In this video, we will learn about proteins, their structure and their function. We will learn how amino acids bond into polypeptide chains which are then shaped into functional proteins. And we’ll learn how proteins are classified by their structure.
Proteins are some of the most important and complex biological macromolecules because proteins carry out such a wide variety of functions within organisms. Proteins function as enzymes, as hormones, and as pigments. They help to protect our body from infection, and they function in the transport of materials. They play roles in signaling, they give cells and organisms structure, and they allow movement.
Proteins and nucleic acids are closely related. In organisms, the instructions for making proteins are stored in DNA, which is transcribed into RNA, which is then translated to make proteins. Proteins are polymers. Polymers are macromolecules made up of several repeating subunits, or monomers. The monomers of proteins are called amino acids.
This diagram shows the general structure of an amino acid. It’s formed around essential carbon atom called the alpha carbon. And like all carbon, this atom is able to form four chemical bonds. The alpha carbon is bonded with one hydrogen atom, a carboxyl group, an amino group, and what we call the R group or side chain. The R group of the amino acid varies. Here, I’ve drawn a second amino acid complete with side chain. And this amino acid has a side chain that identifies it as leucine. Leucine is one of 20 different standard amino acids, the names of which are often abbreviated to one letter for quick reference. I’ve finished this amino acid within R group that identifies it as threonine.
When monomers bond together, we call it polymerization. And the polymerization of amino acids is the first step to making a protein. When two amino acids bond together, a condensation reaction, also sometimes called dehydration synthesis, occurs between the amino group of one amino acid and the carboxyl group of the other. The atoms rearrange. One oxygen and two hydrogen atoms are released, and they bond together to form one molecule of water. The bond that forms between the two amino acids is called a peptide bond.
This molecule is called a dipeptide because it contains two amino acids. A functional protein starts with a string of amino acids, which we call a polypeptide because it contains many peptide bonds.
Now that we know how polypeptides are formed, we’re ready to look at the overall structure of a finished protein. The properties of a functional protein don’t only rely on the sequence of the amino acids. A protein’s function is also determined by its final shape and structure. You can think of the amino acids as an alphabet, which is used to build words. And those words are composed into sentences or even paragraphs which give the words meaning. This is similar to how polypeptides are eventually formed into functional proteins.
The sequence of amino acids in the polypeptide chain is what we refer to as the primary structure of the protein. And we’ve already learned that this chain is held together by peptide bonds. The polypeptide chain can then take on either of two secondary shapes. It can spiral into what we call an 𝛼-helix, or it can fold into what we call a 𝛽-sheet. Both the 𝛼-helix and 𝛽-sheet structures are held in place by hydrogen bonds.
The tertiary structure of the protein is the three-dimensional arrangement that the 𝛼-helices and 𝛽-sheets take on. One polypeptide chain can have different secondary structures in different places. The tertiary structure of a protein is held in place by several different types of bonds, which include hydrogen bonds, ionic bonds, and disulfide bonds. Hydrophobic interactions are the attraction between nonpolar parts of certain amino acids. They also contribute to the tertiary structure of the protein.
The quaternary structure of the protein is made up of more than one folded polypeptide bonded together. They’re three-dimensional structures fit together like puzzle pieces, and they’re held together by bonds and interactions between their various parts. The combination of different amino acid sequences, along with different three-dimensional structures, means that the possibilities for different proteins are nearly infinite, which is why this macromolecule is able to carry out so many different functions.
Proteins are commonly classified by their structure. Three common structural classifications are fibrous, globular, and conjugated. Fibrous proteins are more or less what they sound like. They are long, strands-like proteins with repetitive structures. Fiberous proteins are insoluble in water, and their stable structure adapts them to functions having to deal with structure and movement. Some examples can be found in the cytoskeleton within cells. Specific examples include the myosin and actin that are responsible for the contraction of muscle cells.
