IGEM:IMPERIAL/2009/Encapsulation/PKU

=Phenylketonuria - PKU=

Absorption in gut
Phenylalanine is absorbed as an amino acid in our gut. absorption This is because the hydrolysis of peptides involving phenylalanine is fast enough. It takes 175 minutes or approximately 3 hours for all phenylalanine related peptides to be digested, after which, phenylalanine will be present as an amino acid to be absorbed. absorption1 For this experiment, volunteers are fed a milk-protein meal.

absorption2



Great book on digestion 

Metabolism of Phenylalanine
Phenylalanine is fully catabolised to CO2 and water, after its hydroxylation to tyrosine. This is the rate limiting step in the whole process. The complex system requires PAH, tetrahydrobiopterin co-enzyme (BH4) and several enzymes for the regeneration of BH4. The characteristics of phenylalanine metabolism have been modelled MetabolismModel MetabolismModel2.

Conversion to tyrosine





Phenylalanine is converted to tyrosine by phenylalanine hydroxylase. Tyrosine synthesis in the body depends on the availability of phenylalanine. When phenylalanine intake is limited, tyrosine can become a conditionally indispensable amino acid.



The carboxyl carbon of the tyrosine is released as CO2 when tyrosine degrades. The remainder of the molecule is converted to fumarate or acetoacetate, which enter the tricarboxylic acid cycle.

 

PAH PhenylAlanine Hydroxylase
This is the enzyme that catalyses the reaction from phenylalanine to tyrosine. It is naturally produced by the liver. However, it was expressed in E. Coli and shown to be functional at pH 7.0 PAH1. Furthermore, phosphorylation by the C-subunit kinase rendered the enzyme resistant to the Xa protease (fig 1 below) which is a member of the serine protease family PAH1. The serine protease family includes some of the most abundant proteases present in the human gastro-intestinal tract such as chemotrypsin-like, signal peptidases (More Info on Serine Proteases). Therefore we think that by fully phosphorylating the PAH enzyme during its production process, we can protect it against the protease-rich environment of the small intestine. The PAH enzyme is activated by L-Phenylalanine PAH1 and thus is expected to be activated in the environment of the small intestine where food is naturally degraded into amino acids. These amino acids are subsequently absorbed by the gut.