Introduction to Proteins and Amino acids

Introduction to Proteins and Amino acids

Objective

At the end of this lecture, student will be able to

–      Explain general concept of proteins

–      Classify amino acid

–      Explain properties of amino acid 

–      Explain the significance of amino acid

•       Proteios: holding the first place

•       10 – 12kg in adult body proteins

•       Proteins are the most abundant organic molecules of living system and forms the fundamental basis of structure and function of life

•       Large molecules

•       Made up of chains of amino acids

•       Are found in every cell in the body

•       Functions are broadly grouped as structural and dynamic function

•       Proteins are polymers of amino acid

Elements of proteins

•       Carbon– 50%

•       Hydrogen– 6%

•       Oxygen– 19%

•       Nitrogen– 13%

•       Sulfur– 5%

•       Other elements are P, Fe, Cu, I, Mg, Mn, Zn etc

Classification of proteins

Structure of Proteins

•       Proteins on complete hydrolysis with conc. HCl yield L-α-amino acid and is the common properties of all proteins

•       As many as 300 amino acid occur in nature, out of these only 20 are known as standard amino acid and are repeatedly found in structure of proteins

•       Made up of chains of amino acids; classified by number of amino acids in a chain

–      Peptides: fewer than 50 amino acids

•       Dipeptides: 2 amino acids

•       Tripeptides: 3 amino acids

•       Polypeptides: more than 10 amino acids

–      Proteins: more than 50 amino acids

•       Typically 100 to 10,000 amino acids linked together

•       Chains are synthesizes based on specific bodily DNA

•       An adult has about 100g of free amino acid, which represents amino acid pool of the body

•       Four levels of structure

–      Primary structure

–      Secondary structure

–      Tertiary structure

–      Quaternary structure

Any alteration in the structure or sequencing changes the shape and function of the protein

Denaturing

•       Alteration of the protein’s shape and thus functions through the use of

–      Heat

–      Acids

–      Bases

–      Salts

–      Mechanical agitation

•       Primary structure is unchanged by denaturing

Functions of protein

–      Provide structural and mechanical support

–      Maintain body tissues

–      Functions as enzymes and hormones

–      Help maintain acid base balance

–      Transport nutrients

–      Assist the immune system

–      Serve as a source of energy when necessary

Best Sources of Protein

•       Proteins are abundant in

–      Dairy foods

–      Meats

–      Poultry

–      Meat alternatives such as dried beans, peanut butter, nuts, and soy

•       Cooked meat, poultry, or fish

–      Provides 21–25 grams of protein

–      About 7 g

–      About the size of a deck of cards

–      Adequate amount for one meal

Adults should consume 0.8 g/kg/d of protein

Amino acid

•       Amino acid are the group of organic compounds containing two functional groups ie. Amino and carboxyl groups

•       Amino group is basic and carboxyl group is acidic in nature

•       Amino acid mostly exist in ionized form in biological system

•       α– amino acid: If both –COOH and – NH₂ is attached to same carbon atom

Amino acid structure

Peptide Bonds Link Amino Acids

•       Form when the acid group (COOH) of one amino acid joins with the amine group (NH₂) of a second amino acid

•       Formed through condensation

•       Broken through hydrolysis

Condensation and Hydrolytic Reactions

Classification of Amino acid

•       Amino acids are classified into different ways based on structure, Polarity, nutritional requirement etc..

1. Classification based on structure:

•       Each amino acid is assigned with 3 letter or 1 letter symbol and are commonly used to represents the amino acid in proten structure

•       20 amino acid found I proteins are divided into seven groups

I. Amino acid with aliphatic side chain

a. Glycine – Gly or G

b. Alanine – Ala – A

c. Valine – Val – V

d. Leucine – Leu – L

e. Isoleucine – Ile – I

II. Amino acid containing hydroxyl group

f. Serine – Ser – S

g. Threonine – Thr – T

III. Sulfur containing amino acid

h. Cysteine – Cys- C                        Cystine – Cys- C

I. Methonine – Met- M

IV. Acidic amino acid and their amides

j. Asparitc acid – Asp – D

k. Asparagine – Asn – N

l. Glutamic acid – Glu – E

m. Glutamine – Gln – Q

V. Basic amino acid

n. Lysine – Lys – k

o. Arginine – Arg – R

p. Histidine – His – H

IV. Aromatic amino acid

q. Phenylanaline – Phe – F

r. Tyrosine – Tyr – Y

s. Tryptophane – Trp – W

VII. Imino acid  

t. Proline – Pro –P

2. Classification based on polarity: 4 types

a. Non Polar amino Acid: Amino acid are also referred as hydrophobic (water heating). They have no charge in R^l group. e.g.: Alanine,  Leucine, Isoleucine, Valine etc

b. Polar amino acid with no charge on R^l group: amino acid with no charge on R^l group. However they possess other groups such as hydroxyl, sulfhydryl & amide and precipitate in hydrogen bonding of protein synthesis. e.g.: glycine, serine, threonine, cystine, tyrosine

c. Polar amino acid with + ve charge on R^l group: e.g. Leucine, Argenine, Histidine

d. Polar amino acid with – ve charge on R^l group: e.g. Glutamic acid, Aspartic acid

