In the alpha amino acids, the α–carbon is a chiral carbon atom, with the exception of glycine .

2. Amino acids are biologically important organic compounds made from amine (-NH2) and carboxylic acid (-COOH) functional groups, along with a side-chain specific to each amino acid. • The key elements of an amino acid are carbon, hydrogen, oxygen, and nitrogen, though other elements are found in the side-chains of certain amino acids.

3. In the form of proteins, amino acids comprise the second largest component (after water) of human muscles, cells and other tissues. • Outside proteins, amino acids perform critical roles in processes such as neurotransmitter transport and biosynthesis. • These are all L-stereoisomers ("left-handed" isomers) although a few D-amino acids ("right-handed") occur in bacterial envelopes and some antibiotics.

4. Many important proteinogenic and non-proteinogenic amino acids also play critical non-protein roles within the body. • For example: • in the human brain, glutamate and GABA are respectively the main excitatory and inhibitory neurotransmitters • Hydroxyproline (a major component of the connective tissue collagen) is synthesised from proline • The standard amino acid glycine is used to synthesise porphyrins used in red blood cells • The non-standard carnitine is used in lipid transport.

5. 9 of the 20 standard amino acids are called "essential" for humans because they cannot be created from other compounds by the human body, and so must be taken in as food.

6. In the structure shown, R represents a side-chain specific to each amino acid. The carbon atom next to the carboxyl group is called the α–carbon and amino acids with a side-chain bonded to this carbon are referred to as alpha amino acids. • These are the most common form found in nature.

7. In the alpha amino acids, the α–carbon is a chiral carbon atom, with the exception of glycine. • In amino acids that have a carbon chain attached to the α–carbon, the carbons are labeled in order as α, β, γ, δ, and so on • In some amino acids, the amine group is attached to the β or γ-carbon, and these are therefore referred to as beta or gamma amino acids.

8. Amino acids are usually classified by the properties of their side-chain into four groups. The side-chain can make an amino acid a weak acid or a weak base, and a hydrophile if the side-chain is polar or a hydrophobe if it is nonpolar. • The phrase "branched-chain amino acids" or BCAA refers to the amino acids having aliphatic side-chains that are non-linear; these are leucine, isoleucine, and valine.

9. Proline is the only proteinogenic amino acid whose side-group links to the α-amino group and, thus, is also the only proteinogenic amino acid containing a secondary amine at this position. • In chemical terms, proline is, therefore, an imino acid, since it lacks a primary amino group, although it is still classed as an amino acid in the current biochemical nomenclature, and may also be called an "N-alkylated alpha-amino acid".

10. Proteins are made of Amino Acid

12. 1-Classification according to the structure of the side chain R. a)Aliphatic amino acids ; They are amino acids with aliphatic groups in their side chains . b)Aromatic amino acids; They are amino acids with aromatic groups in their side chains (contain a phenyl group). 2-Classification according to the polarity of the side chain R. a) Polar amino acids ; 1-Polar uncharged amino acids . 2-Polar charged amino acids which can be; 3-Polar acidic or positively charged amino acids. 4-Polar basic or negatively charged amino acids. b) Non-polar amino acids. 3- Classification based on their nutritional value. Essential and non Essential 4-Classification based on their metabolic fate. Ketogonic and glucogonicand both

13. Classification of Amino Acids • As the properties of the amino acids and their role in proteins are determined by the side chain , therefore amino acids are classified according: 1-Polarity of the side chain. -Acid -Basic -Neutral 2- Structure of side chain - Aliphatic - Aromatic

17. 1-Essential amino acids ;They are those amino acids that cannot be synthesized in the body thus they are essential in the diet, inculde: 2- Non-essential amino acids; They are those amino acids that can be synthesized in the body thus they are non-essential in the diet.

18. AMINO ACID CLASSIFICATION side Depending on chain structure • Aliphatic side chain • aromatic side chain • Sulphur – containing side chain • Acidic side chain • Basic side chain • Amide side chain • Hydroxylic side chain • Imino Side Chain

19. Of the standard α-amino acids, all but glycine can exist in either of two enantiomers, called L or D amino acids, which are mirror images of each other. • While L-amino acids represent all of the amino acids found in proteins during translation in the ribosome, D-amino acids are found in some proteins produced by enzyme posttranslational modifications after translation and translocation to the endoplasmic reticulum, as in exotic sea-dwelling organisms such as cone snails. They are also abundant components of the peptidoglycan cell walls of bacteria, and D-serine may act as a neurotransmitter in the brain.

20. Zwitter ions • Thus, at pH between 2.2 and 9.4, the predominant form adopted by α-amino acids contains a negative carboxylate and a positive α-ammonium group, as shown in structure (2) , so has net zero charge. This molecular state is known as a zwitter ion .

21. Amino acids have zero mobility in electrophoresis at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isolectric point and some amino acids (in particular, with non-polar side-chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point.

26. Proteins functions • Most abundant and functionally diverse group of molecules • Indispensable for life • Have several diverse functions: - Catalytic functions [enzymes] - Receptor [insulin receptor] - Structural function [collagen] - Transport [haemoglobin, myoglobin] - Protective functions [immunoglobulins] - Hemostasis [clotting factors] - Hormonal functions [insulin, glucagon, GH] - Control of gene expression [transcription factors] - DNA packing [histones] - Act as buffers