"
        hydrophilic residues occur on exterior,
                participate in reactions in aqueous environments
         hydrophobic residues occur in interior,
                interact with membrane lipid bi-layer
         gly fits in both hydrophobic & hydrophilic environments

     Quaternary Structure - assembly of multiple subunits [iG1 7.11]
            monomers / dimers / oligomers
               e.g., hemoglobin is a tetramer: two alpha + two beta chains
           charged residues (asp, glu, lys, arg, his) form ionic bonds bx subunits


Post-translational processing (IG1 Table 8.6)
   Chemical modification of amino acids
          addition of formyl group to met  fmet
   Addition of carbohydrate side chains (glycosylation) (IG1 8.21)
          e.g., ABO blood group antigen proteins
   Amino acids may be cleaved out of primary structure (IG1 8.22,23,25)
          e.g., biologically active insulin is less than half the primary  sequence (IG1 8.24)

Insulin Processing

    preproinsulin  proinsulin  insulin    HOMEWORK
        (110 aa's)            (86 aa's)        (51 aa's)


Detection of genetically-based variation in Proteins

       allelic variation translates to proteins with minor changes in electric charge
       allozymes arise from different alleles of the same protein gene locus
          substitution of alternatively charged amino acids changes overall charge
             [cf: Isozymes are homologous proteins encoded by different gene loci]

  Ex.: Sickle-cell hemoglobin (HbS) is a variant of standard HbA beta globin protein
            glu  val  in sixth residue of beta chain
             because GAG  GUG in sixth codon of beta-chain mRNA
                  HOMEWORK: What is the corresponding mutation in the DNA?
             "minus "neutral" charge net negative charge decreased
              val stabilizes crystalline form of hemoglobin    'sickling' of rbc's

        Homework: Critique the following statement:
           "
Electrophoresis of hemoglobin separates two alleles, F and S, of the Hb gene."


Overview of protein function


  
Enzymatic catalysis of biological reactions

       Substrates are bound in active sites: the Induced-Fit Model
        Lowered energy of activation
            biological reactions occur at body temperature
                                                          with lower energy input
        Anabolic - synthesis of complex molecules from simpler components
             Ex., transferases synthesize peptide bonds
                     synthetases attach tRNA to amino acid

       Catabolic - break-down of complex molecules into simpler components
             Ex., dehydrogenases remove protons (H+)
                     proteases split peptide bonds

    Structural motifs recur in proteins with similar functions

        Identification of motifs allows inferences about function (IG1 Res Briefing 9.1, pp. 170-171)
             Helix - turn - helix motifs binds Ca++ (IG1 RB9.1) (cf. iG1 7.12]
 
            Zinc - finger motifs binds major & minor DNA grooves (IG1 RB9.2)
             Leucine Zipper motif binds DNA and forms 'zippable' dimer (
IG1 RB9.3)

    Other protein functions
        Structural
            Collagen constitutes 25% of human protein
            Histones are the major components of chromosomes
                [online MGA animation of DNA packing into chromosomes]

       Nucleic Acid binding proteins
            Polymerases, nucleases, helicases, ligases, etc.

        Transport
            Hemoglobin in blood & myoglobin in muscle bind O2

        Miscellaneous
            immunoglobulins, hormones , etc.
            Major Histocompatibility Complex (MHC) determines transplant success

       Drosophila Genome Project has cataloged >11,000 genes with protein products
            > 1/2 have unknown functions

        Human Genome comprises ~20,500 protein-coding genes: why so few?

                Protein-coding regions may be transcribed in different ways from different promoters
                mRNAs may be spliced in different wasy to combine different exons


All text material 2013 by Steven M. Carr