Polypeptides and a carboxyl group. A carboxyl group

Polypeptides are chains of amino acids and essential portions of proteins in cells. In this video, learn how polypeptides are formed and what’s unique to their structure.

Proteins

Proteins are extremely important to cellular function.

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They make up half of the mass of a cell. Protein comes from the Greek word ‘proteios,’ which means ‘first place.’ Proteins help support structure in the cell and with transport and storage of substances. They also work to help to defend the cell and control metabolic functions.

Structurally, proteins are very diverse and complex. Each type of protein holds its own specific shape, and that’s where polypeptides come in.

Polypeptides

Polypeptides help make up proteins by bonding numerous amino acids together. Proteins are created by the bonding of two or more polypeptides, which are then folded into a specific shape for a particular protein.

Amino Acids

Polypeptides are similar in structure to their particular amino acid groups, except that they’re connected by covalent, or electron-sharing bonds. Amino acids are the basic building blocks of the polypeptide. There are 20 different amino acids, all with specific structures. By understanding the structure of the amino acids needed for whatever protein you are building, you can grasp the idea of how they will bond together with other amino acids.

Carboxyl Groups

Carboxyl groups appear in all amino acids since the latter are made up of an amino group and a carboxyl group. A carboxyl group consists of a hydroxyl (OH) bonded to a carbon, which is double bonded to an oxygen.

This creates a negative group, thereby allowing the group to bond with another carbon atom and create a covalent bond. This group bonds to an amino group, which leads to the creation of an amino acid.Polypeptides become increasingly complex and diverse as they form proteins. At one end of the polypeptide is the carboxyl group called the c-terminal. On the opposite end is the amino terminal, or n-terminal.

Formation

So, how are polypeptides formed? It’s kind of a ‘wet process,’ actually. Why? Because it involves water. Here’s what this means: each amino acid has what’s known as an amino group. An amino group is one that looks like this: NH2, where the N stands for nitrogen and the H stands for hydrogen. Amino acids also have a carboxyl group, which looks like this: COOH. The C stands for carbon, the O stands for oxygen, and the H you already know about. When the polypeptide is formed, at least two amino acids saddle up next to one another.

When they do so, they reach out to each other (chemically speaking, of course). One reaches out with the amino group and the other reaches out with the carboxyl group.They then chemically join hands to form a peptide bond (a.k.a. amide bond), which is the covalent bond between two amino acids. You know how if you hold someone’s hand for too long, some watery sweat begins to build up between the hands? Well, when the two amino acids join hands to form a peptide bond, they actually release water as well.

This is known as a condensation reaction because two or more molecules (in this case amino acids) combine to form an even larger molecule (a dipeptide, in this case) with a concurrent release of a smaller molecule (water, in our example). The amino acid reaching out with the carboxyl end (COOH) lets go of an OH, while the amino acid reaching out with the amino end (NH2), lets go of an H. OH + H = H20, which equals water.

When two amino acids form a peptide bond, they form a molecule known as a dipeptide, where ‘di’ means two. A peptide is a molecule that is loosely defined as being composed of 2-50 amino acids, sometimes called an oligopeptide. A polypeptide is then loosely seen as a molecule composed of 50 or more amino acids.

That being said, technically speaking, a polypeptide is a peptide composed of many (‘poly-‘) peptides and that could mean just a few amino acids linked together.Each amino acid unit in a polypeptide is called a residue. As such, all proteins are polypeptides, but not all polypeptides are proteins.

The discussion for why this is the case is beyond this lesson’s scope, but involves factors such as function, mass, and structure that are found in a protein but not in a simple and short polypeptide chain.

Lesson Summary

Let’s review. A polypeptide is a chain of amino acids. Amino acids bond together with peptide bonds in order to form a polypeptide.

The n-terminal (amino terminal) is located at one end of the polypeptide while the c-terminal (carboxyl terminal) is located at its other end. Two or more polypeptides can bond together to form proteins and contribute to each one’s unique shape.The formation of a polypeptide involves the formation of a peptide bond (a.k.a. amide bond), a type of covalent bond between one amino acid’s amino group (NH2) and another amino acid’s carboxyl group (COOH).

This is known as a condensation reaction because two or more molecules combine to form an even larger molecule with a concurrent release of a smaller molecule (water, in this case). Two amino acids linked together are called a dipeptide, or generally just a peptide. A peptide is a molecule typically composed of between 2 and 50 amino acids, sometimes called an oligopeptide. A polypeptide is, very generally, seen as a molecule composed of 50 or more amino acids.

Each amino acid unit in a polypeptide is called a residue.

What to Remember About Polypeptides

Polypeptides
  • Polypeptides are amino acid chains that make up proteins
  • Polypeptides are formed trough the formation of peptide bonds between an amino acid’s amino group and another amino acid’s carboxyl group
  • All polypeptides are not proteins, but all proteins are polypeptides

Learning Outcomes

Once you’ve finished, you should be able to:

  • Explain the importance of proteins
  • Describe the structure of a polypeptide
  • Summarize how polypeptides are formed
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