Proteins

• Proteins are made up of C, H, O, N. There a group of large and complex polymer molecules, made up of long chains of amino acids.
• There roles include structural, catalytic, signaling and immunological.
• All amino acids have the same general structure of a carboxylic acid group, an amino group and an R group (which defines an amino acid).
• Amino acids join by condensation reactions to form peptide bonds.
• Two amino acids form a dipeptide, three form a tripeptide and many form a polypeptide.
• Proteins have four structural levels, primary, secondary, tertiary and quaternary.
• The primary structure is the sequence of amino acids in the chain, determining the eventual shape and function.
• The secondary structure is whether the protein coil into an a helix or fold into a b pleated sheet due to many hydrogen bonds.
• The tertiary structure is the further coiling or folding of structures. More bonds form due to interactions between the R-groups of the polypeptide chain.
• The quaternary structure is how several polypeptide chains are held together by bonds.
• The 3D shape of a protein is maintained by hydrogen bonds, hydrophobic interactions and disulfide bonds.
• Peptide bonds are primary. Hydrogen bonds are secondary and tertiary. Ionic, disulfide and hydrophobic and hydrophilic interactions are all tertiary.
• If the bonds that maintain a protein’s shape are broken, the protein will stop working properly and is denatured.
• Changes in temperature, pH or salt concentration can all denature a protein.
• Globular proteins are round, compact and easily soluble so they can be transported in fluids. Eg haemoglobin and enzymes.
• Haemoglobin is a globular protein. Hydrophilic side chains face out and hydrophobic side chains face in. Haemoglobin's soluble because there folded so hydrophobic groups are on the inside and hydrophilic groups on the outside. There therefore good for transport in the blood.
• Fibrous proteins are tough and rope-shaped. They are formed from parallel polypeptide chains held together with cross links. They tend to be found in connective tissues such as tendons.
• Collagen is made of three polypeptide chains tightly coiled in a strong triple helix. The chains are interlinked by strong covalent bonds.
• Keratin has lots of disulfide bridges between it's polypeptide chains alongside hydrogen bonds making the molecule very strong. It's found in fingernails, hair and horns.
• Elastin is strong and flexible due to cross linking and coiling. When stretched the elastin proteins elongate but remain attached to each other.
• Fibrous proteins are long and narrow with little tertiary structure whereas globular are round and spherical (3D). Fibrous are structural repetetive amino acid sequences whereas globular are functional/metabolic irregular amino acid sequences.