3. Nutritional classification of amino acid

20 amino acid are required for the synthesis of various proteins

Based on nutritional requirement. They are 2 types

a. Essential amino acid: Arginine, Valine, Histidine, Isoleucine, Leucine, Lysine, Methonine, Phenylalanine, Threonine and Tryptophan

b. Non-essential amino acid: Remaining all

4. Amino acid classification based on their metabolic fate

Carbon skeleton of amino acid can serve as a precursor for the synthesis of glucose, fats or both

a. Glycogenic Amino acid: amino acid are precursor for the formation of glucose or glycogen e.g.: Alanine, Aspartate, Glycine

b. Ketogenic amino acid: amino acid are precursor for the formation of fats. e.g.: Leucine and Lysine

c. Glycogenic & ketogenic amino acid: amino acid are precursor for the formation of glucose and fats e.g.: Isoleucine, Phenylalanine, Tryptophan, Tyrosine etc.

Properties of Amino acid

Amino acid differ in their physiochemical properties which determine the characteristics of proteins

I. Physical properties

1. Solubility:  Most are soluble in water and insoluble in organic solvent
2. Melting point: melts at high temperature (above 200^oc)
3. Taste: may be sweet, tasteless, or bitter
4. Optical properties: All amino acid except glycine possess optical isomers due to presence of asymmetric carbon atom
5. Amino acid as ampholytes: it contains both functional group i.e. acidic & basic group. They can donate a proton or accept a proton. Hence regarded as ampholytes
6. Zwitterion or Dipolar ion: Zwitter means hybrid.
7. zwitterion
   is a hybrid molecule contain both +ve and –ve ionic group.
8. Isoelectric P^H (P^I): is defined as the P^H at which molecules exists as Zwitterion, thus molecules are electrically neutral

II. Chemical properties

Based on two functional group

1. Reaction due to –COOH group

1.    Amino acid forms salt (-COONa) with base & esters with alcohol (-COOR^I)

2.    Decarboxylation: Amino acid undergoes decarboxylation to form amines

3.       Reaction with ammonia: the carboxylic group of dicarboxylic amino acid react with NH₃ to form amide

                Aspartic acid + NH₃                                          Asparagine

Glutamic acid + NH₃                                        Glutamine

b. Reaction due to – NH₂ group:

1. Amino acid group behaves as bas and combined with acid to form salts
2. Reaction with Ninhydrin: Amino acid react with ninhydrin to form a purple, blue or pink colour complex
3. Transamination: transfer of amino group from one amino acid to a ketoacid to form a new amino acid. Imp in amino acid metabolism
4. Oxidative deamination: Amino acid undergoes oxidative deamination to liberate free ammonia

•       Amino acid are chiral molecules. Only L-amino acid are found in proteins, D form occurs in bacterial peptides

Peptides

•       Peptides (from Greek word meaning “digested”) are biologically occurring short chains of amino acid monomers linked by peptide (amide) bonds

Functions

•       Act as biological catalyst called enzymes

•       Provide structural frame work of cells and tissues

•       Act as transport media in blood stream

•       Act as hormones or regulatory proteins for controlling biological process

•       Perform mechanical work (skeletal muscle contraction, pumping of heart)

•       Serve as essential nutrient

•       Act as antibodies in the blood stream

•       Act in clotting mechanism

Some important biologically active peptides in human body

Protein Turnover

•       Protein turnover is the balance between protein synthesis and protein degradation.

•       More synthesis than breakdown indicates an anabolic state that builds lean tissues, more breakdown than synthesis indicates a catabolic state that burns lean tissues.

Nitrogen Balance

•       Nitrogen balance is the measure of nitrogen input with the nitrogen output subtracted from it

•       Nitrogen Balance = Nitrogen intake – Nitrogen loss

•       Blood urea nitrogen can be used in estimating nitrogen balance, as can the urea concentration in urine

•       A positive value is often found during periods of growth, tissue repair or pregnancy.

•       A negative value can be associated with burns, fevers, wasting diseases and other serious injuries and during periods of fasting.

•       This means that the amount of nitrogen excreted from the body is greater than the amount of nitrogen ingested

•       A negative Nitrogen balance can be used as part of a clinical evaluation of malnutrition

Summary

•       Protein are polymers of amino acid

•       Amino acid are the group of organic compounds containing two functional group i.e. amino and carboxyl group

•       Amino acid exhibit zwitterion

•       Nitrogen balance is the measure of nitrogen input with the nitrogen output subtracted from it

FAQs

1. What is the difference between a protein and an amino acid?
   • Proteins are complex molecules composed of amino acids, while amino acids are the building blocks of proteins.
2. Can you get enough protein from a vegetarian diet?
   • Yes, it is possible to obtain sufficient protein from a vegetarian diet by including a variety of plant-based protein sources.
3. How much protein do I need daily?
   • The recommended daily intake of protein varies depending on factors such as age, sex, weight, and activity level. Generally, adults should aim for 0.8 grams of protein per kilogram of body weight per day.
4. Are protein supplements necessary for muscle building?
   • While protein supplements can be convenient for meeting increased protein needs, they are not essential for muscle building. A well-balanced diet that includes adequate protein from whole food sources can support muscle growth and recovery.
5. What are the signs of too much protein in the diet?
   • Excessive protein intake may lead to symptoms such as dehydration, kidney damage, digestive issues, and weight gain.

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