Your cells use various types of molecules in order to perform their daily functions. One of the most important classes of molecules are called proteins. Proteins are molecules that have several different functions within the cell. Such functions include transporting things in and out of the cell, speeding up chemical reactions, and helping cells recognize and join to one another. Therefore, proteins are really versatile molecules that can be used for multiple purposes. Just like a Swiss army knife has multiple tools that can be used in different ways, proteins have different structures that specify their particular function.
Proteins consist of one or more polypeptide chains. Each polypeptide chain consists of smaller sub-units or amino acids that are linked together. Amino acids serve as the building blocks of polypeptides, and polypeptides serve as the building blocks of proteins. Think of an individual amino acid as a paper clip. When you link several paper clips together, you make a paper clip chain. Joining dozens of amino acids together makes a polypeptide chain. A polypeptide chain can serve as a protein on its own. However, lots of proteins are made up of several polypeptide chains.
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There are 20 naturally-occurring amino acids, and each one is a little different. All amino acids have the same basic structure; however, something called an R group distinguishes one amino acid from another. Here, you can see the chemical structures of all 20 amino acids.
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Although the black parts of each amino acid are the same, the R groups (shown in brown) are all different from one another. The differences in these R groups are what help give a polypeptide chain its structure and, ultimately, its function.
The order and arrangement of these amino acids in a polypeptide chain will produce different chemical structures. A protein's structure is essential to its function. So, multiple amino acids can be strung together to make a particular polypeptide chain with a specified function. Similarly, letters of the alphabet can be rearranged to make different words, each with its own meaning. This image is a simple representation of a polypeptide chain made of several different amino acids. Each pattern represents a different amino acid.
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You know now that a polypeptide chain consists of a bunch of different amino acids linked together. But, how are these amino acids linked together? Actually, the process called dehydration synthesis is not that difficult to understand.
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Dehydration synthesis involves the removal of a water molecule (H2O) from the two amino acids that are to be linked together. A hydrogen (H) is removed from one amino acid, and an oxygen and hydrogen (OH) are removed from the other amino acid. The removal of water creates a peptide bond, which holds two amino acids together. Each time an amino acid is added to the polypeptide chain, another water molecule is removed. Amino acids are continually joined to one another using dehydration synthesis. However, polypeptide chains can come in all different lengths. For example, insulin is one of the shortest proteins in the human body and consists of 51 amino acids. Titin, on the other hand, is the longest known protein and consists of multiple polypeptide chains and over 34,000 amino acids!
Proteins are multifunctional and are essential to everyday cellular functions. All proteins are different from one another and each consists of one or more polypeptide chains. Your cells use dehydration synthesis to link multiple amino acids together and form a polypeptide chain. During this process, a water molecule is removed which creates a peptide bond between two amino acids. Each polypeptide chain is a series of amino acids joined together in a specific sequence. Because each amino acid has a unique R group, a particular amino acid sequence creates a polypeptide whose function is dependent upon its chemical structure. So although each polypeptide chain is composed of the same 20 amino acids, there are almost an infinite number of combinations, each of which can produce a different polypeptide chain.
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| Vocabulary | Definitions |
|---|---|
| Proteins | molecules made up of amino acids; essential to cell function |
| Polypeptide chains | a series of amino acids that are joined together in a specific sequence |
| R-groups | they give a polypeptide chain its structure and function |
| Dehydration synthesis | involves the removal of a water molecule from two amino acids that are to be linked together |
| Peptide bond | holds amino acids together |
| Insulin | one of the shortest proteins in the human body; made up of 51 amino acids |
| Titin | 34,000 amino acids and many polypeptide chains make up this, the longest protein |
The process of learning about the polypeptide chain will increase your capacity to:
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Given an unknown polypeptide, you might want to find out its sequence using chemical reactions to break the molecule into smaller pieces. The N-terminal residue (the first amino acid in the sequence) can react with phenylisothiocyanate to form a compound that can be analyzed to determine what that residue was. The rest of the original sequence is usually left intact. The process can be repeated many times to get the identity of the next amino acid. This is called the Edman degradation.
Certain chemicals react with specific amino acids of the chain. Trypsin (Tryp) breaks the peptide bonds on the carboxyl side of lysine and arginine. Chymotrypsin breaks the peptide bonds on the carboxyl side of phenylalanine, tyrosine, and tryptophan. Cyanogen bromide (CNBr) cleaves the peptide bond on the carboxyl side of methionine.
A peptide was broken into two smaller peptides by CNBr and into two different peptides by Tryp. The sequences are as follows:
CNBr 1: Gly-Thr-Lys-Ala-Glu
CNBr 2: Ser-Met
Trp 1: Ser-Met-Gly-The-Lys
Trp2: Ala-Glu
What was the sequence of the parent polypeptide?
By arranging the four sequences in an overlapping set, we can read off the parent. The dots in the sequences below are only intended to provide spacing, so as to line things up.
CNBr2: Ser-Met
Trp 1: .. Ser-Met-Gly-Thr-Lys
CNBr 1: ............. Gly-Thr-Lys-Ala-Glu
Trp2: ...................................... Ala-Glu
Parent: Ser-Met-Gly-Thr-Lys-Ala-Glu
What peptides would be released by trypsin treatment of the peptide?
Ala-Ser-Thr-Lys-Gly-Arg-Ser-Gly
Trypsin breaks the bond at the carboxyl side of lysine and arginine residues. It will produce Ala-Ser-Thr-Lys and Gly-Arg and Ser-Gly
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