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More than one polypeptide chain
- When a protein contains more than one polypeptide chain, each chain is called a subunit.
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A protein molecule containing multiple copies of a single protein subunit. The enzyme neuraminidase exists as a ring of four identical polypeptide chains. The small diagram shows how the repeated use of the same binding interaction forms the structure.
- Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, Peter Walter
- 2002
- 2002
- Overview
- Introduction
- Translation: The big picture
- The genetic code
- Codons to amino acids
- Translation: Beginning, middle, and end
- Initiation
- Elongation
- Termination
- Epilogue: Processing
An in-depth look how polypeptides (proteins) are made. Initiation, elongation, and termination.
Introduction
Ever wonder how antibiotics kill bacteria—for instance, when you have a sinus infection? Different antibiotics work in different ways, but some attack a very basic process in bacterial cells: they knock out the ability to make new proteins.
To use a little molecular biology vocab, these antibiotics block translation. In the process of translation, a cell reads information from a molecule called a messenger RNA (mRNA) and uses this information to build a protein. Translation is happening constantly in a normal bacterial cell, just like it is in most of the cells of your body, and it's key to keeping you (and your bacterial "visitors") alive.
When you take certain antibiotics (e.g., erythromycin), the antibiotic molecule will latch onto key translation molecules inside of bacterial cells and basically "stall" them. With no way to make proteins, the bacteria will stop functioning and, eventually, die. That's why infections clear up when they're treated with the antibiotic.1,2
[Why doesn't erythromycin hurt humans?]
Ever wonder how antibiotics kill bacteria—for instance, when you have a sinus infection? Different antibiotics work in different ways, but some attack a very basic process in bacterial cells: they knock out the ability to make new proteins.
To use a little molecular biology vocab, these antibiotics block translation. In the process of translation, a cell reads information from a molecule called a messenger RNA (mRNA) and uses this information to build a protein. Translation is happening constantly in a normal bacterial cell, just like it is in most of the cells of your body, and it's key to keeping you (and your bacterial "visitors") alive.
When you take certain antibiotics (e.g., erythromycin), the antibiotic molecule will latch onto key translation molecules inside of bacterial cells and basically "stall" them. With no way to make proteins, the bacteria will stop functioning and, eventually, die. That's why infections clear up when they're treated with the antibiotic.1,2
[Why doesn't erythromycin hurt humans?]
Translation involves “decoding” a messenger RNA (mRNA) and using its information to build a polypeptide, or chain of amino acids. For most purposes, a polypeptide is basically just a protein (with the technical difference being that some large proteins are made up of several polypeptide chains).
In an mRNA, the instructions for building a polypeptide come in groups of three nucleotides called codons. Here are some key features of codons to keep in mind as we move forward:
•There are 61 different codons for amino acids
•Three “stop” codons mark the polypeptide as finished
•One codon, AUG, is a “start” signal to kick off translation (it also specifies the amino acid methionine)
These relationships between mRNA codons and amino acids are known as the genetic code (which you can explore further in the genetic code article).
[See the full codon table]
In translation, the codons of an mRNA are read in order (from the 5' end to the 3' end) by molecules called transfer RNAs, or tRNAs.
Each tRNA has an anticodon, a set of three nucleotides that binds to a matching mRNA codon through base pairing. The other end of the tRNA carries the amino acid that's specified by the codon.
[What are 5', 3', and base pairing?]
tRNAs bind to mRNAs inside of a protein-and-RNA structure called the ribosome. As tRNAs enter slots in the ribosome and bind to codons, their amino acids are linked to the growing polypeptide chain in a chemical reaction. The end result is a polypeptide whose amino acid sequence mirrors the sequence of codons in the mRNA.
A book or movie has three basic parts: a beginning, middle, and end. Translation has pretty much the same three parts, but they have fancier names: initiation, elongation, and termination.
•Initiation ("beginning"): in this stage, the ribosome gets together with the mRNA and the first tRNA so translation can begin.
