Download Proteins - CasimiroSBI4U

Proteins
 Proteins makeup 50% -60% of your dry weight
 Composed of C H O N (and sometimes S and P)
 They are very large molecules
ex. β- lactoglobulin – C1642H2652O550N420S18
Insulin – C254H227O75N65S6
Functions of Proteins
1.
2.
3.
4.
5.
6.
7.
8.
enzymes
oxygen transport (haemoglobin)
blood clotting (fibrin)
immunological defense (antibodies)
food reserves (albumin, casein in milk)
hormones
structural (hair, nails, skin)
muscular movement (actin/myosin)
Composition of Proteins
 Proteins are composed of amino acids = monomer
General structure  note





A central C
An H group
An amino group
A carboxyl group
A variable ‘R’ group
 For each AA there is a different ‘R’ group which lends that
AA certain chemical properties
ex. Glycine – R = H
Alanine – R = CH3
Serine – R= CH2OH
 In solution, the carboxyl group acts as a weak acid, and the
amino group acts as a weak base.
 Thus, amino acids can exist in three different ionic states
depending on the pH. (varied properties)
 There are 20 common types of AA’s can be grouped by
properties of side chains:
1. nonpolar side groups (hydrophobic) – less soluble in
water
2. polar side groups (hydrophilic) –soluble in water;
grouped further into:
a. charged
b. uncharged
 There are 8 essential AA’s in human adults (9 in children)
and the body must get these in the diet  the others can be
synthesized
Protein Structure
 Polypeptide chains = polymers of amino acids, linked by
peptide bonds, arranged in a specific linear sequence.
 Peptide bond = covalent bond formed by condensation
reaction that links carboxyl group of one amino acid to
amino group of another.
 Peptides = are folded and coiled into 3D conformational
structures
 Polypeptide chains range in length and have unique linear
sequences.
3D Conformation = Protein Function
A protein’s 3D structure:
 enables a protein to recognize and bind specifically to
another molecule
 is a consequence of the specific linear sequence of amino
acids in the polypeptide
 is produced as a result of the folding and coiling of peptide
 is stabilized by chemical bonds and weak interactions
between neighboring regions of folded protein
 The properties of R-groups influence the folding and coiling
of peptide.
Primary (1o) Structure
 Unique sequence of AA in protein
 Determined by genes
 Slight change can affect protein’s conformation and
function (e.g. Sickle Cell Anemia –only 1 AA is
substituted)
Secondary (2o) Structure
 Chemical interactions between AA’s determine
configuration of molecule in 3D space
 H-H bonds along the backbone of the protein
Types of 2o structure:
i) α helix  hair, wool
ii) sheets  silk, spider webs
iii) “seemingly” random coils
Tertiary (3o) Structure
 further chemical interactions between R-groups of
certain AA’s lend further 3D shape:
1) weak interactions stabilize protein:
i.
hydrophobic interactions between nonpolar Rgroups
ii.
hydrogen bonding between polar side chains
iii.
ionic bonds between charged side chains
2) covalent linkages:
i.
S-S bridges form between 2 cysteine AA’s  these
‘hold’ the 3D structure together Very Important
Quaternary (4o) Structure
 Interactions between polypeptide chains to form
larger protein molecules
 Not all proteins undergo 4o structure
ex. Haemoglobin
Collagen (triple helix)
Protein Function
 ‘Shape determines function’  a proteins function id
determined by it’s configuration
 denaturation (destruction of shape) will result in a loss
of function
 denatured proteins will halt the biochemical pathways
they catalyze
Factors which will Denature Proteins
 pH change
 salt concentration
 various chemicals (organic solvents)
 temp change
 digestive system (ex. gastrin)
Enzymes
Enzymes are biological catalysts
The rates of reaction depends on:
 concentration of enzymes
 concentration of substrate
 presence of coenzymes/cofactors/inhibitors
 temperature
 pH