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1.6 Proteins (STRUCTURE OF PROTEINS (PRIMARY STRUCTURE - The primary…
1.6 Proteins
STRUCTURE OF PROTEINS
PRIMARY STRUCTURE - The primary structure determines the ultimate shape and function of the protein. One amino acid change (unintentional) can alter the shape and change the way it functions. The shape is specific to the function and crucial! Alterations can render the protein useless.
How it forms: polypeptide chains form when amino acids bond together in a particular sequence. The primary structure of a protein is the number, type and sequence of amino acids that make up this linear chain together with the peptide bonds that hold them together. Different proteins have different primary structures. Different proteins are made up of different types, numbers and sequences of animo acids making up the primary chain.
SECONDARY STRUCTURE - The secondary structure of proteins is the arrangement in space of the atoms that form the backbone or linear chain of the protein. The amino acids chain can coil into a helix shape, due to positive charge on
-NH and negative charge on C=O group on either side of the peptide bond forming a H bond.
The helix and beta pleated sheet shapes are secondary structures of protein molecules.
BETA-PLEATED SHEET - the amino acids chain folds back upon itself many times forming anti-parallel chains. The oxygen and hydrogen atoms that have been brought into close proximity form cross linking hydrogen bonds. The hydrogen bonds help to stabilise this secondary structure.
TERTIARY STRUCTURE - The secondary structure can be twisted even more giving it a more complex shape maintained by several bonds. Where these bonds occur depends on the primary structure of the protein. This stage also creates the proteins distinctive recognition.
QUATERNARY STRUCTURE. Large proteins from complex molecules with a number of individual polypeptide chains linked in various ways. It can be a non-protein (prosthetic group) for example the iron haem group found in haemoglobin protein structures.
The 3D structure is important to how proteins function the sequence of amino acids (primary structure) is what determines the structure in the first place!
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BIURET TEST
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3) Mix gently. It will become a purple colour to indicate the presence of a protein. If it remains blue there is no protein present.
AMINO ACIDS
Amino acids are a monomer unit which combine for form polymers called polypeptides. They are made up of a central carbon atom that attaches to an amino group, a carboxylic acid group, a hydrogen atom and a R group.
R group = the side chain from the central carbon atom. It can be anything from a simple hydrogen atom to a more complex ring structure.
PEPTIDE BONDS
2 amino acids can join together to form a dipeptide linked by a peptide bond. Peptide bonds form between the carboxyl (COOH) group on one of the amino acids and the animo (NH2) group on the other.
This produces water - it is therefore a condensation reaction.
A dipeptide can be split again back into two amino acids by a hydrolysis reaction.
POLY PEPTIDES - if more amino acids are added to a dipeptide it becomes a polypeptide. It happens in the polymerisation process (several condensation reactions to make a large polymer)
A protein consists of one or more polypeptide chains folded into highly specific 3D shapes.
There is four levels of protein structure: primary, secondary, tertiary and quaternary
Function of proteins
Proteins are large and complex polymers made up of long chains of amino acids. It has a wide range of roles.
STRUCTURAL - proteins are the main component of body tissues like muscle, skin and hair
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BONDS THAT STABILISE SECONDARY AND TERTIARY STRUCTURES - as the chains of amino acids bend and fold to form secondary and tertiary structures, various atoms are brought into close proximity and form bonds. Hydrogen and oxygen atoms from both the main chain and the R groups may form hydrogen bonds. The R group of two amino acids contain sulphur atoms. When these atoms are in close proximity they form disulphide bridges. Many carboxylic acid and amino acid groups form charged groups n solution Oppositely charged groups form ionic bonds. Many hydrophobic R groups tend to cluster towards the interior of the protein molecule forming hydrophobic interactions.