amino acids structure and function

Mansoura UniversityFaculty of PharmacyBiochemistry Department

AMINO ACIDS STRUCTURE & FUNCTION

PROF\DR. MAMDOUH ELSHISHTAWY

BY STUDENTS\ IBRAHIM MAHMOUD IBRAHIM

IBRAHIM RAGAB IBRAHIM

IBRAHIM BAHGAT IBRAHIM

IBRAHIM AYMAN IBRAHIM

IBRAHIM ELDOSOKY ELNAGAR

Introduction

AMINO ACIDS PLAY CENTRAL ROLES BOTH AS BUILDING BLOCKS OF PROTEINS AND AS INTERMEDIATES IN METABOLISM. THE 20 AMINO ACIDS THAT ARE FOUND WITHIN PROTEINS CONVEY A VAST ARRAY OF CHEMICAL VERSATILITY.THE PRECISE AMINO ACID CONTENT, AND THE SEQUENCE OF THOSE AMINO ACIDS, OF A SPECIFIC PROTEIN, IS DETERMINED BY THE SEQUENCE OF THE BASES IN THE GENE THAT ENCODES THAT PROTEIN. THE CHEMICAL PROPERTIES OF THE AMINO ACIDS OF PROTEINS DETERMINE THE BIOLOGICAL ACTIVITY OF THE PROTEIN .

PROTEINS NOT ONLY CATALYZE ALL (OR MOST) OF THE REACTIONS IN LIVING CELLS, THEY CONTROL VIRTUALLY ALL CELLULAR PROCESS. IN ADDITION, PROTEINS CONTAIN WITHIN THEIR AMINO ACID SEQUENCES THE NECESSARY INFORMATION TO DETERMINE HOW THAT PROTEIN WILL FOLD INTO A THREE DIMENSIONAL STRUCTURE, AND THE STABILITY OF THE RESULTING STRUCTURE. THE FIELD OF PROTEIN FOLDING AND STABILITY HAS BEEN A CRITICALLY IMPORTANT AREA OF RESEARCH FOR YEARS, AND REMAINS TODAY ONE OF THE GREAT UNSOLVED MYSTERIES. IT IS, HOWEVER, BEING ACTIVELY INVESTIGATED, AND PROGRESS IS BEING MADE EVERY DAY.

IMPORTANCE OF AMINO ACIDSWHEN PROTEINS ARE DIGESTED OR BROKEN DOWN, AMINO ACIDS ARE LEFT. THE HUMAN BODY USES AMINO ACIDS TO MAKE PROTEINS TO HELP THE BODY:

BREAK DOWN FOOD

GROW

REPAIR BODY TISSUE

PERFORM MANY OTHER BODY FUNCTIONS

AMINO ACIDS CAN ALSO BE USED AS A SOURCE OF ENERGY BY THE BODY.

The Chemistry of Amino Acids

AMINO ACIDS  ARE BIOLOGICALLY IMPORTANT ORGANIC COMPOUNDS COMPOSED OF AMINE (-NH2) AND CARBOXYLIC ACID (-COOH) FUNCTIONAL GROUPS, ALONG WITH A SIDE-CHAIN SPECIFIC TO EACH AMINO ACID. THE KEY ELEMENTS OF AN AMINO ACID ARE CARBON,HYDROGEN, OXYGEN, AND NITROGEN, THOUGH OTHER ELEMENTS ARE FOUND IN THE SIDE-CHAINS OF CERTAIN AMINO ACIDS .

ABOUT 500 AMINO ACIDS ARE KNOWN AND CAN BE CLASSIFIED IN MANY WAYS. THEY CAN BE CLASSIFIED ACCORDING TO THE CORE STRUCTURAL FUNCTIONAL GROUPS' LOCATIONS AS ALPHA- (Α-), BETA- (Β-), GAMMA- (Γ-) OR DELTA- (Δ-) AMINO ACIDS; OTHER CATEGORIES RELATE TO POLARITY, PH LEVEL, AND SIDE-CHAIN GROUP TYPE (ALIPHATIC, ACYCLIC,AROMATIC, CONTAINING HYDROXYL OR SULFUR, ETC.). IN THE FORM OF PROTEINS, AMINO ACIDS COMPRISE THE SECOND-LARGEST COMPONENT (WATER IS THE LARGEST) OF HUMAN MUSCLES, CELLS AND OTHER TISSUES. OUTSIDE PROTEINS, AMINO ACIDS PERFORM CRITICAL ROLES IN PROCESSES SUCH AS NEUROTRANSMITTER TRANSPORT AND BIOSYNTHESIS.

The generic structure of an alpha amino acid in its un-ionized form

Functions of Amino acids in Man

Precursors for protein synthesis:

OF more than 10.000 protein molecules as enzymes, hormones, serum, albumin,

immuno-globulins, clotting factors, etc.

Act as Buffers:

Since they are amphoteric molecules .. Because of weak positively charged

amino group & weak negatively charged carboxylic group.

In Detoxification of drugs & metabolic by-products :

e.g. Benzoic acid (toxic) + glycine → Hippuric acid (non-toxic) water

soluble → excreted in urine .

