We’ve already looked at the monomers that make up a protein: amino acids. But fully functioning proteins are often much more complex in structure than a simple polypeptide chain. Today we will look at the different levels of structure in proteins and some specific examples.
Levels of protein structure
So let’s start with a quick recap of the primary structure of a protein. A polypeptide chain is a sequence of amino acids joined with peptide bonds. Make sure to revise the previous article if you’re feeling a bit lost already.
Once the polypeptide chain is made, it can start to form the next level of structure. There are two types of secondary structure that can be formed: a coiled α-helix or a folded β-pleated sheet. Both of these structures are caused by hydrogen bonding between N-H groups and C=O groups on different amino acids in the chain. Take a look here at the detailed diagram.
The tertiary structure forms when the α-helices or β-pleated sheets coil or fold up even further and more bonds form between the different amino acids. There are several types of bond that can form:
- Hydrogen bonds: form between partial positive and partial negative charges in different R groups.
- Disulfide bonds: in some amino acids (cysteine for example) the R group contains a sulfur atom which can bond to sulfur atoms in the R group of another amino acid.
- Ionic bonds: you’ll know about these from chemistry. If an R group is negatively charged, it can bond ionically to a positively charged R group.
As well as these bonds, there can be interaction between hydrophobic and hydrophilic regions of the polypeptide chain. Hydrophobic R groups clump together on the inside of the protein away from water, and hydrophilic R groups are happily pushed to the outside as they are attracted to water.
Lastly, the quaternary structure can be formed. This is when multiple polypeptide chains come together to form a protein. The classic example of this is haemoglobin (see below) which is made of four polypeptide chains. The quaternary structure is influenced by the tertiary structure, so all the types of bond we’ve mentioned so far come into play.
Types of protein
The final structure of a protein determines its function. There are two main groups of proteins:
- Globular proteins: round, compact and soluble
- Fibrous proteins: long rope-like structure, strong and insoluble
Globular proteins are soluble due to hydrophobic and hydrophilic interactions as mentioned above. This makes them good for being transported in fluids, so hormones and enzymes are globular proteins. Some specific examples are:
- Haemoglobin: found in red blood cells and carries oxygen. Made up of four polypeptide chains, each of which has a iron-containing non-protein prosthetic group called haem. Haem binds oxygen. Because it has a non-protein component it is called a conjugated protein.
- Amylase: an enzyme found in saliva and the small intestine which hydrolyses starch to maltose. This enzyme is a single polypeptide chain.
- Insulin: a hormone secreted by the pancreas to help control blood glucose concentration. Insulin is two polypeptide chains joined with disulfide bonds.
Fibrous proteins have more of a structural role, so they don’t need to be easily transported or reactive. Some examples are:
- Elastin: found in connective tissue that needs to be elastic and spring back into shape e.g. arteries, ligaments, skin.
- Keratin: often found externally e.g. nails, hair, horns, feathers, skin. Can be hard or flexible.
- Collagen: found in connective tissue e.g. bone (where it binds to minerals to make it extra strong), muscle, skin.
Proteins are very diverse and versatile molecules with all sorts of uses. You will come across antibodies when you study the immune system, and transport proteins found in the cell membrane will pop up in a number of topics.
Summary
- Primary structure is the sequence of amino acids in a polypeptide chain.
- Secondary structure can be an α-helix or a β-pleated sheet. These are formed through hydrogen bonding.
- Tertiary structure is further coiling and folding caused by various types of interactions and bonds between R groups.
- Quaternary structure is when multiple polypeptide chains combine to produce one protein.
- Globular proteins are round, compact and soluble which makes them great for transport in fluids.
- Fibrous proteins are rope-like, strong and insoluble which makes them great for providing structure.
