Metabolic fuels and Dietary components Lecture - 2 By Dr. Abdulrahman Al-Ajlan

Function of Protein  Every life process depend on this class of molecules, ex  Enzymes and polypeptide hormones direct and regulate metabolism in the body.  Contractile proteins in muscle permit movement.

Continue  In bone, the protein collagen forms a framework for deposition of calcium phosphate crystals.  In bloodstream, proteins such as hemoglobin and plasma albumin shuttle molecules essential to life. Where immunoglobulin's destroy infectious bacteria and viruses.

 More than 300 different amino acids have been described in nature. Only 20 are commonly found as constituents of mammalian proteins.  Each amino acid (except for praline) has a carboxyl group, an amino group and a distinctive side chain (R-group) bonded to the α- Carbon atom. Amino Acids

General structure of α- amino acid.

Continue  At physiologic pH (pH=7.4), the carboxyl group is dissociated, forming the negatively charged carboxyl ate ion (-COO - ), and the amino group is protonated (- + NH 3 ).

Classification of amino acids  The amino acids are classified according to their side chain 1.Amino acids with non polar side chains Glysine = Gly Alanine = Ala Valine = Val Leucine = Leu Isoleucine = Ile Phenylalanine = Phe Tryptophan = Trp Methionine = Met Proline = Pro

Glycine = Gly- G Alanine = Ala – A Valine = Val – V Leucine = Leu - L

Isoleucine = Ile-I Pyenylalanine = phe – F Tryptophan = trp – W Methionine = met - M

Continue Proline = pro – p A.Each of these amino acids has a hydrophobic side chain that does not bind or give of protons.

Continue B.They are not participating in hydrogen or ionic bonds. C.The side chains of these aa tend to cluster together in the interior of the protein. D.They play important role in stabilizing protein structure. E.Oily or lipid-like

Continue… 2.Amino acids polar but uncharged R-groups. Asparagine = Asn Glutamine = Gln Cysteine = Cys Serine = Ser Threonine = Thr Tyrosine = Tyr Asparagine = Asn - N Glutamine = Gln - Q

Continue Cysteine = Cys- C Serine = Ser- S Threonine = Thr –T Tyrosine = Tyr- Y

Continue… A.They have zero net charge at neutral pH. B.Serine, threonine, tyrosine each contain a polar hydroxyl group that can participate in hydrogen bond formation. C.The side chain of cysteine contains a sulfhydryl group (-SH). In protein the –SH groups of two cysteines can become oxidized to form a dimmer, cystine, which contains a covalent cross-link called a disulfide bond (-S-S-).

Continue 3.Amino acids with acidic side chains. Glutamic acide = Glu –E Aspartic acid = Asp - D Aspartic acid = Asp Glutamic acid = Glu A.They are proton donors. B.At pH 7 the side chain of these amino acids are fully ionized and contain a negative charged carboxylate group (-COO). They are called aspartate or glutamate. C.These amino acids are negatively charged at physiologic pH ( )

Continue 4.Amino acids with basic side chains. Arginine = Arg- R Lysine = Lys-K Histidine-= His- H

Continue A.The side chains of the basic amino acids accept protons B. At physiologic pH the side chain of lysine and arginine are fully ionized and positively charged

Continue  In general amino acids are amphoteric compounds. They contain acidic (-COO) and basic (-NH 2 ) groups in the same molecule. In solution, the -NH 2 group can accept a proton and exist as –NH 3, resulting in the amino acid containing two weakly acidic groups (-COO) and (-NH 3 )