a)structure:- *as the name implies
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A)Structure:- *as the name impliescontain:-
*carboxylic acid and amino groups areattached to the α carbon atom(carbon 2).
*aside chain (R group ) also attached to theα carbon ,R group differ for each of the 20common amino acid(protein).
*functions of the protein are related to thechemical properties of R group.
ids, thus the (R) instead of (S).
An amino acid in its (1) un-ionized and (2) zwitterionic forms
B)Classification:-
Based on R group :-1)Non polar ,Aliphatic side chains amino acid:*Glycine) Gly(-is not asymmetric.-greatest structural flexibility.*proline(Pro):--called imino acid.*Alanine(Ala);-*Branched chain amino acid:-Valine(Val),leucine(leu),iso leucine(lle).
2)Aromatic side chain amino acid:-
*Phenyl alanine(Phe):---phenyl group.-Very hydrophobic.*tyrosine(Tyr) and Tryptophan(Trp);--hydrophobic (more polar)-tyrosine contain phenolic group.-tryptophan contain N in its indol group.
3)Polar ,Uncharged side chains amino acid:-
-hydrophilic (water loving)-found on the surface of globular protein(interact with water).
*Serine(Ser) and Threonine (Tre):--contain polar hydroxyl group so form H bond water or other polar compounds that bind to protein.
*Asparagine (Asn),Glutamine (Gln):--are amides of amino acid aspartate and glutamate.-they are polar because of the carbonyle and nitrogen atoms in their amide groups.
*Cystein(Cys) and Methionine(Met):--polar because they each contain asulfur atom-More hydrophobic because of its sulfhydryl group interact with other sulfhydrl group to form disulfides.
4)Charge side chain amino acid:-
*The negative amino acid:-
*Aspartic(Asp) and Glutamate(Glu):--contain carboxylic acids.-called acidic amino acids
4)Charge side chain amino acid:-
*The negative amino acid:-
*Aspartic(Asp) and Glutamate(Glu):--contain carboxylic acids.-called acidic amino acids
*The positive amino acid:-
*Arginine(Arg) and Lysine(Lys) and Histidine(His):
-side chain contain nitrogen.-ph is 7.4.-Arg side chain it is guanidinium.-Lys side chain it is amino on the € epsilon carbon.-His has an imidazole.
GlycineAlanine
ValineLeucine
Isoleucine Proline
Phenylalanine
Tryptophan
Tyrosine
Glutamine
Asparagine
Serine
Threonine
Cysteine
Methionine
Aspartic
Acid
Glutamic
Acid
Arginine
Lysine
Histidine
C)Optical Properties of the amino acid:-
-The one amino acid not exhibiting is glycinesince its R group is a hydrogen atom.
-The ability of a molecule to rotate the plane of polarized lighteither to the right (dextrorotatory) or to the left(levorotatory).
-All amino acid are L-α-amino acids
-D-amino acids found in polypeptide antibiotics
-D-amino acid are never found in proteins
-The ability of protein to absorb ultravioletlight is predominantly due to the presence of thetryptophan which strongly absorbs U.V.
D)ACID –base Properties:-
*amino acids that do not ionizable side chains are titrated,two pKа values are observed:-
-At low ph ,both groups(α-carboxyl/pk₁,α-amino/pk₂) carry protons .amino group has appositive charge ,carboxyl has zero charge{pkа=1}.
-pH is increased by the addition of alkali(oH),amino group positivecharge,carboxyl is negative{pkа=0}.
-pH is increased by the addition of alkali(oH),amino group positivecharge,carboxyl is negative{pkа=0}.
-Isoelectric point(pI):-the pH at which the net charge on themolecules in solution is zero is called pI.zero the pH will be
equivalent to the isoelectric point: pI.
-At this pH the molecules called zwitterions will not migrate in anelectric field because negative charge equal positive charge., Electricallyneutral at this pH.
-Association constant(kа)is acidic strength of the carboxyl, amino andionizable R-group in amino acids.
-pkа = -log kа.
-Net charge is sum of all charge in amino acid , peptide or protein. itsdepend on ph.
-During the titration of any amino acid and protein the Net chargedepend on pH(Net charge=0 ,the pH is PI.
E)Functional significance of amino acid R.group:-
In solution it is the nature of amino acid R groups that dictate structure-function relationships of peptides and protein.
-the hydrophobic amino acids will be found on the interior of protein(un contact with water).
-The hydrophilic amino acids are found on the exterior(active side ofenzymatic ally active protein).
-The imidazole ring of histidine act proton donor or acceptor atphysiologicalPH.found in the reactive side of enzyme.important in hemoglobin to buffer the H ions fromcarbonic acid ionization in RBCs .(Hb exchange O₂ &CO₂ at the tissuesor lungs ,respectively.
-primary alcohol of serine & threonine as well as the thiol(-SH) ofcysteine act as nucleophiles during enzymatic catalysis.the thiol of cysteine is able to form adisulfidebond with othercysteines(cysteine-S-S-cysteine)this simple disulfideisidentified as cystine.
-Disulfide bond present in a large number of proteins.(formation activestructural domains).
-Disulfide bond in cysteines in different polypeptide chain of oligomericprotein play a crucial role in ordering the structure of complex proteins(insulin receptor).