PROTEIN CHEMISTRY-2STRUCTURE & FUNCTION

Dr. Vijay Marakala, MBBS, MD.

BIOCHEMISTRY

IMS, MSU.

OUTLINE

Structural organization of protein

Classification of protein

Isoelectric pH of protein

Protein denaturation

Structural organization of protein

A protein molecule is a linear chain of amino acids.

However, under natural conditions a protein chain spontaneously folds into a three-dimensional structure/conformation.

Native protein.

Structural organization of protein

I. Primary structure

II. Secondary structure

III. Tertiary structure

IV. Quaternary structure

Structural organization of protein

Structural organization of proteinPRIMARY STRUCTURE

The sequence of the amino acids in a polypeptide chain

Bonds responsible for primary structure

i. Peptide bond Covalent bonds

Primary structure of human Insulin

Clinical importance of primary structure

Primary structure determines the biological activity of proteins.

A change of even a single amino acid - seen in mutation E.g;-

SECONDARY STUCTURE

Secondary structure means folding and twisting (conformation) of the polypeptide chain.

Spatial arrangement of amino acid residues that are adjacent in the primary structure

Bonds responsible

Hydrogen bonding between the hydrogen of NH (amide) and oxygen of C=O

(carbonyl) groups within the polypeptide chain

SECONDARY STUCTURE

1. -helix

2. -pleated sheet

3. β-bend or β-turn

4. loops

Most commonly foundsecondary structures

SECONDARY STUCTURE-helix

-helix

Regular

Generally right handed (clockwise)

Each turn has 3.6 amino acid residues [0.54nm]

The axial distance between adjacent amino acids is 1.5Å[0.15nm]

SECONDARY STUCTURE-helix occurrence

Fibrous protein

α-keratin of hair, nail and

skin

Myosin and tropomyosin

of muscle

Globular protein

Hemoglobin

SECONDARY STUCTURE-pleated sheet

Formed between different segments of a protein chain

The polypeptide chains in the -pleated sheet may be arranged either Parallel or

Antiparallel

-pleated structure is stabilized by H-bondsbetween peptide bonds of adjacent segments of the protein chain

Parallel Antiparallel

OCCURRENCE OF -pleated sheet

Many protein contains - β-pleated sheet

both in fibrous and globular proteins.

Fatty acid-binding proteins

Carbonic anhydrase

Silk protein

SECONDARY STRUCTURE

TERTIARY STRUCTURE

Amino acids that are far apart in the polypeptide sequence and that reside in different types of secondary structure may interact within the completely folded structure of a protein.

Further folding of protein chain with its secondary structures to form final

three-dimensional conformation of the protein

TERTIARY STRUCTURE

Bonds stabilizing the tertiary structure are Hydrogen bond

Ionic bond

Hydrophobic bond

Disulfide bond

Non-covalent bonds

Covalent bond

TERTIARY STRUCTURE

QUTERNARY STRUCTURE

Refers to spatial relationship between subunits in an oligomeric protein.

Bonds stabilizing quaternary structure are -

- Hydrogen bonds,

- Ionic bonds and

- Hydrophobic bonds.

QUTERNARY STRUCTURE

CLASSIFICATION OF PROTEIN

I. Based on functions

II. Based on composition

III. Based on shape

CLASSIFICATION OF PROTEIN

Based on functions

1. Catalytic proteins or Enzymes

Glucokinase

Dehydrogenase

2. Transport proteins

Hemoglobin

Transferrin

3. Storage proteins

Ferritin, Myoglobin

4. Contractile proteins

Actin, Myosin

5. Structural proteins

Collagen, Keratin,

6. Defense proteins

Immunoglobulins

7. Regulatory protein

Hormones

insulin, GH

Simple proteins

Conjugated proteins

Derived proteins

SIMPLE PROTEINS

Upon hydrolysis yield only amino acids or their derivatives.

egg albumin, serum albumin

Albumin

Serum globulinsGlobulin

Simple protein combined with non protein substance.

Nucleoprotein

Glycoprotein

Chromoprotein

Phosphoprotein

Lipoprotein

They are denatured or degraded products of simple and conjugated proteins

Primary derived proteins

Secondary derived proteins

DERIVED PROTEINS

Proteins

Aminoacids

Primary derivatives

Secondary derivatives

CLASSIFICATION OF PROTEIN

BASED ON SHAPE

Soluble

Spherical or oval in shape

Axial ratio<10

Low molecular wt

Eg: albumin

globulin

histones

Insoluble

Elongated or needle shaped

Axial ratio > 10

High molecular wt

E.g: collagen

elastin

ISOELECTRIC pH(pI) OF PROTEINS

The pH of the solution influences net charge of protein molecules dissolved in it.

At lower pH, proteins carry relatively more of +ve charge and at higher pH, relatively more of –ve charge.

Isoelectric pH (pI) of a protein molecule is the pH at which it

carries no net charge.

ISOELECTRIC pH (pI) OF PROTEINS

At the isoelectric pH

The net charge is zero.

minimum water solubility.

Maximum precipitation

Isoelectric pH (pI) of a protein is determined by amino acid composition.

all the ionizable groups

Number of anions = Number of cations

pI of Casein = 4.6pI of Albumin = 4.7

Protein denaturation

That is, it involves loss of secondary, tertiary and quaternary structure of proteins but not primary structure.

Unfolding of proteins

Process of disorganization of higher orders of protein structure without

altering primary structure.

Protein denaturation

Protein denaturationDenaturing agents

Physical agents Heat

X-ray

UV radiation

Chemical agents Acids

Alkalies

Urea

Protein denaturation

Loss of 3-dimentional structure

Loss of biological activity

Usually irreversible, some cases reversible

Decreases solubility, causes precipitation

Increases digestibility of proteins

Protein structureIsoelectric pHProtein denaturation

IMPORTANT