Aminotransferases and Transaminations

Protein Degradation

• The half-life of a cytosolic protein is determined by its amino-terminal residue, known as the N-terminal rule.
• A yeast protein with methionine at its N terminus has a half-life of more than 20 hours, whereas one with arginine at this position has a half-life of about 2 minutes.
• E3 enzymes are the readers of N-terminal residues, and they favor rapid ubiquitination if the residue is destabilizing (e.g., arginine or leucine) and do not favor ubiquitination if the residue is stabilizing (e.g., methionine or proline).
• Other signals that identify proteins for degradation include cyclin destruction boxes and PEST sequences.

Human Papilloma Virus (HPV)

• HPV encodes a protein that activates a specific E3 enzyme, which ubiquitinates the tumour suppressor p53 and other proteins that control DNA repair, leading to their destruction.
• The activation of this E3 enzyme is observed in more than 90% of cervical carcinomas.

Proteasome

• The proteasome is the protein that executes proteolysis, using the energy of ATP to hydrolyze peptide bonds.
• The proteasome has a sedimentation coefficient of 26S and is composed of 2 subunits: a 20S proteasome and a 19S regulatory subunit.
• The 20S complex contains 2 copies of 14 subunits and has a mass of 700 kD.

Transaminations

• Aminotransferases catalyze the transfer of an α-amino group from an α-amino acid to an α-ketoacid.
• The α-amino group of many amino acids is transferred to α-ketoglutarate to form glutamate.
• Glutamate can transfer the amino group to oxaloacetate to form aspartate or glutamine can undergo oxidative deamination to produce α-ketoglutarate and ammonia.

Aspartate Aminotransferase (AST)

• AST catalyzes the transfer of an amino group from aspartate to α-ketoglutarate to form oxaloacetate and glutamate.
• This transamination reaction is reversible.

Alanine Aminotransferase (ALT)

• ALT catalyzes the transfer of an amino group from alanine to α-ketoglutarate to form pyruvate and glutamate.

Glutamate Dehydrogenase

• Glutamate dehydrogenase catalyzes the conversion of glutamate to α-ketoglutarate and ammonia.
• The enzyme can use NAD+ or NADP+ as the oxidizing agent.
• The oxidation produces α-iminoglutarate, which is hydrolyzed to form ammonia and α-ketoglutarate.

Regulation of Protein Degradation

• The degradation of proteins regulates a number of functions, including gene transcription, cell-cycle progression, organ formation, circadian rhythm, inflammatory response, tumor suppression, cholesterol metabolism, and antigen processing.
• The degradation of proteins is dynamically controlled by altering the stability and abundance of regulatory proteins.
• For example, the control of the inflammatory response is regulated by the degradation of an inhibitory protein, I-kB, which allows the transcription factor NF-κB to stimulate transcription of target genes.