Amino Acids (Physical Therapy)

Amino Acids Physical therapy Level I Prof. Fathia El Sharkawi * Topics of first and second lectures: *What are proteins? *Functions of proteins *Composition of proteins *Classifications of amino acids *Properties of amino acids *Zewitter ion *Structure of proteins *Classifications of proteins *Physical and chemical properties of proteins *Denaturation of proteins AMINO ACIDS AND PROTEINS • Proteins are the most characteristic chemical compounds found in the living cells. They have high molecular weights and each protein is composed of approximately 20 different kinds of amino acids linked to each other in large numbers. Many proteins contain all of the 20 amino acids and therefore differ from each other with respect to the number and sequence of their constituent amino acids. Functions of proteins 1. Some proteins serve as important structural elements of the body, e.g. as hair, wool, collagen and important constituents of the connective tissues. 2. Enzymes are biologically active proteins. 3. Some hormones as insulin, glucagon and some electron carriers e.g. cytochromes are proteins or containing protein structure. 4. Other proteins participate in muscular contraction and some are associated with the genes. 5. The antibodies (gama globulins) are proteins. 6. The plasma proteins (albumins and globulins) are involved in regulation of the osmotic pressure of the blood. 7. Blood clotting factors are proteins Composition of proteins: .All proteins contain carbon, hydrogen, oxygen and nitrogen, most of them also contain sulphur and some contain phosphorus. • All proteins by decomposition (hydrolysis) yield cheifly various α-amino acids as final products: • Classification of amino acids 1- According to the polarity of the R- group:a. Non polar (hydrophobic) amino acids: • the amino acid contain aliphatic side chains (e.g. valine, alanine, leucine, isoleucine, & methionine) or which contain aromatic side chains (e.g. Phenylalanine & tryptophan). a. Polar amino acids: • the amino acids contain hydroxyl group (tyrosine, serine & threonine) or sulfhydryl group (cysteine) or amide group (asparagine) or CooH group (aspartic) or basic group (lysine). Classification of amino acids • 2- According to their chemical structure: • The usual classification of amino acids depends on their chemical structure. • They are classified as amino acids containing aliphatic, aromatic, hydroxyl, sulphohydryl, acidic and basic side chains. Classification of amino acids 3-According to nutritional requirements: Essential amino acids are those which cannot be synthesized in the body so they are only supplied by diet. Other amino acids can be synthesized in the body so not essential in diet so called non-essential amino acids. • The essential amino acids are: valine, leucine, isoleucine, threonine, methionine, lysine, histidine, phenylalanine and tryptophan. It must be noticed that arginine is essential in infants as it is synthesized only in adults liver. Essential amino acids maintain the normal growth and the nitrogen balance in the body. Classification of amino acids • 4- According to their biochemical fate in the body: a. Glycogenic amine acids: are amino acids that can be transfered in the body into glucose as glycine, alanine, serine, valine ... etc. b. Ketogenic amino acids: are amino acids that only can be transfered to ketone bodies in the body e.g. leucine and lysine c. Mixed amino acids: amino acids that are both glycogenic and ketogenic e.g. phenylalanine, tyrosine tryptophan and isoleucine. Properties of amino acids: • Amino acids are soluble in water, acidic or basic aqueous solutions but are insoluble in organic solvents being precipitated from aqueous solutions by alcohol. Their high melting point (>200 °C) reflect the presence of charged groups i.e. the high energy needed to disrupt the ionic forces maintaining the crystal lattice. • Amphoteric properties of amino acids” • Amino acids are ampholytes i.e. they behave both as a weak acid and as a weak base since they each contains at least one carboxyl group and one amino group. In alkaline solution amino acids behave like an acid: Zwitter ion • In acid solution amino acids behave like bases: Monoamino-monocarboxylic amino acids exist in aqueous solution as dipolar molecules (Zwitter ions). The α-carboxyl group is dissociated and negatively charged while the α-amino group is protonated and positively charged thus the overall molecule is electrically neutral Zwitter ion (Zero charge) R Isoelectric pH (pI) I +H N - C 3 +H R Aqueous soln I pH 7 (neutral) H2N - C - H -H I I COO- OHCOOH H2O Buffer soln pH 3 (acidic) R I +H N - C - H 3 [+1] I COOH R I H2N - C - H I COO [-1] Buffer soln pH 10 (alkaline) PH of amino acids • The pH at which the number of +ve charges on an amino acid equals the number of –ve charges is called the isoelectric point (pI). At this pH the amino acid has a net charge zero and do not migrate in an electric field • Net charge of amino acids at neutral pH (physiological pH): • At physiologic pH, all amino acids have a negatively charged group (-COO-) and a positively charged group (- NH3 +), both attached to the α-carbon. • [Note: Glutamate, aspartate, histidine, arginine, and lysine have additional potentially charged groups in their side chains.] • Substances, such as amino acids, that can act either as an acid or a base are defined as amphoteric, and are referred to as ampholytes (amphoteric electrolytes). •Effect of pI on amino acid solubility • Most amino acids are less soluble at their pI. • This can be attributed to the fact that the Zwitter ion has zero net charge and can be crystallized.