Peptides acids Proteins

As constituents of proteins the amino-acids are important constituents of the food of animals. Certain amino-acids can be made in the body from ammonia and non-nitrogenous sources others can be made from other amino-acids, e.g. tyrosine from phenylalanine and cystine from methionine, but many are essential ingredients of the diet. The list of essential amino-acids depends partly on the species. See also peptides and proteins. 

Besides such textual databases that provide bibhographic information, sequence databases have attained an even more important role in biochemistry. Sequence databases are composed of amino add sequences of peptides or proteins as well as nudeotide sequences of nudeic acids. The 20 amino adds are mostly represented by a three-letter code or by one letter according to the biochemical conventions) the four nudeic adds are defined by a one-letter code. Thus the composition of a biochemical compound is searchable by text retrieval methods. 

The relationship between structure and function reaches its ultimate expression in the chemistry of ammo acids peptides and proteins... 

CHAPTER TWENTY SEVEN Ammo Acids Peptides and Proteins... 

Individual ammo acids differ m their acid-base properties This is important m peptides and proteins where the properties of the substance depend on its ammo acid constituents especially on the nature of the side chains It is also important m analyses m which a complex mixture of ammo acids is separated into its components by taking advantage of the differences m their proton donating and accepting power... 

Electrophoresis is used primarily to analyze mix tures of peptides and proteins rather than individual ammo acids but analogous principles apply Because they incorporate different numbers of ammo acids and because their side chains are different two pep tides will have slightly different acid-base properties and slightly different net charges at a particular pH Thus their mobilities m an electric field will be differ ent and electrophoresis can be used to separate them The medium used to separate peptides and proteins is typically a polyacrylamide gel leading to the term gel electrophoresis for this technique... 

The amount of sample required is quite small as little as 10 mole is typical So many peptides and proteins have been sequenced now that it is impossible to give an accurate count What was Nobel Prize winning work m 1958 is routine today Nor has the story ended Sequencing of nucleic acids has advanced so dramatically that it is possible to clone the gene that codes for a particular protein sequence its DNA and deduce the structure of the protein from the nucleotide sequence of the DNA We 11 have more to say about DNA sequencing m the next chapter... 

---