3) Stereoisomerism of - amino acids
The four bonds of the central (alpha) carbon (C) of an amino acid are directed towards the four corners of a tetrahedron. With respect to the carboxyl (COOH) and amino(NH2) groups, there are two possible arrangements of the H and Radical group. These arrangement are literally mirror images of each other, and are called stereoisomers (AKAenantiomers) Stereoisomers are designated D (dextro-rotatory) or L (levo-rotatory) according to the direction in which the crystalline forms rotate polarized light, to the right and left, respectively. Naturally-occuring proteins comprise exclusively the L forms of amino acids.
In
this example, Alanine has
a CH3 as
a radical group. If you imagine holding the model with the COOH at
the top and the NH2 at
the bottom, the CH3 radical
group in the D form
will be on your right.
In the mirror image L form,
it will be on the left.
4) Synthesis of amino acids in living organism:
a) transamination reaction of amino-acids
в) reductionaminationof- keto acids
a) Transamination as the name implies, refers to the transfer of an amine group from one molecule to another. In the degradation of most standard amino acids, an early step in degradation consists in transamination, which is the transfer of the α-amino group from the amino acid to anα-keto acid. This reaction is catalyzed by a family of enzymes called transaminases. Actually, the transamination reaction results in the exchange of an amine group on one acid with a ketone group on another acid. It is analogous to a double replacement reaction.
The most usual and major keto-acid involved with transamination reactions is alpha-ketoglutaric acid, an intermediate in the citric acid cycle. A specific example is the transamination of alanine to make pyruvic acid and glutamic acid.
The mechanism of transamination for alanine, yet is the same with all transaminases. The reaction occurs in two stages:
Transfer of the amino group from alanine to the enzyme, which releases pyruvate, and
Transfer of the amino group from the enzyme to α-ketoglutarate, which releases glutamate.
b)Reductive amination- is a means of converting an aldehyde or a ketone into an amine. The reaction begins with nucleophilic addition of ammonia or an amine to the carbonyl group of an aldehyde or a ketone. The imine or enamine derivative formed is subsequently reduced by hydrogen to form the amine product. If the nucleophile is ammonia, a primary amine will be formed in reductive amination. If the nucleophile is a primary amine, the product will be a secondary amine. With a secondary amine nucleophile, a tertiary amine product will be obtained. Recall that the nucleophilic addition of ammonia or primary amine results in imine formation and addition of a secondary amine to an aldehyde or ketone results in enamine formation. Both are ultimately reduced to form amines in reductive amination.
5. Chemical properties of amino acids.
a) acid-base properties
b) Chirality in Amino Acids
c) Desamination reaction;
d) Formation of ester,halogenanhydrates, decarboxylation (Acetylation, Reduction, Reaction with Nitrous acid, Reaction involving both the carboxyl & the amino group)