Ammo acids

Example Miller and Rich investigated the conformational consequences of substitution s on an amino acid in cyclosporin A. an importan t iininunosnppressive dnig. One of th e ammo acids iii this cyclic iin decapeptide is (2.v, ir. 4r, 5f )-3-nydroxy-4-methyl-2-(methylamino)-6-octenoic acid (MeBmtli. It is essential for biological activitv. 

Some amides, ammo acids, di and polycimino compounds, amino alcohols... 

You will have noticed that, throughout this chapter, the heteroatom has always been hie nucleophile. There is one way to use nitrogen as an electrophile however and this provides a good synhion for ammo acid synthesis ... 

Some of the most important organic compounds in biochemistry are the a ammo acids rep resented by the general formula shown... 

A quote from a biochemistry text IS instructive here This IS not an easy reaction in or game chemistry It is how ever a very important type of reaction in metabolic chemistry and is an integral step in the oxidation of car bohydrates fats and several ammo acids G L Zubay Biochemistry 4th ed William C Brown Publishers 1996 p 333... 

This reaction is catalyzed by an enzyme called a dehydratase and is one step along the pathway by which plants convert glucose to certain ammo acids... 

The term chiral recognition refers to a process m which some chiral receptor or reagent interacts selectively with one of the enantiomers of a chiral molecule Very high levels of chiral recognition are common m biological processes (—) Nicotine for exam pie IS much more toxic than (+) nicotine and (+) adrenaline is more active than (—) adrenaline m constricting blood vessels (—) Thyroxine an ammo acid of the thyroid gland that speeds up metabolism is one of the most widely used of all prescription... 

An unknown acetylenic ammo acid obtained from the seed of a tropical fruit has the molec ular formula C7H11NO2 On catalytic hydrogenation over platinum this ammo acid yielded homoleucme (an ammo acid of known structure shown here) as the only product What is the structure of the unknown ammo acid" ... 

An example of a biologically important aide hyde is pyridoxal phosphate which is the active form of vitamin Bg and a coenzyme for many of the reac tions of a ammo acids In these reactions the ammo acid binds to the coenzyme by reacting with it to form an imine of the kind shown in the equation Re actions then take place at the ammo acid portion of the imine modifying the ammo acid In the last step enzyme catalyzed hydrolysis cleaves the imme to pyridoxal and the modified ammo acid... 

A standard method for the preparation of an a ammo acid uses a bromo carboxylic acids as the substrate and aqueous ammonia as the nucleophile... 

An imidazole ring is a structural unit m the ammo acid histidine (Section 27 1) and is involved m a large number of biological processes as a base and as a nucleophile... 

Nucleophilic substitution by ammonia on a halo acids (Section 19 16) The a halo acids obtained by halogenation of car boxylic acids under conditions of the Hell-Volhard-Zelinsky reaction are reac tive substrates in nucleophilic substitu tion processes A standard method for the preparation of a ammo acids is dis placement of halide from a halo acids by nucleophilic substitution using excess aqueous ammonia... 

Although this reaction is useful for preparing a ammo acids (Table 22 3 fifth entry) it IS not a general method for the synthesis of amines Its major limitation is that the expected primary amine product is itself a nucleophile and competes with ammonia for the alkyl halide... 

Carey Organic Chemistry I 27 Ammo Acids Peptides I Text Fifth Edition and Proteins... 

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

Ammo acids are carboxylic acids that contain an amine function An amide bond between the carboxylic acid function of one ammo acid and the ammo nitrogen of another is called a peptide bond... 

A dipeptide is a molecule consisting of two ammo acids joined by a peptide bond A tnpeptide has three ammo acids joined by two peptide bonds a tetrapeptide has four ammo acids and so on Peptides with more than 30-50 ammo acids are polypeptides Proteins are polypeptides that have some biological function... 

As m most aspects of chemistry and biochemistry structure is the key to function We 11 explore the structure of proteins by first concentrating on their fundamental building block units the a ammo acids Then after developing the principles of peptide structure we 11 see how the insights gamed from these smaller molecules aid our under standing of proteins... 

Ammo acids are classified as a p 7 and so on according to the location of the amine group on the carbon chain that contains the carboxylic acid function... 

Aminocyclopropanecarboxylic acid an a ammo acid that is the biological precursor to ethylene in plants... 

Allhough more lhan 700 differenl ammo acids are known lo occur nalurally a group of 20 of Ihem commands special allenlion These 20 are Ihe ammo acids lhal are normally presenl m proteins and are listed m Table 27 1 All Ihe ammo acids from which proteins are derived are a ammo acids and all bul one of Ihese conlam a primary ammo funclion and conform lo Ihe general slruclure... 

Table 27 1 includes Ihree teller and one teller abbrevialions for Ihe ammo acids Bolh enjoy wide use... 

Our bodies can make some of Ihe ammo acids shown m Ihe fable The olhers which are called essential amino acids, we have to gel from whal we eal... 

Ammo acids with nonpolar side chains... 

Proline is relatively compact because of the cyclic nature of its side chain It has less conformational flexibility than the other ammo acids and the presence of proline affects the shape of a peptide more than other ammo acids... 

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... 

Ammo acids with polar but nonwmzed side chains Among ammo acids with polar side chains serine is the smallest it is not much larger than alanine With a —CH2OH side chain serine participates well m hydrogen bonding and often occurs m regions of a peptide that are exposed to water... 

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