Download Aromatic compounds of biological importance

Peptides.
Structure and functions of proteins
Department of General Chemistry
Poznań University of Medical Sciences
MD 2015/16
1. Peptides (glutathione, insulin)
2. Three dimensional structure of proteins:
- primary,
- secondary,
- tertiary,
- quaternary structure
3. Forces stabilizing the native conformation
4. Classification of proteins based on their composition
Glutathione – reduced GSH and oxidized GSSG
(g-glutamylcysteinylglycine)
Structure of proteins
- Primary – the linear sequence of amino acids linked together covalently.
The primary structure is the one-dimensional first step in specyfying the
three-dimensional structure of a protein.
The strategy for determining the primary structure:
- hydrolysis in HCl (concentrated HCl, 110oC, 24 h),
- Sanger reaction
- Edman degradation
- carboxypeptidases
Sanger’s reaction
Sanger was the first to determine the sequence of a polypeptide.
He reduced the disulfide bonds, separated the A and B chains, and cleaved each
chain into smaller peptides using trypsin, chymotrypsin, and pepsin.
Each peptide reacts with 1-fluoro-2,4-dinitrobenzene, which derivatizes the
exposed a-amino group of amino terminal residues.
Structure of proteins
- Secondary – refers to the arrangement in space of the atoms in the
backbone of the polypeptide chain
- Side chain groups are not included at the level of secondary structure
- Forces stabilizing the secondary structure – hydrogen bonds of the peptide bond atoms
Hydrogen bonds in different helices
Hydrogen bonds formed
between H and O atoms
stabilize a polypeptide in an
a-helical conformation
Amphipatic faces of a helice
Pleated b-sheets
antiparallel
Pleated b-sheets
parallel
A b-turn that links segments of antiparallel b sheet
Supersecondary structure of proteins
bab unit
bb unit
aa unit
Greek key
b-meander
Collagen –
connective tissue protein
The repeating unit: (Gly-X-Pro) or
(Gly-X-Hyp)
Three individual helices are twisted
around each other to form a stiff
rod. Triple helical molecule ics
called tropocollagen.
The three strands are held together
by hydrogen bonds.
Cross-link in collagen
Cross-link in elastin
Silk fibroin
The regularly repeating sequence: (Ser-Gly-Ala-Gly)n
Antiparallel pleated sheet
Structure of proteins
Tertiary structure of proteins
- includes the three-dimensional arrangement of all atoms
in the protein, including the atoms in the side chains and any prosthetic
groups (ones other than amino acids)
- In very large proteins, the folding of parts of the chain can occur
independently of the folding of other parts. Such independently folded
portions of proteins are referred to as DOMAINS
- Forces stabilizing – hydrogen bonds between side chains, electrostatic
attractions (salt, ionic bonds), hydrophobic interactions,
Myoglobin
One
polypeptide
chain, a-helix
Quaternary structure of proteins
Proteins can consist of more than one polypeptide chain; they have
more than one subunit.
Quaternary structure – refers to the arrangement in space of
subunits with respect to one another.
Interaction between subunits is mediated by noncovalent forces,
such as hydrogen bonds and hydrophobic interactions.
Myoglobin: one polypeptide chain
(tertiary structure)
Hemoglobin: 4 polypeptide chains – 4 subunits
(quaternary structure)
Heme
Hemoglobin
Forces involved in protein folding
CLASSIFICATION OF PROTEINS
according to their shape
• Globular proteins – spherically shaped, compact, water soluble,
most of globular proteins are soluble in the cytosol or in the lipid phase of
biological membranes.
Axial ratios (ratios of length to breadth) of less than 10 and generally not
greater than 3-4.
They are primary agents of biological actions: enzymes, transport molecules,
hormones, membrane-bound receptors, immunoglobulins.
• Fibrous proteins – have relatively low water solubility, higher amount of
the secondary structure, elongated „rodlike” shape, high tensile strength,
unusual covalent cross-links. Fibrous proteins are generally insoluble in the
cytosol.
The axial ratios are greater than 10.
They have mechanical and structural functions, provide mechanical
support, a structural matrix, to individual cells and tissues of the
mammalian organisms.
CLASSIFICATION OF PROTEINS
ACCORDING TO THEIR COMPOSITION
Simple proteins: contain only α-amino acids or their derivatives and
occur as such in nature.
Conjugated proteins: those which are united with some nonprotein
substance (the prosthetic group)
SIMPLE PROTEINS:
• Albumins
• Globulins
• Histones
• Scleroproteins (albuminoids)
• Glutelins
• Prolamines
• Protamines
CONJUGATED PROTEINS:
• Nucleoproteins
• Glycoproteins and mucoproteins
• Phosphoproteins
• Chromoproteins
• Lipoproteins
• Metalloproteins
Denaturation of proteins
Denaturing agents:
- form new hydrogen bonds
stronger than native ones
- reduce disulfide bonds
Separation, isolation, purificaton of proteins
Electrophoresis of blood seum proteins
Albumin
a-
b-
and
g-
globulins
Electorophoretic separation of serum proteins