Synthesis of Polypeptides

1
Synthesis of Polypeptides

• Polypeptide is synthesized by dehydration
  reaction
• Chain grows from amino terminus to carboxy
  terminus
• Chain has a repetitive backbone with variable
  side groups
• R groups frequently interact with others

2
Four Levels of Protein Structure

• Biological activity of protein is determined by
  these levels
• Primary structure is the sequence of amino acids
  in a polypeptide (Usually read N-C)
• Secondary structures are localized folds or
  helices that form within a region of a
  polypeptide
• Tertiary structures are larger folding events
  that are stabilized by interactions between R
  groups
• Quaternary structure is the interaction of
  multiple polypeptides within one active proteins

3
Primary Structure

• Sequence of amino acids within a single
  polypeptide
• Are often similar among proteins of similar
  function
• Usually written from amino terminus to carboxy
  terminus
• Can also provide some insight into additional
  structures by the position of particular groups
  of amino acids

4
Secondary Structure

• Localized within regions of polypeptide
• Stabilized by hydrogen bonding
• a helix-stabilized by frequent polar groups
• Right handed helices
• b-pleated sheets are formed by consecutive polar
  groups on two regions of polypeptide

5
Tertiary Structure

• Large folding events that are stabilized by
  interactions between amino acids
• Hydrophobic interactions
• Nonpolar regions generally internalize in
  structure
• Disulfide bridge
• Very stable bond formed between two distant
  cysteine residues
• Ionic interactions
• Strong bond between oppositely charged side groups

-Hydrogen bonds form between polar groups
6
Quaternary Structure

• Only seen in compound proteins
• Interactions are maintained between polypeptide
  chains by bonds similar to tertiary structure
• Function is often unique to quaternary structure
• Individual components are unable to accomplish
  task alone

7
Protein Structure Revisited
8
Protein Conformation

• The 3D structure in which the protein is
  biologically active is called the active
  conformation
• Denatured protein has lost its active
  conformation
• Shape of a protein is consistent under identical
  conditions
• Proteins will attempt to find the lowest energy
  form under conditions
• Conditions that affect conformation
• Solvent (polar versus non-polar),pH, temperature
  and chemical agents (2-mercaptoethanol)

9
Protein summary

• Very important biological macromolecules that
  perform a wide array of functions
• Polymers of amino acids
• 20 natural amino acids that have distinct R side
  groups
• The side groups determines the shape and function
  of a polypeptide
• There are four levels of structural organization
  of proteins

10
Nucleic Acids

• Deoxyribonucleic acid (DNA) and ribonucleic acid
  (RNA)
• Polymer of nucleotides
• Store cellular information
• Molecules of inherited information
• Basis for genetics
• Often large, very complex molecules
• Serve as templates for proteins
• Help control regulation of cellular functions

11
Nucleotides

• Comprises
• A pentose sugar
• A nitrogen containing base
• An organic phosphate group

12
Bases

• Two kinds of bases
• Pyrimidines are single ring structures
• cytosine, thymine and uracil
• Purines are double ring structures
• adenine and guanine
• In DNA G-C andA-T interactionss

13
Pentoses

• Deoxyribose in DNA and Ribose in RNA
• Only difference is the lack of an Oxygen at
  carbon 2 in deoxyribose
• Ring structures

14
Nucleic Acid Structure

• Phosphate group is attached to carbon 5 of
  pentose
• Oxygen of Phosphate forms a phosphodiester bond
  with the OH group on carbon 3 of the pentose
• Strands have a 5 end and a 3 end

15
DNA vs RNA

• Both are important for cell function
• DNA is double stranded and longer-lived
• RNA is single stranded and has shorter life
• Both can serve as a template for the synthesis of
  the other
• DNA to RNA is very common (transcription)
• RNA to DNA occurs only with retroviruses (reverse
  transcription)
• Both have 5-3 orientations

16
DNA Double Helix

• Structure elucidated by Watson and Crick
• From work of Wilkens and Franklin
• Two strands are anti-parallel
• 5end of each strand aligns with 3 end of
  complementary strand
• Bases pair by hydrogen bonding
• A-T (2 bonds)and G-C (3 bonds)
• Since strands are complementary they serve as a
  template for opposite strand
• Strands make Double Helix structure (twisted
  ladder)