Amino Acids Peptides & Proteins

BIOCHEMISTRY
AMINOACIDS
PEPTIDS and PROTEINS

AGENDA:
AMINO ACIDS – essencial, non essencial
PROTEINS – structure: primary,
secondary, tertiary and quaternary
physical and chemical properties;
simple proteins;
proteinogramm
compaund proteins
chromoproteins, citochroms,
lipoproteins α- β- preβ- khilomicrons
metaloproteins

All proteins, which may find in living bodies, are
consists of amino acids
Degradation of proteins into smaller fragments –
polypeptide chains may be broken by proteolytic
enzymes such as trypsin, chymopripsin, pepsin,
elastase – the final point of protein degradation is
amino acid.
All amino acids occur in proteins are
L-, and α- (alpha)- amino acids

Electrophoresis (electrophoregram)

Sponsored
Medical Lecture Notes – All Subjects
USMLE Exam (America) – Practice

Classification of Amino Acids
A.Classification based on structure:
Each amino acid is assigned a 3 letter or 1 letter
symbol
These symbols are commonly used to represent the
amino acids in protein structure
e.g.:
ANGIOTENSIN II (octapeptide)
[Asp-Arg-Val-Tyr-Ile-His-Pro-Phe]
or simply
[D-R-V-Y-I-H-P-F]

Classification of amino acids based on polarity
There are four groups of AA depending topolarity
1.Non-polar AA (also referred as hydrophobic [water hating]) nocharge
on ‘R’group
[Alanine, Leucine, Isoleucine, Valine, Methionine, Phenylalanine,
Tryptophan, Proline]
2.PolarAAwith no charge on ‘R’group
[Glycine, Serine, Threonine, Cysteine, Glutamine, Asparagine.Tyrosine]
– they however possess groups such as hydroxyl, sulfhydryl, amide –
and participate in hydrogen bonding of protein structure.
3.PolarAAwith positive ‘R’group
[Lysine, Arginine, Histidine]
4.PolarAAwith negative ‘R’group
[Aspartic acid, GlutamicAcid]

Amino
Acids
non-polar,
aliphatic

Positively
charged
Amino
Acids

Negatively
charged
Amino
Acids

Amino Acids found in proteins are also can be
divided into seven distinct groups
-AA with aliphatic sidechains
-Hydroxyl group containingAA
-Sulfur containingAA
-Acidic AA and theiramides
-BasicAA
-AromaticAA
-Imino acids

Aliphatic amino acids
[Glycin, Alanin, Valin, Leucin, Isoleucin]

Hydroxyl group containing AA
[Serine, Threonine, Tyrosine]

Sulfur containing AA
[Cystein (with sulfhydryl group), Methionine (with thioether group)]

Acidic Amino acids and their amides
[Aspertic acid (Aspartate), Glutamic acid (Glutamate)] – dicarboxylic
monoamino acids
[Aspargine, Glutamine] – their respective amide derivatives
all these fourAApossess distinct codons for their incorporation into
proteins

Basic AA
[Lysine, Arginine (with gunidino
group), histidine (imidazole ring)]
dibasic monocarboxylic acids (they
are highly basic in character)

Aromatic AA
[Phenilalanine, Tyrosine, Tryptophan (with indolering)]

Imino acids
[Proline (containing pyrrolidine ring)] – it has an imino group (=NH)
instead of amino group (–NH2). Therefore Proline is α-imino acid

monoamino-monocarboxylic amino acids

monoamino-dicarboxylic amino acids

diamino monocarboxylic amino acids

heterocyclic amino acids
Histidine, Tryptophan, Proline – can be considered as heterocyclicAA

Amino
Acids
variant of
classification

Nutrificational classification of amino acids
The 20AA are required for the synthesis of variety proteins, besides
other biologicalfunctions.
However, all these 20 AA need not be taken in thediet.
Based on the nutritional requirements, amino acids are grouped into two
classes:
Essential and Non-essential.

Essential and non-essential amino acids
The AA which are cannot be synthesized by the body, and therefore,need to be supplied
through the diet are called essentialAA.
They are required for proper growth and maintanance of the individual.
Arginine
Valine
Histidine
Isoleucine
Leucine
Methionine
Phenylalanine
Threonine
Tryptophan
Arginine and Histidine can be synthesized by adults and not by growing childrenthey
are considered as semi-essentialAA.
Thus 8 AA absolutely essentialwhile 2 are semi-essential.
Non-essential (or dispensable)AA:
Glycine,Alanine, Serine, Cysteine,Aspartate,Asparagine, Glutamate, Glutamine,
Tyrosine, Proline.

AA classification based on their metabolic fate
Carbone skeleton of AA can serve as a precursor for the synthesisof
glucose (glycogenic) or fat (ketogenic) or both.
1.Glycogenic AA (can serve as precursors for the formation of glucoseor
glycogen)
[Alanine, Aspartate, Glycin, Methionineetc]
2.Ketonic AA (fat can besynthesized)
[Leucine, Lysine]
3.Glycogenic and kethogenicAA
[Isoleucine, Phenylalanine, Tryptophan, Tyrosine] – precursors for
synthesis of glucose as well as fat.