•Elongation ("middle"): in this stage, amino acids are brought to the ribosome by tRNAs and linked together to form a chain.
•Termination ("end"): in the last stage, the finished polypeptide is released to go and do its job in the cell.
In order for translation to start, we need a few key ingredients. These include:
•A ribosome (which comes in two pieces, large and small)
•An mRNA with instructions for the protein we'll build
•An "initiator" tRNA carrying the first amino acid in the protein, which is almost always methionine (Met)
During initiation, these pieces must come together in just the right way. Together, they form the initiation complex, the molecular setup needed to start making a new protein.
[Does that just happen by itself?]
I like to remember what happens in this "middle" stage of translation by its handy name: elongation is when the polypeptide chain gets longer.
But how does the chain actually grow? To find out, let's take a look at the first round of elongation—after the initiation complex has formed, but before any amino acids have been linked to make a chain.
Our first, methionine-carrying tRNA starts out in the middle slot of the ribosome, called the P site. Next to it, a fresh codon is exposed in another slot, called the A site. The A site will be the "landing site" for the next tRNA, one whose anticodon is a perfect (complementary) match for the exposed codon.
[How is the right tRNA chosen?]
Once the matching tRNA has landed in the A site, it's time for the action: that is, the formation of the peptide bond that connects one amino acid to another. This step transfers the methionine from the first tRNA onto the amino acid of the second tRNA in the A site.
Not bad—we now have two amino acids, a (very tiny) polypeptide! The methionine forms the N-terminus of the polypeptide, and the other amino acid is the C-terminus.
Polypeptides, like all good things, must eventually come to an end. Translation ends in a process called termination. Termination happens when a stop codon in the mRNA (UAA, UAG, or UGA) enters the A site.
Stop codons are recognized by proteins called release factors, which fit neatly into the P site (though they aren't tRNAs). Release factors mess with the enzyme that normally forms peptide bonds: they make it add a water molecule to the last amino acid of the chain. This reaction separates the chain from the tRNA, and the newly made protein is released.
Our polypeptide now has all its amino acids—does that mean it's ready to do its job in the cell?
Not necessarily. Polypeptides often need some "edits." During and after translation, amino acids may be chemically altered or removed. The new polypeptide will also fold into a distinct 3D structure, and may join with other polypeptides to make a multi-part protein.
Many proteins are good at folding on their own, but some need helpers ("chaperones") to keep them from sticking together incorrectly during the complex process of folding.
Some proteins also contain special amino acid sequences that direct them to certain parts of the cell. These sequences, often found close to the N- or C-terminus, can be thought of as the protein’s “train ticket” to its final destination. For more about how this works, see the article on protein targeting.
Aug 10, 2022 · When a protein contains more than one polypeptide chain, each chain is called a subunit. The arrangement of multiple subunits represents a fourth level of structure, the quaternary structure of a protein.
Apr 24, 2023 · Subunit structure is determined by the sequence and characteristics of amino acids in the polypeptide chain. The active site is a groove or crevice on an enzyme in which a substrate binds to facilitate the catalyzed chemical reaction.
- Theodore Lewis, William L. Stone
- 2023/04/24
- 2021
Jul 26, 2022 · When a protein contains more than one polypeptide chain, each chain is called a subunit. The arrangement of multiple subunits represents a fourth level of structure, the quaternary structure of a protein.
Transcript. The four levels of protein structure are primary, secondary, tertiary, and quaternary. It is helpful to understand the nature and function of each level of protein structure in order to fully understand how a protein works. By Tracy Kovach. Created by Tracy Kim Kovach. Questions. Tips & Thanks. Want to join the conversation? Log in.
- 9 min
- Tracy Kim Kovach
The quaternary structure of a native conformation refers to the three dimensional organization of all the atoms in a multi-subunit protein. Multi-subunit proteins consist of two or more individual amino acid chains, each with their own primary, secondary, and tertiary structures.
