protein chemistry 2
TRANSCRIPT
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