Phenylalanine protein incorporation

Phenylalanine and tryptophan have side chains that incorporate aromatic rings which are large and hydrophobic The aromatic portion of tryptophan is bicyclic which makes it larger than phenylalanine Tryptophan also has a more electron rich aromatic ring and is more polarizable than phenylalanine Its role is more specialized and it is less abundant m proteins than most of the other ammo acids... 

Cystic fibrosis, a disease of the Caucasian population, is associated with defective CL regulation and is essentially a disorder of epithehal cells (113,114). The defect arises at several levels in the CL ion transporter, ie, the cystic fibrosis transmembrane regulation (CFTR), and is associated with defective CL transport and defective processing, whereby the protein is not correctiy incorporated into the cell membrane. The most common mutation, affecting approximately 60% of patients, is termed F 608 and designates the loss of phenylalanine at this position. This mutation appears to be at least 50,000 years old, which suggests that its survival may have had evolutionary significance (115). 

The specific ribonucleotide sequence in mRNA forms a message that determines the order in which amino acid residues are to be joined. Each "word," or codon, along the mRNA chain consists of a sequence of three ribonucleotides that is specific for a given amino add. For example, the series UUC on mRNA is a codon directing incorporation of the amino acid phenylalanine into the growing protein. Of the 43 = 64 possible triplets of the four bases in RNA, 61 code for specific amino acids and 3 code for chain termination, fable 28.1 shows the meaning of each codon. 

Selection of these regulatory mutants is often done by using toxic analogues of amino adds for example p-fluoro-DL-phenylalanine is an analogue of phenylalanine. Mutants that have no feedback inhibition or repression to the amino add are also resistant to the analogue amino add. They are therefore selected for and can be used to overproduce the amino add. Some amino add analogues function as false co-repressors, false feedback inhibitors or inhibit the incorporation of foe amino acid into foe protein. 

The cells of all contemporary living organisms are surrounded by cell membranes, which normally consist of a phospholipid bilayer, consisting of two layers of lipid molecules, into which various amounts of proteins are incorporated. The basis for the formation of mono- or bilayers is the physicochemical character of the molecules involved these are amphipathic (bifunctional) molecules, i.e., molecules which have both a polar and also a non-polar group of atoms. Examples are the amino acid phenylalanine (a) or the phospholipid phosphatidylcholine (b), which is important in membrane formation. In each case, the polar group leads to hydrophilic, and the non-polar group to hydrophobic character. 

Figure 17.4 Ketone derivatives of phenylalanine and mannose can be fed to cells to incorporate the monomers into proteins and glycans. The resultant modifications can be probed using hydrazide-containing reagents.

In the chemical communication many peptide-protein (e.g., peptide hormone agonist) or protein-protein signaling interaction take place. Various photoreactive amino acids, e.g., azido-phenyalanines (Apa, TFApa) [40, 41], benzoyl-phenyalanines (Bpa,p-OH-Bpa) [41,42],trifluorometil-diazirine-phenylalanine (TMDPhe) [43], were developed (Fig. 3), which can be incorporated into any places in peptide sequences by standard solid-phase synthetic techniques. 

Tyrosinase is a monooxygenase which catalyzes the incorporation of one oxygen atom from dioxygen into phenols and further oxidizes the catechols formed to o-quinones (oxidase action). A comparison of spectral (EPR, electronic absorption, CD, and resonance Raman) properties of oxy-tyrosinase and its derivatives with those of oxy-Hc establishes a close similarity of the active site structures in these proteins (26-29). Thus, it seems likely that there is a close relationship between the binding of dioxygen and the ability to "activate" it for reaction and incoiporation into organic substrates. Other important copper monooxygenases which are however of lesser relevance to the model studies discussed below include dopamine p-hydroxylase (16,30) and a recently described copper-dependent phenylalanine hydroxylase (31). 

Aromatic amino acids are biogenetic precursors of neuroamines (dopamine, serotonin, histamine, etc.). On the other hand, phenylalanine (Phe) is frequently present in peptide sequences, while tyrosine is an important site of phosphorylation of proteins. Aromatic amino acids and neuroamines fluorinated on the aromatic ring have been the focus of many investigations. Indeed, after incorporation in polypeptides and proteins, they can be used as probes in NMR and in PET. 

Similarly, lysine has been incorporated into gluten hydrolyzate and lysine, threonine and tryptophan have been individually incorporated into zein hydrolyzates. Lysine, methionine, and tryptophan were incorporated simultaneously into hydrolyzates of protein from photosynthetic origin. A very interesting application of this procedure involved the preparation of low-phenylalanine plasteins from a combination of fish protein concentrate and soy protein isolate by a partial hydrolysis with pepsin then pronase to liberate mainly phenylalamine, tyrosine, and tryptophan, which were then removed on sephadex G-15. Desired amounts of tyrosine and tryptophan were added back in the form of ethyl esters and a plastein suitable for feeding to infants afflicted with phenylketonuria was produced. 

Many biologically important routes of amino acid utilization, other than those leading to incorporation into proteins, are known. Some of these routes are distinctly anabolic pathways in which the amino acids serve as an initial substrate in an independent biosynthetic pathway. Other simple pathways involve the conversion of one amino acid to another, such as the formation of tyrosine from phenylalanine. The utilization of glycine in the formation of porphyrin derivatives occurs by very complex highly branched pathways. Some other biologically important pathways lead to the biosynthesis of small peptides as in the biosynthesis of glutathione. 

Dopamine may alternatively be formed from tyrosine via hydroxylation of L-dopa which is decarboxylated. However, inverse isotope dilution experiments to study the formation of dopamine and dopa have shown that this is probably a minor pathway in peyote (176). It has been shown that L-tyrosine is incorporated into alkaloids in peyote three times more efficiently than into protein (344). 4-Hydroxy-3-methoxyphenethylamine can be methylated to 3,4-dimethoxy-phenethylamine (homoveratrylamine), which may be viewed as a dead-end product in Scheme 2 (10, 203). Phenylalanine is probably not a precursor of the... 

Figure 3 Phosphonomethyl phenylalanine (Pmp) and phosphonodifluoromethyl phenylalanine (F2Pmp) are nonhydrolyzable phosphotyrosyl (pTyr) mimetics. Flexameric peptide sequences that incorporate either Pmp or F2Pmp inhibit protein tyrosine phosphatase 1B. The difluoro- analogs are most potent, presumably because of their reduced pKa relative to the phosphonate.

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