2. Protiens Functions of proteins. Structure– primary, secondary, tertiary and quaternary structure. Medical importance of the protein structure. Analysis of proteins.

Question 1

Primary Structure of Proteins

Answer 1

Includes:
- A sequence of AA (amino acids), which determines the shape/ function of the protein
- Number of AA
- Arrangement of AA in the chain

It is determined by the corresponding gene

Supported by peptides and covalent bonds betw adjacent AA

Example 1: PROTAMINE
- found in head of sperm
- contains 49-50 AA, 38 of them are basic
- ARGANINE is predominant
- FUNCTION - packages DNA and stabilizes it
- basic AA bind to acidic DNA and form stable complexes, therefore displacing histones and transition proteins

Example 2: Glycophorin A (in membrane of RBC)
- transmembrane protein
- contains 131 amino acids.
- 15 (found in blood plasma) of serine and threonine residues carry SIALIC ACID which gives a negative surface charge to RBC
- 16 of hydrophobic AA are located where protein passes through membrane

Question 2

Secondary Structure

Answer 2

Uniform folding of the polypeptide chain in separate sections as a result of the formation of Weak HYDROGEN BONDS between N-H and C=O groups of diff AA within backbone.

Formation of (alpha) A- helix sheet and (Beta) B- pleated sheet

Question 3

Tertiary Structure

Answer 3

• Spontaneous folding of polypeptide chain, resulting from interactions between the R grps of the various AA.
• Tertiary structure is promoted by the chaperons.

Types of interactions in Tertiary structure
Noncovalent:
- Hydrophobic (betw nonpolar side chains)
- van der Waals interactions
- Hydrogen bonds (betw polar side chains)
- Ionic bonds (betw +ly and -ly charged side chains)
Covalent
- Disulfide bridges (-S-S-) (betw 2 cysteine residues)

Question 4

Globular Proteins

Answer 4

Structure
- hydrophobic AA residues are orientated towards the insides, whilst the hydrophilic ones remain on surface

Function
Include:
- Enzymes
- Hormones - insulin, thryoglobulin
- Hemoglobin, myoglobin, fibrinigen, egg albumin (ovalbumin), membrane proteins

Question 5

Fibrous Proteins

Answer 5

Structure
- Filamentous structure
- Insoluble in water
- Have a structure-forming function

Function
- ELASTIN - builds tendons
- FIBRIN - in blood clotting
- KERATIN - in epidermis of skin, hair and nails
- COLLAGEN - in tendons

Question 6

Quaternary Structure of P

Answer 6

• Binding of 2+ identical polypeptide chain into a functional complex
• Individual polypeptide are called sub units

Example 1: GLYCOPHORIN A

Example 2: HEMOGLOBIN
- Is a globular protein with quaternary structure
- Is a Tetramer
- Each subunit has a non polypeptide component, called heme, with an iron atom
- In lungs, oxygen binds reversibly to these iron atoms and is transported through blood.
- In the tissues the oxygen is released from the
hemoglobin molecule.

Question 7

Medical Importance

Answer 7

• Sickle Cell Disease (an inherited blood disorder) caused by substitution of one AA (valine) for the normal one (glutamic acid)

Circumstances of incorrect protein folding in the cell:

• Accumulation of insoluble aggregates in neurons -> neurodegenerative disorders, e.g. Alzheimer’s, Parkinson’s diseases
• Prions – infectious agents consisting of misfolded protein → enters the neuron
  → binds to a cellular protein, causing it to misfold → chain of reactions of misfold
  and amyloid formation → loss of cell function and fatal diseases e.g.
  Scapie (sheep),
  Bovine spongiform encephalopathy (BSE, “mad cow disease”),
  Creutzfeldt-Jakob disease (CJD) in humans

Question 8

Summary of P structure

Answer 8

Primary P structure - sequence of a chain of AA

Secondary P structure - hydrogen bonding of peptide backbone causes AA to fold into a repeating pattern

Tertiary P Structure - 3d folding pattern of a protein due to side chain interactions

Quaternary P Structure - P consisting of 1+ AA

Question 9

Analysis of P

Answer 9

• Recombinant protein production
• Protein sequencing
• Biophysical methods - x-ray, NMR (nuclear magnetic resonance), spectrscopy, etc
• Mass determination
• Mass spectrometry
• Immunohistochemistry - using of antibodies
• Functional analysis