Protein Half-Life and Amino Acid Residues Quiz

Amino Acids

• Collagen is an exception to the standard amino acid composition, with one-third of its composition being glycine.
• There are 20 standard amino acids and 11 non-standard amino acids.
• Pyrrolysine is an example of a non-standard amino acid, used by some methanogenic bacteria in enzymes to produce methane.
• Selenocysteine is incorporated into some proteins at a UGA codon, which is normally a stop codon.

Classification of Amino Acids

• Based on structure:
  • Aliphatic (non-aromatic hydrocarbon compounds)
  • Aromatic (having aroma/fragrant)
  • Heterocyclic (cyclic compounds in which at least one of its ring members is not a carbon atom)
• Imino acid (simple amino acids)
• Branched chain amino acids: valine, leucine, isoleucine
• Hydroxyl amino acids: serine, threonine
• Sulphur-containing amino acids: cysteine, methionine
• Amino acids with amide group: asparagine, glutamine

Amino Acid Properties

• Histidine:
  • Can boost histamine levels in the body when taken with vitamin B3 and vitamin B6
  • Important for normal sexual functioning
  • Low levels contribute to the development of rheumatoid arthritis and nerve damage-related deafness
• Aromatic amino acids:
  • Tyrosine, tryptophan, phenylalanine
  • Least hydrophobic to most hydrophobic

Protein Structure

• Proteins are polymers of amino acids linked by peptide bonds.
• All proteins are polymers, with the monomers being α-amino acids.
• Most amino acids in the body are α-amino acids.
• There are 20 different kinds of amino acids coded for in the genes and incorporated into proteins.

Amino Acid Abbreviations

• Alanine: A or Ala
• Arginine: R or Arg
• Asparagine: N or Asn
• Aspartic acid: D or Asp
• Cysteine: C or Cys
• Glutamine: Q or Gln
• Glutamic acid: E or Glu
• Glycine: G or Gly
• Histidine: H or His
• Isoleucine: I or Ile
• Leucine: L or Leu
• Lysine: K or Lys
• Methionine: M or Met
• Phenylalanine: F or Phe
• Proline: P or Pro
• Serine: S or Ser
• Threonine: T or Thr
• Tryptophan: W or Trp
• Tyrosine: Y or Tyr
• Valine: V or Val

α-Carbon

• The α-carbon is the carbon next to the carboxyl group.
• Amino group is attached to the α-carbon.
• To the α-carbon of every amino acid are also attached a hydrogen atom and a side chain (R).
• Different α-amino acids are distinguished by their different side chains.

Chirality

• Whenever a carbon atom has 4 different substituents attached to it, it becomes a center of chirality (stereocenter or asymmetric carbon).
• All of the amino acids incorporated by organisms into proteins are of the L form.
• Most naturally occurring polysaccharides involve D-sugars, but the L isomer is preferred.

Glycine

• Glycine is a nonpolar amino acid.
• Its side chain is a hydrogen atom.
• Because there is a second hydrogen atom at the α-carbon, glycine is not optically active.
• Glycine is very evolutionarily stable at certain positions of some proteins.

Protein Folding

• Secondary and tertiary levels of protein structure are reserved by noncovalent forces or bonds.
• Types of bonds:
  • Hydrogen bonds
  • Electrostatic bonds
  • Hydrophobic interactions
• Leptin folding pattern is compatible with the helical cytokines.

Plasma Proteins

• Principal plasma proteins:
  • Prealbumin
  • Albumin
  • α1-globulin
  • α2-globulin
  • β-globulin
  • γ-globulin
• Albumin:
  • Most abundant plasma protein
  • Major contribution to oncotic pressure of plasma
  • Important determinant of the distribution of extracellular fluid between the intravascular and extravascular compartments
  • Synthesized in the liver
  • Concentration reflects the functional capacity of the organ
  • Functions: regulates colloidal osmotic pressure of blood, binds water, cations, fatty acids, hormones, bilirubin, thyroxine (T4), and drugs.