CHEM 445 / BIOL 445
Biochemistry II

J. D. Cronk
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26. header

Lecture 26. PLP-dependent reactions. The urea cycle.

Friday 30 March 2007

The versatility of PLP as a coenzyme is demonstrated by the variety of transformations at the a, b, and g carbons of amino acid substrates.

Reading: BTS6, Ch.23, pp.658-672.

 
25. Summary

Lecture 26 Summary

The aminotransferase reaction, as well as many other metabolic reactions involving amino acids or amines, is dependent upon enzymes using pyridoxal phosphate (PLP) as a cofactor.

In most typical circumstances, an animal must excrete excess nitrogen from amino acids. Metabolically, this is equivalent to ammonia or ammonium ion. The latter is assimilated into urea and excreted as such by most terrestrial vertebrates. A metabolic cycle called the urea cycle is linked to the citric acid cycle, and is responsible for the synthesis of urea.

Pyridoxal phosphate (PLP)

  
Structural diagram of pyridoxal phosphate (PLP)   Pyridoxal phosphate (PLP) is derived from pyridoxine (or Vitamin B6). This impressively versatile cofactor catalyzes reactions involving amino acid substrates. The common feature of all these reactions is the formation of a Schiff base linkage between the a-amino group of the amino acid and the 4' aldehyde group of PLP (red in figure at left). This substrate-PLP Schiff base is often referred to as the external aldimine (see figure below), distinguishing it from the internal aldimine, the resting state of PLP enzymes in which PLP is covalently linked as a Schiff base with a lysine residue.  

 

 

Study questions

  • Diagram a scheme for each of the following PLP-mediated reactions: i. racemization of an a-amino acid, ii. a-decarboxylation of an a-amino acid, and iii. a, b elimination from an a-amino acid.

Page updated 12-27-06

References

  1. Berg, Tymoczko, and Stryer. Biochemistry (BTS): 6th edition (2007, Freeman) Ch.23, pp.656-660.
  2. Staunton J. Primary Metabolism: A Mechanistic Approach (1978, Oxford University Press)
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