formed when more than two amino acids join together
Amino acids have the same general structure
● a carboxyl group (-COOH)● an amine or amino group (-NH2)● an R group (also known as a variable side group)— attached to a carbon atom
R groups generally contain carbon. The only exception is
● glycine — its R group consists of just one hydrogen atom
All living things share a bank of
only 20 amino acids— only difference between them is what makes up their R group
Amino acids are linked together by
● condensation reactions— to form dipeptides and polypeptides— A molecule of water is released during the reaction
Peptide bonds
The bonds formed between amino acids
The reverse reaction (hydrolysis) happens when
dipeptides and polypeptides are broken down
Hydrogen bond
is a relatively weak bond formed between hydrogen atoms and other atoms (FON)— e.g. fluorine, oxygen, nitrogen
Primary structure
● is the number and sequence of amino acids in the polypeptide chain— can determine the 3D shape or tertiary structure— therefore affect the shape of the active site in enzymes
Proteins are much easier to explain if you describe their structure in four levels
is specific for each protein — one alteration in the sequence of amino acids can affect the function of the protein
Secondary structure
● Hydrogen bonds form between the amino acids in the chain● makes it automatically coil into— an alpha (a) helix or— fold into a beta (b) pleated sheet
The many hydrogen bonds make these structures
stable— Most channel proteins are made of alpha helices
Disulfide bridges
are covalent bonds between sulfur atoms— a lot stronger than the ionic and hydrogen bonds in proteins
Tertiary structure
● The coiled or folded chain of amino acids is often coiled and folded further● More bonds form between different parts of the polypeptide chain— including hydrogen bonds and ionic bonds — attractions between negative and positive charges on different parts of the molecule
Tertiary Structure
● Disulfide bridges also form whenever two molecules of the amino acid cysteine come close together— the sulfur atom in one cysteine bonds to the sulfur atom in the other
The tertiary structure is
the 3D shape of the polypeptide chain— All enzymes, antibodies and some hormones have this
The tertiary structure
creates a specific shape due to the sequence of amino acids in the chain as ● hydrogen bonds● ionic bonds● disulfidebridges (covalentbonds) — form between R groups
A change to the sequence of amino acids would affect
the secondary and tertiary structure — as these bonds would form in different places
Quaternary structure
proteins made of more than one polypeptide chain joined together
Examples of quaternary structures
●Antibodies●Haemoglobin●Insulin●Collagen ●Other non-protein groups may also be associated — e.g the haem group in haemoglobin
Quaternary Structure
is the way these polypeptide chains are assembled together— Some proteins are made of several different polypeptide chains held together by bonds
The quaternary structure is the proteins
final 3D structure
Not all proteins have a quaternary structure — some are made of only one polypeptide chain.
some are made of only one polypeptide chain
A protiens shape determines
its function — e.g. haemoglobin is a compact, soluble protein— makes it easy to transport— makes it great for carrying oxygen around the body
Proteins found in living organisms have all got different structures and shapes, which makes them
specialised to carry out particular jobs
Enzymes
● roughly spherical in shape — due to the tight folding of the polypeptide chains● soluble ● often have roles in metabolism — e.g. some enzymes break down large food molecules (digestive enzymes)● other enzymes help to synthesise (make) large molecules
Antibodies
● are involved in the immune response ● are found in the blood● made up of two light (short) polypeptide chains two heavy (long) polypeptide chains— bonded together● have variable regions — the amino acid sequences in these regions vary greatly.
Transport proteins
● E.g. channel proteins are present in cell membranes— Channel proteins contain hydrophobic (waterhating) and hydrophilic (waterloving) amino acids— cause the protein to fold up and form a channel● These proteins transport molecules and ions across membranes
Structural proteins
● are physically strong● consist of long polypeptide chains lying parallel to each other with cross‑links between them● include :— keratin (found in hair and nails) — collagen (found in connective tissue)
Collagen has
● three polypeptide chains tightly coiled together — makes it strong— makes it a great supportive tissue in animals
Proteins are also used as
chemical messengers in the body— e.g. as hormones like insulin
To test for a protien
use the biuret test
The biuret test for proteins
● add a few drops of sodium hydroxide solution — test solution needs to be alkaline● add some copper(II) sulfate solution