10 Questions
Which type of linkage does the amide-bearing side chain of asparagine form with oligosaccharide units in glycoproteins?
Glycosidic linkage
What is the main reason for the ultraviolet absorption of most proteins in the 275-285 nm range?
Tryptophan content
In proteins, which group is subject to phosphorylation in some enzymes for the regulation of metabolic pathways?
Hydroxyl group of tyrosine
Which ionizable group enables amino acids to act as both acids and bases?
Carboxyl group
What effect does the imidazole group of histidine have on the buffering activities of proteins?
Stabilizes the protein structure
What is the role of the butylammonium side chain of lysine in proteins?
Binds with co-enzymes
Which side chain introduces a bend in the peptide chain?
Proline's cyclic pyrrolidine group
What is the primary form of interaction between reducing ends of oligosaccharides and serine/threonine residues?
Covalent bonding
How does tryptophan absorption at 280 nm help indirectly measure protein concentration?
It correlates with protein amount
What makes an amino acid an amphoteric substance?
It can act as both an acid and a base
Study Notes
Stereochemistry of Amino Acids
- The α-carbon of amino acids has four different groups attached, making it a chiral or asymmetric carbon, resulting in two possible enantiomers (L and D) that are non-superimposable mirror images and optically active
- Nearly all amino acids occurring in proteins are of the L-form, while D-amino acids are rare in nature
- Glycine is the only amino acid that lacks a mirror image pair of enantiomers and is not optically active
Classification of Amino Acids
- The 20 amino acids in proteins are encoded by DNA and can be designated by a three-letter abbreviation
- Classification schemes include polarity and charge on R groups, structure of side chains, catabolic fate of the amino acid, and the body’s ability to synthesize the amino acid
Classification based on Polarity and Charge on R Groups
- Amino acids are classified into three subgroups based on polarity and charge on their R groups
- Non-polar R Groups: eight amino acids with non-polar side chains, including alanine, valine, leucine, isoleucine, methionine, proline, phenylalanine, and tryptophan
- Uncharged Polar R Groups: amino acids with uncharged, polar side chains, including serine, threonine, glycine, asparagine, glutamine, and tyrosine
- Charged Polar R Groups: five amino acids with charged side chains, including lysine, arginine, histidine, glutamic acid, and aspartic acid
Classification based on Structure of Side Chain
- Aliphatic amino acids: branched chain amino acids (valine, leucine, isoleucine), sulphur-containing amino acids (methionine, cysteine), amide group-containing amino acids (asparagine, glutamine), hydroxy amino acids (serine, threonine, tyrosine), and simple amino acids (glycine and alanine)
- Aromatic amino acids: phenylalanine, tyrosine, and tryptophan
- Imino acid: proline
- Heterocyclic amino acids: histidine and tryptophan
- Dicarboxylic amino acids: aspartic acid and glutamic acid
Classification based on Catabolic Fate of Amino Acid
- Amino acids can be divided into three categories based on their catabolic fate: glucogenic, ketogenic, and those that can give rise to both glucose and ketone bodies
Explore the concept of stereochemistry in amino acids, focusing on the chiral nature of the α-carbon and the enantiomers L and D. Learn why most amino acids in proteins are of the L-form and the rarity of D-amino acids in nature.
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