Properties of Amino AcidsDr. Kiran Meena

11/9/2019

3:00 to 4:00 PM

Specific learning objectives

Properties of amino acids:

• Amino Acids have an Asymmetric Center

• D and L stereoisomerism of amino acids

• Acid-Base Properties of Amino Acids

• Titration of amino acids

• Absorption

• Solubility

• Chemical properties of amino acid

Properties of Amino Acids

Amino Acids have an Asymmetric Center

Optically active molecules have an asymmetry such that they are notsuperimposable on their mirror image.

Cα atoms of all aa are asymmetric centers and optically active exceptglycine, in which R=H two of the four substituents on α-carbon atoms arehydrogen.

Cα is a chiral center, this carbon atom is attached to four different groups.

D and L stereoisomerism of amino acids

Fig.6.3: Marks' Basic Medical Biochemistry-A Clinical Approach, 2nd Edi

Acid-Base Properties of Amino Acids

Amino acids in aqueous solution, contain weak acidic α-carboxylgroups and weak basic α-amino groups

Charged and uncharged form of ionizable weak acid groups –COOHand -NH3

+ exist in protonic equilibrium:

R-COOH R-COO- + H+

R- NH3+ R- NH2

+ + H+

Cont--

Henderson-Hasselbalch equation: Quantitative relationship between pH

and concentration of a weak acid (HA) and its conjugate base (A-).

Derivation of Henderson-Hasselbalch equation: Consider release of a

proton by a weak acid (HA):

“salt” or conjugate base (A–) is ionized form of a weak acid (HA).

Cont--

Dissociation constant of acid (Ka): or

By taking negative logarithm of both sides:

Substituting pH = -log[H+] and pKa = -logKa obtain Henderson-

Hasselbalch equation:

Cont--

Larger the Ka, the stronger acid, because most of HA has dissociated

into H+ and A–.

Conversely, smaller the Ka, the less acid has dissociated and,

therefore, the weaker the acid.

pKa values for a particular molecule are determined by titration.

Titration of an Amino Acids

• Carboxyl and amino group of glycine

titrated with a strong base (NaOH)

• 1st , at low pH2.34, -COOH group loses its

proton

• 2nd, at pH5.97,zwitterion/dipolar ion form

• 3rd, at pH9.60, NH3+ group loses its

proton.

• 4th,at pH12.0 titration complete

Fig3.10: Lehninger Principles of Biochemistry by David L Nelson, 6th Ed/

Cont--

Titration curves predict the electric charge of aa:

• For glycine, no ionizable group in its side chain, the isoelectric point

calculated by arithmetic mean of two pKa values:

• pKa for –COOH is pK1 is 2.34, whereas pKa for next pKa for -NH3+ is

pK2 is 9.60

• pI=1/2 (pK1 + pK2) = 1/2 (2.34+9.60)=5.97

Cont--

At physiological pH, carboxyl group exists as R-COO- and amino

group as R-NH3+

Ex. Alanine:

Ultraviolet Spectra of Tyr, Phe and Trp

• Amino acids do not absorb visible

light

• Tyr, Phe, and Trp absorb high-

wavelength (250-290nm) UV light.

• Absorption of light by most proteins at

280nm used to detect presence of a

protein in solution

Fig.3.7. Harper’s Illustrated Biochemistry 30th edition

Solubility

Polar, uncharged R groups includes serine, threonine,

cysteine, asparagine, and glutamine.

• R groups of these aa more soluble in water, or more

hydrophilic, than those of nonpolar aa, because they contain

functional groups that form hydrogen bonds with water.

Chemical properties of amino acids

1. Reactions due to carboxyl group

Decarboxylation: Carboxyl group decarboxylated to give primary amines.

Removal of CO2 from aa with formation of amines. Ex. histidine to

histamine

https://www.slideshare.net/senchiy/amino-acids-metabolism-new-12281450

https://slideplayer.com/slide/7968638/

Formation of amides: Carboxyl group condense with amines to form

amides, and remove ammonia from brain

https://www.materialsworldmodules.org/resources/polimarization/4-condensation.html

https://www.tankonyvtar.hu/hu/tartalom/tamop412A/2011-0016_07_pharmacognosy_1/ch13s03.html

Formation of Peptide bonds: Carboxyl group joins with amino group

of another aa to form peptide bond

Fig3.13: Lehninger Principles of Biochemistry by David L Nelson, 6th Ed/

Cont--

2. Reactions due to amino group

Amino group reacts with Co2 in alkaline pH to form carbamino

compound, serve to transport Co2 from tissues to lung by hemoglobin

(Hb)

Hb-NH2 + Co2 = Hb-NH-COOH (Carbamino Hb)