I.Physical propertie of amino acids1-solubility
1.Solubility
– most of amino acids are usually soluble in water and insoluble in
organic solvents

I.Physical properties of amino acids2-melting
2.Melting point
– amibno acids generally melt at higher temperature, often above 200ºC

I.Physical properties of amino acids3-taste
3.Taste
– amino acids may be
-sweet (Gly, Ala,Val),
-tasteless (Leu),
-bitter (Arg, Ile)
Monosodium glutamate (MSG, ajinomoto) is used as a flavoring agent in
food industry, and Chinese food to increase taste and flavor.
In some individuals intolerant to Monosodium glutamate (MSG),
Chinese syndrome (brief and reversible flulike symptoms) is observed.

I.Physical properties of amino acids4-optical
4.Optical properties – all amino acids (except Glycine)
possess optical isomers
due to the presence of asymmetric carbon atom

D- (right rotating) and L- (left rotating)-forms
of amino acid
(based on the structure of glyceraldehyde)

I.Physical properties of amino acids5-ampholytes
Amiono acids as ampholytes
AA contain both acidic (–COOH) and basic (–NH2) groups.
They can donate a proton or accept a proton – hence AAare regarded as
ampholytes.
Zwitter ion (or dipolar ion) – [name zwitter is from German – means
hybrid].
Is a hybrid molecule containing positive and negative ionic groups.
The AA rarely exist in a neutral form with free carboxylic(–COOH) and
free amino [basic] (–NH2) groups. In strongly acidic pH (low pH), the
amino acid is positively charged (cation), while in strongly alkaline pH
(high pH), it is negatively charged (anion). Each AA has acharacteristic
pH at which it carries positive and negative charges and exists as
zwitterions.

General structure of amino acid and its ion form

Existence of an amino scid as anion and zwitterion

I.Physical properties of amino acids5-isoelectricity
Isoelectric pH (sympol pI) is difined as the pH at which a
molecule exists as a zwitterions or dopolar ion and carries no
net charge. Thus, the molecule is electrically neutral.
The pI value can be calculated by taking the average pKa
values corresponding to the ionizable groups.

Titration curve of amino acid [Leucine]

II1.Chemical properties of amino acids
Amino acids with their acidic part form salts with bases
(e.g. with NaOH forming compouns like R–COONa)
and in reactions with alcohols they form esters (–
COOR´)

II2. Chemical properties of amino acids
Amino acids undergo decarboxylation – to produce
corresponding amines
Reactions due to –COOH group
this reaction assumes significance in the living cells due to the
formation of many biologically important amines [histamine
(from histidine), tyramine (from tyrosine), γ-amino buteric
acid (GABA) – from grutamate]

Reaction with ammonia
carboxyl group of dicarboxylic acids reacts with NH3 to form
amide
Aspartic acid + NH3 → Asparagine
Glutamic acid + NH3 → Glutamine
Reactions due to – NH2 group

The amino group behave as base and combine with acids (e.g
+ –
HCl) to forn salts (–NH3 Cl )

Colour reactions of proteins/amino acids
– are used to identified amino acids

Reaction with ninhydrin.
The α-amino acids react with ninhydrine to form a purple,
blue, or pink colour complex (Ruhemann’s purple)
This reaction used for the quantative determination of amino
acids and protein.
NB Proline and Hydroxyproline give yellow colour with
ninhydrine.

Transamination – very important reaction in amino acid
metabolism – based on transfer of an amino group from an
amino acid to keto acid to form a new amino acid

Amino acid in reaction of oxidative deamination – liberate
free ammonia

Amino acids used in Drugs
D-Penicillamine (D-dimethylglycine)
N-Acetylcysteine
Gabapentine (γ-amonobutyrate linked to cyclohexane)

Dimensions of a fully extended polypeptide chain
(the distance between two adjacent alfa-carbon atoms is 0.36 nm)

Classification of peptides and proteins
Peptides names consist af amino acids and always start
with amino ending amino acid listing the sequence of
all next
Peptides are divided into oligopeptide (40 and less
amino acind)
and polypeptides – more than 40 amino acids

Structure of proteins
Primary structure of protein
Secondary structure of protein
Tertiary structure of proteins
Quaternary structure of proteins

Protein structure
(primary, secondary, tertiary, quaternary)

Secondary structure of protein
A right handed alfa-helix
–CH–R groups of amino acids;
Dotted blue lines are hydrogen
bonds.
[here only a few hydrogen bonds shown
for clarity]

Functional classification of proteins
1.Structural proteins (keratin, collagen)
2.Enzymes (or catalytic) proteins (pepsin, hexokinase)
3. Transport proteins (hemoglobin, serum albumin)
4. Hormonal proteins (insulin, growth
hormone) 5.Contractile proteins (actin,
myosin) 6.Storage proteins (ovalbumin,
glutelin)
7.Genetic proteins (nucleoproteins)
8.Defense proteins (immunoglobulins, snake venoms)
9.Receptor proteins (hormone receptors, authomon nervsystem

Protein classification based on chemical nature and
solubility
1. Simple proteins (only AAresidues)
2.Conjugated proteins(plus non protein moiety –
prosthetic group)
3.Derived proteins – degraded ordenaturated products
of simple and conjugated proteins

Amino Acids Peptides & Proteins

More Related Content

What's hot

Similar to Amino Acids Peptides & Proteins

More from Eneutron

Recently uploaded

Amino Acids Peptides & Proteins