Since certain amino acids can interact with other amino acids in the same protein, this primary structure ultimately determines the final shape and therefore the chemical and physical properties of the protein.
The secondary structure of the protein is due to hydrogen bonds that form between the oxygen atom of one amino acid and the nitrogen atom of another. In globular proteins such as enzymes, the long chain of amino acids becomes folded into a three-dimensional functional shape or tertiary structure.
This is because certain amino acids with sulfhydryl or SH groups form disulfide S-S bonds with other amino acids in the same chain. As will be seen later in this unit, during protein synthesis, the order of nucleotide bases along a gene gets transcribed into a complementary strand of mRNA which is then translated by tRNA into the correct order of amino acids for that polypeptide or protein. Therefore, the order of deoxyribonucleotide bases along the DNA determines the order of amino acids in the proteins.
It is very important for proteins to achieve their native conformation since failure to do so may lead to serious problems in the accomplishment of its biological function. Defects in protein folding may be the molecular cause of a range of human genetic disorders. For example, cystic fibrosis is caused by defects in a membrane-bound protein called cystic fibrosis transmembrane conductance regulator CFTR. This protein serves as a channel for chloride ions. The most common cystic fibrosis-causing mutation is the deletion of a Phe residue at position in CFTR, which causes improper folding of the protein.
Many of the disease-related mutations in collagen also cause defective folding. A misfolded protein, known as prion, appears to be the agent of a number of rare degenerative brain diseases in mammals, like the mad cow disease.
Related diseases include kuru and Creutzfeldt-Jakob. The diseases are sometimes referred to as spongiform encephalopathies, so named because the brain becomes riddled with holes. Prion, the misfolded protein, is a normal constituent of brain tissue in all mammals, but its function is not yet known.
Prions cannot reproduce independently and not considered living microoganisms. A complete understanding of prion diseases awaits new information about how prion protein affects brain function, as well as more detailed structural information about the protein. Therefore, improved understanding of protein folding may lead to new therapies for cystic fibrosis, Creutzfeldt-Jakob, and many other diseases. Privacy Policy. Skip to main content.
Genes and Proteins. Search for:. Ribosomes and Protein Synthesis. Learning Objectives Describe the process of translation. Key Takeaways Key Points Protein synthesis, or translation, begins with a process known as pre-initiation, when the small ribosmal subunit, the mRNA template, initiator factors, and a special initiator tRNA, come together.
Key Terms translation : a process occurring in the ribosome in which a strand of messenger RNA mRNA guides assembly of a sequence of amino acids to make a protein. Protein Folding, Modification, and Targeting In order to function, proteins must fold into the correct three-dimensional shape, and be targeted to the correct part of the cell. Learning Objectives Discuss how post-translational events affect the proper function of a protein. Key Takeaways Key Points Protein folding is a process in which a linear chain of amino acids attains a defined three-dimensional structure, but there is a possibility of forming misfolded or denatured proteins, which are often inactive.
Proteins must also be located in the correct part of the cell in order to function correctly; therefore, a signal sequence is often attached to direct the protein to its proper location, which is removed after it attains its location. Protein misfolding is the cause of numerous diseases, such as mad cow disease, Creutzfeldt-Jakob disease, and cystic fibrosis. Licenses and Attributions. CC licensed content, Shared previously. For example, AUG codes for the amino acid methionine beige.
The codon AUG codes for the amino acid methionine beige sphere. The codon GUC codes for the amino acid valine dark blue sphere. The codon AGU codes for the amino acid serine orange sphere.
The codon CCA codes for the amino acid proline light blue sphere. The codon UAA is a stop signal that terminates the translation process. The idea of codons was first proposed by Francis Crick and his colleagues in During that same year, Marshall Nirenberg and Heinrich Matthaei began deciphering the genetic code, and they determined that the codon UUU specifically represented the amino acid phenylalanine.
Following this discovery, Nirenberg, Philip Leder, and Har Gobind Khorana eventually identified the rest of the genetic code and fully described which codons corresponded to which amino acids.
Reading the genetic code. Redundancy in the genetic code means that most amino acids are specified by more than one mRNA codon. Methionine is specified by the codon AUG, which is also known as the start codon. Consequently, methionine is the first amino acid to dock in the ribosome during the synthesis of proteins. Tryptophan is unique because it is the only amino acid specified by a single codon.
The remaining 19 amino acids are specified by between two and six codons each. Figure 2 shows the 64 codon combinations and the amino acids or stop signals they specify. Figure 2: The amino acids specified by each mRNA codon. Multiple codons can code for the same amino acid. Figure Detail. What role do ribosomes play in translation? As previously mentioned, ribosomes are the specialized cellular structures in which translation takes place. This means that ribosomes are the sites at which the genetic code is actually read by a cell.
