Amino acid 4

Question 1

Proteins and Peptides

Answer 1

Peptides: short polymers of amino acids in a peptide chain (up to about 100
amino acids)

• Proteins: long polymers of amino acids in a polypeptide chain (usually > 100 amino acids)

Question 2

The amino acids in peptides and proteins are bound to each other via

Answer 2

peptide bonds

Question 3

condensation reaction

Answer 3

The net reaction for peptide bond formation between 2 amino acids

— between the αcarboxyl group of one amino acid and the
α-amino group of the next amino acid,

—- producing one molecule of water.

Question 4

AMIDE LINKAGES …TRIPEPTIDE

Answer 4

Amino acids are covalently linked to one another by the formation of peptide bonds. These bonds form amide linkages.

• The linkage of two amino acids together by a single peptide bond creates a dipeptide. The linkage of three amino acids through 2 peptide bonds creates a tripeptide etc etc.

Question 5

Depicting Peptides

Answer 5

By convention when depicting peptides, the free amino group is written on the left hand side and is called the N-terminal amino acid or the amino terminus.

The free carboxyl group is written on the right hand side and is called the C-terminal amino acid or the carboxyl terminus.

Linked amino acids
forming a peptide can
also be depicted using
the three of one letter
codes:
Ex.
Val-Cys-Ala-Glu-His

Question 6

Protein Synthesis – Peptide Bond Formation

Answer 6

Reaction takes place in the ribosome and is catalysed by the peptidyl transferase enzyme component

Question 7

Protein Degradation
Peptide Bond Hydrolysis

Answer 7

Catalysed by peptidase/proteases

*Exopeptidases/exoproteases: catalyse the hydrolysis of either the amino
terminal peptide bond of a polypeptide chain (aminopeptidases) or the
carboxy terminal peptide bond (carboxypeptidases). Not specific for any
particular amino acids

*Endopeptidases/endoproteases: catalyse the hydrolysis of peptide bonds
inside the polypeptide chain and are usually fairly specific e.g.trypsin only
catalyses the hydrolysis of peptide bonds C-terminal to Arg or Lys.

• Proteins are initially acted on by endoproteases to break them up into
  peptides and then by exopeptidases to release the amino acids which are
  then reused for protein synthesis or sometimes are oxidised as metabolic
  fuels.

Question 8

Characteristics of Peptide Bonds

Answer 8

• Peptide bonds have approximately a 40% double-bond character because of electron delocalization.
  Peptide bond resonance structures
• The partial double-bond
  character prevents free
  rotation about the peptide
  bond. Therefore the
  carbon and nitrogen atoms
  of the peptide bond and
  the two atoms to which
  each is attached are held
  in a plane configuration.
• The double bond character of the peptide bond is also
  evident in the bond lengths