Globular proteins have a more globular, or round and lumpy, shape. These proteins tend to be water soluble. And they are less stable and more chemically active than fibrous proteins. These properties make globular proteins especially adapted to participating in chemical and metabolic reactions. Examples of globular proteins are enzymes as well as certain hormones, for example, insulin.
Conjugated proteins are proteins that are attached to a nonprotein chemical. Some examples are glycoproteins, which function in cell communication, and hemoglobin, which allows our blood cells to efficiently carry oxygen.
Now that we’ve learned about proteins, their structure, function, and classification, let’s try a practice question.
The diagram shows a basic outline of an amino acid. Which two parts of amino acids join to form a peptide bond?
This question shows us a simplified chemical diagram of an amino acid. It asks us to recall what we know about how to amino acids bond together. The structure of an amino acid is centered around what we refer to as the alpha carbon. The alpha carbon is bonded with an amino group on one side and a hydrogen atom, a carboxyl group on the other side, and finally what we refer to as an R group or side chain.
The composition and structure of the R group can vary. There are 20 different standard amino acids and their side groups are what give them their unique identities and their properties.
In the first step of protein synthesis, several different amino acids bond together to form a polypeptide chain. The polypeptide chain gets its name from the peptide bonds that join the amino acids together. So how does a peptide bond form? Here, I’ve drawn a slightly more detailed representation of two amino acids so that we can investigate how the peptide bond forms.
The polymerization of amino acids occurs through a chemical process known as a condensation reaction, also sometimes called a dehydration synthesis reaction. The condensation reaction gets its name from the fact that it causes a molecule of water to be released. In order for a water molecule to form, two hydrogen atoms and one oxygen atom must be released. The carboxyl group of one amino acid contributes a hydrogen and an oxygen, and then the amino group of the other amino acid contributes the second hydrogen. One molecule of water is released, and the bonds that forms between the amino acids is called a peptide bond.
So the two parts of amino acids that join to form a peptide bonds are NH2, or the amino group, and COOH, or the carboxyl group.
Let’s try a second practice question.
Keratin is a long protein found in hair and nails, with many repeats of the sulfur-containing amino acid cysteine. Using the table provided, determine the group of proteins that keratin is most likely to belong to.
Our table lists three different groups of proteins and some of their properties. Globular proteins have the properties of being compact, roughly spherical, and water soluble. Conjugated proteins are proteins with a prosthetic group. And fiberous proteins are listed as long and insoluble with a repetitive primary structure.
Our question lists some of the properties of a protein called keratin. And then we’re asked to use the information provided and our prior knowledge to determine the group of proteins that keratin would most likely belong to. And we have three choices: globular, conjugated, or fibrous.
In order to answer this question, we’ll try to match what we know about keratin to the properties listed in this table. We’re told that the protein keratin is long in shape. In our table, we’re also told that fibrous proteins are long, which is a great start, but let’s keep going. We’re also told that keratin has many repeats of a particular amino acid. We know that the sequence of amino acids in a polypeptide chain are what make up the primary structure of a protein. This piece of information confirms that keratin has a repetitive primary structure, just like a fibrous protein.
Based on these observations, we’re able to conclude that the group of proteins that keratin is most likely to belong to is fibrous.
Let’s wrap up our lesson by taking a moment to review what we’ve learned. In this video, we learned that proteins are polymers made up of monomers called amino acids. We learned that an amino acid is a molecule with an amino group at one end and a carboxyl group at the other. The amino and carboxyl groups between two amino acids can join by condensation reaction to form a peptide bond.
There are 20 different standard amino acids that are characterized by the structure and composition of their side chain or R group. Proteins can be classified as fibrous, globular, or conjugated based on their overall structure.
We describe the structure of proteins at four different levels. The primary structure is the sequence of amino acids in the polypeptide chain. The secondary structure’s either 𝛼-helix or 𝛽-sheet. The tertiary structure is the three-dimensional shape that the helices and sheets along an amino acid sequence will fold into. The quaternary structure occurs when multiple folded polypeptides bond together.