[Music] hi and welcome back to free science lessons by the end of this two-part video you should be able to describe the features of globular proteins you should then be able to describe how the structures of three globular proteins are linked to their functions so far in this topic we've had a good look at the structure of proteins now one key idea you need to understand is that the structure of any protein is critical to its function scientists divide proteins into two very broad groups based on their structure these groups are called globular proteins and fibrous proteins and in this video we're looking at globular proteins i'm showing you the simplified structure of a globular protein here as you can see globular proteins tend to have an approximately spherical shape now the key feature of globular proteins is that they're soluble in water and you have to be able to explain why we've already seen that some amino acids have got r groups which are attracted to water these are called hydrophilic amino acids globular proteins have got hydrophilic amino acids on their surface this means that the hydrophilic r groups can interact with water molecules and this makes globular proteins soluble in water other amino acids have got r groups which are not attracted to water and these are called hydrophobic amino acids in globular proteins we find the hydrophobic amino acids deep in the center of the protein well away from any water molecules coming up we're going to look at the structure of the globular protein hemoglobin [Music] okay now if you're following the ocr biology spec then you need to be able to describe how the structures of three globular proteins link to their functions but even if you're not following the ocr spec then i would still try to get the general ideas as they'll help you gain a deeper understanding of biology we're going to start with hemoglobin and in the next video we look at insulin and an enzyme okay now hemoglobin is a globular protein with four polypeptide subunits two are called the alpha subunits and two are called the beta subunits now we find hemoglobin in red blood cells and the function of hemoglobin is to bind reversibly to oxygen hemoglobin binds to oxygen in the lungs and then it releases the oxygen in the body tissues now a key feature of hemoglobin is that each subunit contains the prosthetic group heme and this means that hemoglobin is an example of a conjugated protein each heme group contains an fe2 plus ion and this is where the oxygen binds so this means that one hemoglobin molecule can bind to four oxygen molecules now hemoglobin has a really interesting feature when oxygen attaches to hemoglobin the quaternary structure of the protein changes slightly and this makes it easier for more oxygen to attach we'll be looking at how hemoglobin carries oxygen in much more detail in later videos in the next video we look at the structures of two more globular proteins