In Neurotransmitters:

As Endorphins ”31 a.a” & Encephalins ”5 a.a ”Some amino acid derivatives are used as direct

neurotransmitters : e.g. Tyrosine → Dopamine, Epinephrine, NE .Tryptophan → Serotonin , Melatonin

Precursors for some biologically active oligopeptides :

Example : oxytocin, vasopressin & GSH

And also for biological molecules like : Histamine, NAD+ , etc .…

Classification of amino acids

They are classified according to :

Chemical structure of side chain

Polarity of side chain

Biological value

Nutritional value

1 )According to chemical structure:

1 )According to chemical structure:

1)Amino acids with aliphatic side chains : Alanine (Ala,A) Glycine (Gly,G)

2 )Amino acids with aliphatic – branched side chainsIsoleucine

(Ile,I) Leucine (Leu,L)

Valine (Val,V)

3 )A.A with aliphatic side chains containing OH group 4 )A.A with aliphatic side chains containing sulfur

Threonine (Thr,T)

Serine (Ser,S) Methionine (Met,M)

Cysteine (Cys,C)

5 )A.A with acidic side chains 6 )A.A with amide side chains

Glutamic acid (Glu,E)

Aspartic acid (Asp,D)

Glutamine (Gln,Q)

Aspargine (Asn,N)

7 )A.A with aromatic side chains

Tyrosine (Tyr,Y) Phenylalanine (Phe,F)

Arginine (Arg,R) Lysine (Lys,K)

8 )A.A with basic side chains

9 )A.A with heterocyclic side chains Histidine (His,H) Proline (Pro,P) Tryptophan (Trp,W)

2 )According to polarity of side chain:

They are classified into :

A) NON-POLAR SIDE CHAINS :

THEY ARE WATER INSOLUBLE(HYDROPHOBIC) , CONTAIN NO ELECTRON RICH ATOMS (O , N, S, ..) , NOT GAIN OR GIVE PROTONS , NOT FORM HYDROGEN OR IONIC BONDS.

IN AQUEOUS MEDIA :

HYDROPHOBIC SIDE CHAINS CLUSTER IN THE CORE OF MOLECULE & TIGHTLY PACKED INTO THE INSIDE , BY HYDROPHOBIC INTERACTION {VAN DER VAL FORCES} → COALESCE IN AQUEOUS SOLUTION & CHARGED SIDE CHAINS ON THE SURFACE → LIKE MICELLE FORMATION.

B) POLAR , CHARGED SIDE CHAINS :

THEY ARE WATER SOLUBLE AND FORM IONIC BONDS & H-BONDS THEY ARE:

1 )ACIDIC A.A THAT DONATE PROTONS AND FULLY IONIZED & (-) CHARGED AT NEUTRAL PH

2 )BASIC A.A THAT BIND PROTONS AND FULLY IONIZED & (+) CHARGED AT NEUTRAL PH

C) POLAR , UNCHARGED SIDE CHAINS :

THEY ARE WATER SOLUBLE CONTAIN ELECTRON RICH ATOM & NOT GIVE OR ACCEPT PROTONS & PARTICIPATE H-BOND.

Notes :- It is not necessary for the two cysteine a.a to be beside each other to form disulfide linkage , if distant from each other → refolding of molecule to form disulfide linkage (3D structure) ex: Insulin .. That’s is responsible for activity of the protein .Reduction of disulfide bond of insulin in acidic PH → loss of activity .

Protein contain free SH → can act as anti-oxidant as GSH .

Disulfide bonds provide covalent links that give the rigidity to the structure.

3 )According to the nutritional value:

They are classified into :

A) ESSENTIAL AMINO ACIDS (INDISPENSABLE) :

CAN NOT BE SYNTHESIZED IN HUMAN BODY .. SO MUST BE TAKEN IN DIET .

FAILURE TO OBTAIN ENOUGH OF EVEN 1 OF THE 10 ESSENTIAL A.A → DEGRADATION OF THE BODY PROTEIN , A.A MUST BE IN FOOD EVERY DAY (55-56 GRAM PROTEIN RDA) .

B) SEMI-ESSENTIAL AMINO ACIDS :

CAN BE FORMED IN BODY IN SMALL QUANTITIES NOT SUFFICIENT FOR PHYSIOLOGICAL REQUIREMENTS OF THE BODY DURING INFANCY ..SO IT MUST BE TAKEN IN DIET .

BUT , SYNTHESIZED IN ADULTHOOD (NON-ESSENTIAL).

EX : ARGININE & HISTIDINE.

C) NON-ESSENTIAL AMINO ACIDS :

SYNTHESIZED IN THE BODY IN SUFFICIENT AMOUNTS → NOT ESSENTIAL TO BE TAKEN IN DIET.

4 )According to biological value:

They are classified to :

I) Pure ketogenic : that give ketone bodies only in the body .

II) Mixed glucogenic & ketogenic : that give ketone bodies and glucose .

III) Pure glucogenic : that give glucose only .

Thank you