Figure 3: A tRNA molecule combines an anticodon sequence with an amino acid. These nucleotides represent the anticodon sequence. The nucleotides are composed of a ribose sugar, which is represented by grey cylinders, attached to a nucleotide base, which is represented by a colored, vertical rectangle extending down from the ribose sugar.
The color of the rectangle represents the chemical identity of the base: here, the anticodon sequence is composed of a yellow, green, and orange nucleotide. At the top of the T-shaped molecule, an orange sphere, representing an amino acid, is attached to the amino acid attachment site at one end of the red tube. During translation, ribosomes move along an mRNA strand, and with the help of proteins called initiation factors, elongation factors, and release factors, they assemble the sequence of amino acids indicated by the mRNA, thereby forming a protein.
In order for this assembly to occur, however, the ribosomes must be surrounded by small but critical molecules called transfer RNA tRNA. Each tRNA molecule consists of two distinct ends, one of which binds to a specific amino acid, and the other which binds to a specific codon in the mRNA sequence because it carries a series of nucleotides called an anticodon Figure 3. In this way, tRNA functions as an adapter between the genetic message and the protein product.
The exact role of tRNA is explained in more depth in the following sections. What are the steps in translation? Like transcription, translation can also be broken into three distinct phases: initiation, elongation, and termination. All three phases of translation involve the ribosome, which directs the translation process. Multiple ribosomes can translate a single mRNA molecule at the same time, but all of these ribosomes must begin at the first codon and move along the mRNA strand one codon at a time until reaching the stop codon.
The variation of the R group side chains alters the chemistry of the amino acid molecule. Most amino acids have side chains that are non-polar do not have positive and negative poles. Others have positively or negatively charged side chains.
Some have polar side chains that are uncharged. The chemistry of the side-chain affects how the amino acids bond together when forming the final protein structure. If the amino acids have charged side chains, they can form ionic bonds. If the side chains are hydrophobic , they can join with van der Waals interactions. Polar amino acids can join with hydrogen bonds. Therefore, side-chain interactions of a long chain of amino acids, and their order in the chain determines how the protein molecule is formed, i.
More information regarding the different bonds and interactions between the amino acids will be discussed later in this section. Proteins have 4 levels of structure: the primary structure, the secondary structure, the tertiary structure, and the quaternary structure.
What is a polypeptide sequence? In simple terms, polypeptides are chains of amino acids. The primary structure of a protein begins with peptide bond formation between amino acids resulting in the creation of a peptide.
What is a peptide bond? This forms a stable two-dimensional structure with side chains extending out from the polypeptide chain. This allows the side chains to interact with other molecules.
This act of joining smaller units together to create a longer polymer is known as polymerization. How are peptide bonds formed? The reaction of two amino acids joining is a condensation reaction.
This is because a hydrogen and oxygen molecule is lost from the carboxyl group of 1 amino acid, and a hydrogen molecule is lost from the amino group of another amino acid. This produces a water molecule H 2 O , hence the term condensation reaction. The secondary structure forms when hydrogen bonds arise between atoms in the backbone of the polypeptide this does not include the side chains.
This protein is found in hair and nails. This occurs when two polypeptide chains lie next to each other and hydrogen bonds form between them. At the end of a polypeptide, there is either a free carboxyl group or a free amino group.
In this case, the polypeptides run anti-parallel to each other but have also coiled into a barrel shape with hydrogen bonds between the first and last amino acid figure 7.
Although hydrogen bonds in the amino acids are weak, the combination of all the hydrogen bonds together gives the structure stability allowing it to keep its shape. The tertiary structure of the polypeptide is defined as the 3-dimensional structure. The protein begins further folding resulting from side chain R group interactions in the primary sequence. This is via hydrophobic bonds, hydrogen bonds, ionic bonds, disulfide bonds, and Van der Waals interactions.
In the quaternary structure, chains of polypeptides begin to interact together. These protein subunits bind together via hydrogen bonds and van der Waals interactions. Their arrangement allows the specific functionality of the final protein. Changes in conformation can be detrimental to their biological actions. Hemoglobin is an example of a protein with a quaternary structure.
It is worth noting that not all proteins have a quaternary structure, many proteins only have a tertiary structure as their final conformation. Are polypeptides proteins? In some cases, the word polypeptide is used interchangeably with the word protein.
However, a protein may consist of more than 1 chain of polypeptides so using the term polypeptide for all proteins is not always correct. Want to know how our cell create polypeptides and proteins?
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