Amino group reacts with halides or acyl anhydrides, for ex. Reaction

of glycine to give hippuric acid, serves as a method for detoxification

of food additives and drugs to treat hyperammonemia

Cont--

Amino group reacts with fluorodinitrobenzene to form

dinitrophenyl aa, used to identify N-terminal aa in any peptide

chain and identify aa separated by paper chromatography

Cont--

• Transamination: α-NH2 group of one

aa is transferred to a α-ketoacid

resulting in formation of a new aa

and a new ketoacid

• Donor aa (I) becomes a new

ketoacid (I) after losing the α-NH2

group, and recipient ketoacid (II)

becomes a new aa (II) after

receiving the NH2 groupText Book of Medical Biochemistry by Chatterjee & Rana Shinde, 8th Ed

Cont--

• α-amino group from L-amino acid istransferred to α-carbon atom of α-ketoglutarate, produced α-keto acid andglutamate

• Transfer of amino groups from one carbonskeleton to another is catalyzed byaminotransferases

• All aminotransferases have prosthetic group,which is pyridoxal phosphate (PLP),coenzyme form of pyridoxine or vitamin B6

Fig18.4: Lehninger Principles of Biochemistry by David L Nelson

Cont--

• Oxidative Deamination: α-amino

group removed from aa to form

corresponding new aa and α-keto

acid and ammonia.

• Glutamic acid undergo oxidative

deamination

Fig18.7: Lehninger Principles of Biochemistry by David L Nelson

L-Glutamate γ semialdehyde

Cont--

3. Reactions due to side chain

Formation of disulfide bonds: -SH (sulfhydryl) group of two cysteine

can join together to form disulfide bond.

• It provide stability to protein structure by forming intrachain disulfide

bonds

Transmethylation: Terminal –S-CH3 group of methionine, after

activation into S-adenosylmethionine, serves as a major methyl

donor in methylation reactions

Reactions due to –OH group of tyrosine, serine and threonine:

Cont--

–OH group of serine and threonine in proteins is highly reactive and

helps in:

• Phosphorylation: Proteins are phosphorylated at their tyrosine,

serine and threonine residues by kinase using ATP

• Glycosylation: The –OH group join with carbohydrates to form O-

glycosidic bonds of glycoproteins

• N-Glycosidic linkages: Amide group of glutamine and asparagine link

with carbohydrates to form N-glycosidic bonds of glycoproteins

Classification of Proteins

1. Based on Solubility

• Albumins: Soluble in water and salt solutions

• Globulins: Soluble in salt solution but sparingly soluble in water

• Protamines: Soluble in 70-80% ethanol; usually rich in proline

• Protamines: Soluble in water, dilute acids and alkalies; rich in arginine

• Histones: Soluble in salt solutions

• Scleroproteins: Insoluble in water and salt solutions

Cont--

2. Based on Shape

• Fibrous Proteins: Appear like hair with long thin fibers. Ex. Keratin,myosin etc.

• Globular Proteins: Appear like spherical globular drops. Ex. Insulin,albumin, globulins etc.

3. Based on Functions

• Structural: Collagen, Keratin, myosin etc

• Enzyme: Pepsin, amylase, lipase etc

• Hormone: Insulin, glucagon etc

• Transport: Hb, Mb etc

• Storage: Ferritin, ceruloplasmin etc

• Protective: Antibodies

Cont--

3. Based on Composition

• Simple Proteins: Pure proteins and do not contain any non-protein

part, e.g. albumin, globulin, collagen etc.

• Conjugated Proteins: Contains covalently bound non-protein part

called prosthetic group which cannot be separated without loss of

activity of the proteins.

Ex. Lipoproteins have lipids, glycoproteins have carbohydrates,

nucleoproteins have nucleic acid, metalloproteins have metal ions as

the non-protein part.

Properties of Proteins

1. Based on Denaturation: Loss of native form of protein with disruption

of its secondary, tertiary, and quaternary structure leading to changes

in its physical and chemical characteristics and loss of biological

activity

2. Solubility: Depends on its aa composition and 3D conformation,

because, to be soluble, surface amino acids of protein have to interact

with solvent molecules.

Cont--

3. Buffering action of proteins: Buffer solution resists change in its pH by

limited additions of acids or alkalis

• Buffer system composed of a strong acid and its salts with a weak

base or reverse

4. Precipitation: Stability of proteins depends on its capacity to interact

with solvent and its degree of hydration.

• Any force that destabilize this interaction leads to precipitation.

Methods for precipitation, salting out, immunoprecipitation etc.

Bonds stabilizing Proteins Structure

1. Peptide bond

2. Disulfide bond

3. Hydrogen bond

4. Electrostatic interaction

5. Van der Waal’s interaction

6. Hydrophobic interaction

Reference Books

1) Harper’s Illustrated Biochemistry-30th edition

2) Textbook of Biochemistry with Clinical Correlations. 4th edition.Thomas M. Devlin.

3) Biochemistry. 4th edition. Donald Voet and Judith G. Voet.

4) Biochemistry 7th edition by Jeremy M. Berg, John L. Tymoczkoand Lubert Stryer

5) Lehninger Principles of Biochemistry, 6th Ed.

6) Text Book of Medical Biochemistry by Chatterjee & Rana Shinde,8th Ed.

31

Thank you