AMINO ACIDS 5 6 NB 20 amino acids

AMINO ACIDS

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NB: 20 amino acids enter in protein structure. There are other amino acids in the body, but they do not enter in protein e. g. ornithine, citrulline are amino acids with other functions in the body. They enter in formation of urea in the liver.

Free Amino Acid Pool • Free amino acids can come from plasma or muscle • From muscle due to it’s mass that contains ~75% of the total body free AA • Still, free AA are thought to contribute only ~1% to metabolically active AA pool.

. • Amino acids are essential to integrity of skeletal muscle, their use for energy is of concern • Estimated amino acids contribute 5 -15% of energy during prolonged exercise • Because energy demands are so high during exercise, a small percentage is still substantial

General Structure of Amino Acid • Carboxylic acid group • Amino group • Side group R gives unique characteristics

Examples of Amino Acids COOH I H 2 N—C — H I H glycine COOH I H 2 N—C — H I CH 3 alanine 11

Don’t Be Confused COOH I H 2 N—C — H I H glycine H 2 N I H —CH — COOH glycine H I H 2 N—C — COOH I H glycine 12

AMPHOTERIC PROPERTIES OF AMINO ACIDS: that is they have both basic and acidic groups and so can act as a base or acid. This is called a zwitter ion Neutral amino acids (monobasic, monocarboxylic) can exist in aqueous solution as “ Zwitterion” i. e. contain both positive and negative charge. Zwitterion is electrically neutral and can’t migrate into electric field.

The presence of both an acid (carboxyl) and a base (amine) in the same molecule leads to an interaction between the two. This interaction results in a transfer of a hydrogen ion from the acid portion to the base portion of the molecule. 14

The net charge of the zwitterion is 0. This leaves the acid end of the amino acid with a negative charge (–COO-) and a positive charge at the base end (–NH 3 +). The deprotonated portion (portion that has lost a hydrogen ion) is a carboxylate group, and the protonated group (group that has gained a hydrogen ion) is an ammonium group. THE PRESENCE OF A CHARGE ON THE AMINO ACID MAKES THEM WATER-SOLUBLE. Figure shows zwitterion formation. 15

ZWITTERION

Amino acids are polar • Due to presence of – polar covalent bonds N, O and H – Atoms are capable to form hydrogen bonds with water – Carry charges COO- and NH 3+ The water solubility of amino acids vary to some extend, depending of side chain

CLASSIFICATION OF AMINO ACIDS

CLASSIFICATION ACCORDING TO POLARITY OF SIDE CHAIN (R): A- Polar amino acids: in which R contains polar hydrophilic group so can forms hydrogen bond with H 2 O. : Have an UNEVEN distribution of electrons such as Acids and Bases In those amino acids, R may contain: 1 - OH group : as in serine, threonine and tyrosine 2 - SH group : as in cysteine 3 - amide group: as in glutamine and aspargine 4 - NH 2 group or nitrogen act as a base (basic amino acids ): as lysine, arginine and histidine 5 - COOH group (acidic amino acids): as aspartic and glutamic.

CLASSIFICATION ACCORDING TO POLARITY OF SIDE CHAIN (R): B- Non polar amino acids: : Have an EVEN distribution of electrons R is alkyl hydrophobic group which can’t enter in hydrogen bond formation. 9 amino acids are non polar ( glycine, alanine, valine, leucine, isoleucine, phenyl alanine, tryptophan, proline and methionine)

NONPOLAR AMINO ACIDS cont …. Each of these amino acids has a nonpolar side chain that does not gain or lose protons or participate in hydrogen or ionic bonds. The side chains of these amino acids can be thought of as “oily” or lipid-like, a property that promotes hydrophobic inter-actions. 21

C. AMINO ACIDS WITH ACIDIC SIDE CHAINS The amino acids aspartic and glutamic acid are proton donors. At physiologic p. H, the side chains of these amino acids are fully ionized, containing a negatively charged carboxylate group (–COO–). They are, therefore, called aspartate or glutamate to emphasize that these amino acids are negatively charged at physiologic p. H 22

D. AMINO ACIDS WITH BASIC SIDE CHAINS The side chains of the basic amino acids accept protons. At physiologic p. H the side chains of lysine and arginine are fully ionized and positively charged. 23

AMINO ACIDS WITH BASIC SIDE CHAINS …. cont In contrast, histidine is weakly basic, and the free amino acid is largely uncharged at physiologic p. H. However, when histidine is incorporated into a protein, its side chain can be either positively charged or neutral, depending on the ionic environment provided by the polypeptide chains of the protein. This is an important property of histidine that contributes to the role it plays in the functioning of proteins such as hemoglobin 24

NUTRITIONAL CLASSIFICATION: 1 - Essential amino acids: These amino acids can’t be formed in the body and so, it is essential to be taken in diet. Their deficiency affects growth, health and protein synthesis. H LIT TV LAMP H= histidine* L= lysine I = isoleucine T = tryptophan T= threonine V= valine L= leucine A = arginine* M= methionine P= phenyl alanine *= arginine and histidine are semiessential

Nutritional classification: 2 - Semiessential amino acids: These are formed in the body but not in sufficient amount for body requirements especially in children. *= arginine and histidine are semiessential 3 - Non essential amino acids: These are the rest of amino acids that are formed in the body in amount enough for adults and children. They are the remaining 10 amino acids.

METABOLIC CLASSIFICATION: according to metabolic or degradation products of amino acids they may be: 1 - Ketogenic amino acids: which give ketone bodies. Lysine and Leucine are the only pure ketogenic amino acids. 2 - Mixed ketogenic and glucogenic amino acids: which give both ketonbodies and glucose. These are: isoleucine, phenyl alanine, tyrosine and tryptophan. 3 - Glucogenic amino acids: Which give glucose. They include the rest of amino acids. These amino acids by catabolism yields products that enter in glycogen and glucose formation.

Ketogenic amino acids Leucine Lysine