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The key points covered in this video include:

1. Globular Proteins
2. Haemoglobin
3. Insulin
4. Pepsin

Globular Proteins

The overall 3-D structure of a protein determines which type of protein is formed - fibrous and globular. Globular proteins have a few important properties: They are roughly spherical in shape, with hydrophobic R groups on the inside and hydrophilic R groups on the outside, They are therefore soluble in water, They have very specific shapes which allows them to carry out very specific functions. These properties allow them to carry out many important roles in the body. A globular protein is a protein with a spherical shape that is soluble in water. Globular proteins typically have metabolic roles.

Haemoglobin

An important example of a globular protein is haemoglobin which is found in red blood cells. Haemoglobin has an important role in the transport of oxygen from the lungs to the body tissues. Haemoglobin is made up of four polypeptide chains meaning that the protein has a quaternary structure. Haemoglobin is also a conjugated protein as it has a prosthetic group attached to each polypeptide chain. A prosthetic group is a non-protein component that is necessary for the protein to carry out its function. The four prosthetic groups in haemoglobin are called haem groups and contain an iron ion (Fe2+). One oxygen molecule binds to each haem group, allowing each haemoglobin to transport four oxygen molecules.

Insulin

Another important globular protein is the hormone insulin which is secreted by the pancreas. Insulin has an important role in maintaining blood glucose concentration. Insulin is composed of two polypeptide chains - one of which has an α-helix and the other a β-pleated sheet. The two polypeptide chains are joined together by disulphide links. The shape of insulin allows it to specifically bind to receptors on cell membranes to help lower blood glucose concentration. Insulin also has hydrophilic R groups on the outside making it soluble in water. This allows insulin to dissolve in the blood and be easily transported around the body.

Pepsin

Globular proteins can also be enzymes, such as pepsin which catalyses the digestion of proteins. Pepsin is found in the stomach which is an acidic environment - one with a low pH. Many enzymes becomes denatured at low pH because they contain amino acids with basic R groups. Basic R groups accept hydrogen ions (H+) and become positively charged. The positively charged R groups can affect the ionic bonds and hydrogen bonds in the protein- this denatures the enzyme and therefore alters its function. The primary structure of pepsin has very few basic R groups, preventing the tertiary structure from being affected by low pH. The tertiary structure of pepsin is also kept stable by hydrogen bonds and disulphide links.

Summary

Globular proteins are spherical proteins which are soluble in water
Globular proteins can have many roles, including metabolic roles, transport roles and hormonal roles
Haemoglobin is a protein with the role of transporting oxygen around the body
Haemoglobin has prosthetic groups called haem groups where oxygen is able to bind
Insulin is a hormonal protein that regulates blood glucose concentration
Insulin has a specific 3-D structure that allows it to be easily transported in the bloodstream and specifically bind to insulin receptors
Pepsin is an enzyme found in the stomach which catalyses digestion of polypeptides
Pepsin has a primary structure that reduces the effect of low pH

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