Protein Structures: Primary, Secondary, Tertiary, and Quaternary, Slides of Biochemistry

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Peptides Return to TOC

Biochemically Important Small Peptides

• Many rela)vely small pep)des are biochemically ac)ve:
  • Hormones
  • Neurotransmi9ers
  • An)oxidants
• Small Pep)de Hormones:
  • Best-­‐known pep)de hormones: oxytocin and vasopressin
  • Produced by the pituitary gland
  • nonapep)de (nine amino acid residues) with six of the residues held in the form of a loop by a disulfide bond formed between two cysteine residues

Biochemically Important Small Peptides

Biochemically Important Small Peptides

ACTH – Adrenocorticotropic

Hormone

Biochemically Important Small Peptides

• Pep)des with An)bio)c Ac)vity:

Biochemically Important Small Peptides

• Pep)des with An)bio)c Ac)vity: Penicillin
  • Valine and cysteine are found in penicillin but there are linkages other than the pep)de linkage. The NH 2 of cysteine is acylated, in most cases by the benzyl radical.
  • Drugs in the penicillin class work by indirectly burs)ng bacterial cell walls. They do this by ac)ng directly on pep)doglycans, which play an essen)al structural role in bacterial cells.
  • Penicillins block the protein/ enzyme transpep)dases that crosslink the pep)doglycans together. This prevents the bacterium from closing the holes in its cell walls.

Biochemically Important Small Peptides

• Pep)des with An)bio)c Ac)vity: Tyrocidines
  • Mixture of cyclic decapep)des produced by the bacteria Bacillus brevis found in soil
  • It can be composed of 4 different amino acid sequences, giving tyrocidine A–D.
  • Major cons)tuent of tyrothricin which also contains gramicidin
  • First commercially available an)bio)c, but has been found to be toxic toward human blood and reproduc)ve cells.
  • It has a unique mode of ac)on in which it disrupts the cell membrane func)on, making it a favorable target for engineering deriva)ves.[
  • It appears to perturb the lipid bilayer of a microbe's inner membrane by permea)ng the lipid phase of the membrane.

Biochemically Important Small Peptides

Biochemically Important Small Peptides Met-Enkephalin Leu-Enkephalin

Biochemically Important Small Peptides Small Pep)de An)oxidants

• Glutathione (Glu–Cys–Gly) – a tripep:de – is present is in high levels in most cells
• Regulator of oxida:on–reduc:on reac:ons.
• Glutathione is an an:oxidant and protects cellular contents from oxidizing agents such as peroxides and superoxides - Highly reac:ve forms of oxygen oDen generated within the cell in response to bacterial invasion
• Unusual structural feature – Glu is bonded to Cys through the side-­‐chain carboxyl group.

Biochemically Important Small Peptides

General Structural Characteristics of Proteins

• General defini)on: A protein is a naturally-­‐occurring, unbranched polymer in which the monomer units are amino acids.
• Specific defini)on: A protein is a pep)de in which at least 40 amino acid residues are present: - The terms polypep)de and protein are o[en used interchangeably used to describe a protein - Several proteins with >10,000 amino acid residues are known - Common proteins contain 400–500 amino acid residues - Small proteins contain 40–100 amino acid residues
• More than one pep)de chain may be present in a protein:
  • Monomeric : A monomeric protein contains one pep)de chain
  • Mul)meric: A mul)meric protein contains more than one pep)de chain

General Structural Characteristics of Proteins Four Levels of Structures

• Primary Structure
• Secondary Structure
• Ter)ary Structure
• Quaternary
• Primary Structure: Primary structure of protein refers

to the order in which amino acids are linked together

in a protein

• Every protein has its own unique amino acid sequence
  • Frederick Sanger (1953) sequenced and determined the primary structure for the first protein -­‐ Insulin

General Structural Characteristics of Proteins Return to TOC 1 o

• AA seq; all covalent bonds between AA’s