Proteins Flashcards
All proteins contain
C H O N
Some very strong proteins contain sulphur
Proteins are often
Large molecules and are the principle material in enzymes, hormones, nerves
They are polypeptides made up of chains whose monomer units are amino acids
Basic structure of an amino acid
H R O \ | // N —- C — / | \ H H OH
Where NH2 is the amine or basic group
Where COOH is the acidic group (carboxylic acid)
Where R is the variable part of the molecule & represents a different carbon-containing side chain
Number of commonly occurring amino acids
20, plants are able to synthesise all of these
Animals are only able to synthesise some and must obtain the others in their diet (essential amino acids)
Ex. Glycine, valine, methionine
Amino acids are
Small, soluble molecules and a mixture of different amino acids can be separated by electrophoresis or chromatography
Dipeptide formation
Two amino acids join in a condensation reaction to form a dipeptide (products: water and the dipeptide)
Peptide bond between 2 amino acids
Hydrolysed by adding water across the bond
Dipeptide Formation in words
A hydrogen atom from the amine group of one amino acid joins with OH from the carboxyl group of a second amino acid to produce H2O which is released
This forms a peptide bond
Peptide bond
Between carbon & nitrogen
N-terminal
Amine group (H2N…. C)
C-terminal
Carboxylic group (N||||H… OH)
Functional protein may contain
One or more polypeptides
Chains of amino acids shape
Chains of amino acids that make up polypeptides often take up 3D shape
Disulphide bond
Occur between sulphur containing groups in the amino acids cysteine and methionine, strong covalent bonds
Ionic bonds
Free amino and carboxylic acid groups can ionise and then the opposite electrostatic charges can attract
Weaker than disulphide but still strong
Hydrogen bonds
Occur between the amine and the carboxyl groups
The hydrogen bond is the electrostatic attraction between these partial charges
Weakest bond
additional interactions that will contribute to the structure of a protein
Polar/hydrophilic amino acids will move themselves towards water e.g. histidine and glutamine
Non-polar/hydrophobic amino acids will move themselves away from water e.g. tyrosine, phenylalanine
Primary structure
The primary structure of the polypeptide determines its shape and its function,
determined by the sequence of bases in the DNA
Slight change in the primary structure (e.g. substitution of base caused by mutation of DNA) can lead to protein with different properties e.g. sickle cell anaemia
SCA
at 6th amino acid, the glutamic acid changes to valine, changing the shape of red blood cells from biconcave disc to sickle shape
Affects: amount of O2 transported, ability to flexibly move around transport system
Secondary structure
Describes the coiling of the polypeptide chain with hydrogen bonding alone between the amine group of one amino acid and carboxyl of another
Between NH2 and C=O
Structures formed tend to be a-helix or b-pleated sheet
Collagen structure
- 3 helical polypeptide chains which wind around each other to form triple helix
- held together by hydrogen bonds
- at R groups of lysine amino acids, covalent bonds form different triple helices. These cross-links hold many collagen molecules side by side, forming a collagen fibre
- resulting fibres have a large tensile strength
- important structural protein found in skins, tendons and walls of the blood vessels
Tertiary structure describes
Describes the shape of the polypeptide chain with hydrogen, ionic and disulphide bonds between the R groups
Quaternary structure
Describes the shape of the molecule when several polypeptide chains are wound around each other
Often there may be a non-amino acid part to the molecule, a prosthetic group
e.g. haemoglobin has a quaternary structure and is made of four polypeptide chains wound around each other with 4 haem groups that contain iron
Proteins with/without prosthetic groups
Complex
Simple
Haemoglobin
Oxygen carrying pigment found in RBC
4 polypeptide chains: 2 a & 2 b chains
Structure is curled up so that hydrophilic chains are on the outside of the molecule, making it soluble in water
Each polypeptide chain has a haem group which is not made of amino acids (prosthetic group)
Each haem contains a Fe2+. One O2 can bind with each Fe2+ so a complete Hb molecule can carry 8 O2 atoms
Test for proteins
Biuret test
Add biuret reagent
Positive: purple
Negative: no colour change/blue
Main molecular shapes in protein structure
Fibrous such as collagen & keratin that have a structural function
Globular such as enzymes and haemoglobin that carry out metabolic functions
Fibrous proteins
Polypeptide chains form long parallel strands, chains wind around each other in the secondary structure and twist into a second helix in the tertiary structure
Repeated, regular sequences of amino acids
Little variability between proteins
Stable and insoluble
Role in support and structure
Globular proteins
Polypeptide chains folded into a spherical shape
Irregular sequences of amino acids not repeated
Soluble and less stable
Variability between proteins
Metabolic role
Functions of proteins
Enzymes, acting as biological catalysts e.g. amylase, lactase and maltase
Contractile proteins e.g. actin and myosin
Transport proteins, that carry substances around the body e.g. haemoglobin
Structural proteins e.g. collagen, keratin
Hormones, chemical messengers e.g. insulin, glucagon and ADH
Properties of proteins
Insoluble in water - some form colloids
Formed by condensation reactions and broken down by hydrolysis. Enzymes involved in hydrolysis - endopeptidases, exopeptidase and membrane-bound dipeptides
denatured by certain factors
Denatured
Change in shape due to bonds being broken
Factors which denature proteins include
High temperature Acids Alkalis Inorganic solvents Organic solvents Mechanical force
which types of bonds exist in the tertiary structure of a protein that aren’t present in the secondary structure
Ionic bonds, disulphide
