Proteins: Study with Video Lessons, Practice Problems & Examples

Proteins are essential macromolecules composed of amino acids linked in polypeptide chains, exhibiting four structural levels: primary (amino acid sequence), secondary (local structures like alpha helices and beta sheets), tertiary (3D structure of a single chain), and quaternary (assembly of multiple chains). The R group of amino acids determines their properties, influencing protein folding into globular or fibrous shapes. Chaperones assist in proper folding, ensuring functional domains and active sites are formed, critical for biological processes such as enzyme activity and signal transduction.

Proteins

Hi. In this video, we're going to be talking about proteins. So, translation has created some proteins, and we're not going to spend a ton of time talking about proteins in this course because it's a genetics course, and we don't really care as much about them. It's other biology courses, but I do want to briefly go over some of them. So, proteins, they're made up of amino acids, and they're organized into these chains called polypeptide chains. Now there are 4 structural levels of proteins that you need to be aware of. It's called primary, secondary, tertiary, and that makes sense. So 1, 2, 3, 4. Now primary structure refers to just the amino acid sequence. Secondary structure is local structures found within the polypeptide chain, so things close to each other, and these can be divided into 2 structures, which I will show you a picture of to differentiate them, but these are alpha helices and beta sheets, and they actually physically look different. Then you have tertiary structure, and that is going to be the 3D structure of the entire polypeptide chain, and you have quaternary structure, which is the 3D structure of multiple polypeptide chains that make up a single protein. So proteins can be composed of lots of polypeptide chains and quaternary structure deals with that structure. Now every single protein has an amino end and a carboxyl end, so I like to think of this as kind of the start and this is the end, but it just is based on the amino group and the carboxyl group. So here we have primary structure, you can just see these are different amino acids, and that is the sequence of them. Secondary structure is here, and you have alpha helices, which you can see actually looks like a helix, and you have beta sheets. This is antiparallel, those 2 types. Not super important you know about them in this class, but we're going to mention them here, it's not spelled at all correctly. This is antiparallel because the directions of the beta sheets are going opposite, but it could be parallel if they were going the same in either direction. But this is a beta sheet that's antiparallel, and so this is a local or a regional substructure that happens in the polypeptide chain. The tertiary structure deals with the entire polypeptide chain. So you can see there's a lot of beta helices here or beta sheets. There's a couple of alpha helices here. This is the entire polypeptide chain and what its structure looks like, and then you have quaternary structure. So this is various polypeptide chains, so here we have they're all in different colors, so we have the red one, the orange one, the green one, and the blue one. So there are 4 polypeptide chains here, and each of them come together, and their entire structure together makes up the quaternary structure. Now if we talk about the individual amino acids themselves, we like to focus on the R group, and the R group provides is the region of the amino acid that provides the proteins with certain properties. So our R groups can be nonpolar, polar, charged, positive or negative, and these R groups allow for the protein to fold into a bunch of different shapes, and protein shapes are divided into 2 main classes, even though there's a ton of different protein shapes, but we sort of divide them all into 2 classes. You have globular proteins, these are more compact proteins. So if this was a protein, it would look like this or something similar. And you have fibrous proteins, which are linear proteins, so they just look like a line. It might be a squiggle line, but essentially it's just a line, 3D line, because these are three-dimensional things. R groups also allow proteins that have specific domains, which are structural regions that have sometimes could be domains, and these active sites sometimes could be domains, these structural regions with specific functions. And protein folding is really controlled. Proteins mainly can fold on their own, but sometimes they need help, and the proteins that help them are called chaperones, and they help fold proteins correctly. So here is an example of a chemical structure of what an amino acid looks like. You have your carbon, you have your amino group, you have your carboxyl group over here, you have a hydrogen, but the important group here is the R group, and this gives different amino acids or in properties, and when multiple amino acids are attached on here, each with different R groups, that gives an entire polypeptide chain certain properties that help it fold and give it function and give it structure and all sorts of things. So that's a very brief overview of proteins, but this is a genetics class, so that's probably all you're going to get. So, with that, let's now move on.

Which of the following protein structures describes a 3D structure of one polypeptide chain?

• A. Primary structure
• B. Secondary structure
• C. Tertiary structure
• D. Quaternary structure

Which of the following describes the amino acid sequence of a polypeptide chain?

• A. Primary structure
• B. Secondary structure
• C. Tertiary structure
• D. Quaternary structure

Which of the following describes the 3D structure of multiple polypeptide chains in a single protein?

• A. Primary structure
• B. Secondary structure
• C. Tertiary structure
• D. Quaternary structure

Which of the following describes the local structures formed in a single polypeptide chain?

• A. Primary structure
• B. Secondary structure
• C. Tertiary structure
• D. Quaternary structure

Here’s what students ask on this topic:

What are the four structural levels of proteins?

The four structural levels of proteins are primary, secondary, tertiary, and quaternary. The primary structure is the amino acid sequence of the polypeptide chain. The secondary structure refers to local structures within the chain, such as alpha helices and beta sheets. The tertiary structure is the 3D shape of a single polypeptide chain, determined by interactions among R groups. The quaternary structure involves the assembly of multiple polypeptide chains into a functional protein complex.

What is the role of chaperones in protein folding?

Chaperones are proteins that assist in the proper folding of other proteins. While many proteins can fold on their own, chaperones help prevent misfolding and aggregation, ensuring that proteins achieve their correct 3D structure. This is crucial for the protein's functionality, as improper folding can lead to loss of function or diseases. Chaperones bind to nascent or unfolded polypeptides, providing a conducive environment for correct folding.

How do R groups influence protein structure and function?

R groups, or side chains, of amino acids determine the properties of each amino acid, influencing the protein's overall structure and function. R groups can be nonpolar, polar, positively charged, or negatively charged, affecting how the protein folds and interacts with other molecules. These interactions contribute to the protein's 3D shape, stability, and functional domains, which are essential for biological activities such as enzyme catalysis and signal transduction.

What is the difference between globular and fibrous proteins?

Globular proteins are compact, spherical proteins that are generally soluble in water. They perform various functions, including enzymatic activity, transport, and regulation. Examples include hemoglobin and enzymes. Fibrous proteins, on the other hand, are elongated, linear proteins that provide structural support and strength. They are usually insoluble in water and include proteins like collagen and keratin. The distinct shapes of these proteins are due to differences in their amino acid sequences and folding patterns.

What are alpha helices and beta sheets in protein structure?

Alpha helices and beta sheets are types of secondary structures in proteins. An alpha helix is a right-handed coil where the backbone of the polypeptide chain forms a spiral, stabilized by hydrogen bonds between the carbonyl oxygen of one amino acid and the amide hydrogen of another. Beta sheets consist of beta strands connected laterally by at least two or three backbone hydrogen bonds, forming a sheet-like structure. These can be parallel or antiparallel, depending on the direction of the strands.