Proteins are the workhorses of cells. They are important in almost every living process. Proteins are important to life because they help chemical processes happen and give things shape and support. To understand how proteins work and connect in cells, you have to know how they are put together. This study will look at the order of protein structure, the things that affect how proteins fold, and the methods used to figure out protein structure. By the end of this piece, you will know everything there is to know about protein structure and what it means in the world of biology.
2. Hierarchical Organization of Protein Structure
There are four levels in the order of protein structure: the primary, secondary, tertiary, and quaternary levels. Each level of structure tells us important things about how the protein’s parts work, how stable they are, and how they communicate with each other.
2.1 Primary Structure
The primary structure of a protein is the most fundamental level of organization and provides the basis for all higher levels of protein structure. The specific sequence of amino acids in a protein determines its chemical and physical properties, such as its solubility, stability, and reactivity.
The sequence of amino acids in a protein is determined by the genetic code encoded in DNA. Each amino acid is specified by a codon, which is a three-nucleotide sequence in the DNA. The mRNA molecule transcribed from the DNA contains a complementary sequence of codons, which directs the synthesis of a specific sequence of amino acids during protein synthesis.
The peptide bond that joins two amino acids together is formed by a condensation reaction between the carboxyl group of one amino acid and the amino group of another amino acid. This forms a peptide bond and releases a molecule of water. The resulting chain of amino acids is known as a polypeptide.
The primary structure of a protein is typically represented as a linear sequence of amino acid abbreviations, with the N-terminal end (containing the free amino group) on the left and the C-terminal end (containing the free carboxyl group) on the right. For example, the primary structure of the protein insulin is:
The primary structure of a protein is critical to its function because it determines how the protein will fold into its three-dimensional structure. The folding of a protein is driven by interactions between the amino acid side chains, such as hydrogen bonds, electrostatic interactions, and hydrophobic interactions. These interactions are determined by the specific sequence of amino acids in the protein.
The primary structure of a protein is the linear sequence of amino acids that make up the protein. It is determined by the genetic code and is critical to the protein’s function because it determines how the protein will fold into its three-dimensional structure. A change in just one amino acid can significantly alter the protein’s structure and function, which can have important biological consequences.
2.2 Secondary Structure
The secondary structure of a protein refers to the local folding patterns that arise from interactions between neighboring amino acids in the primary structure.
The secondary structure of a protein is stabilized by a variety of non-covalent interactions, including hydrogen bonds, electrostatic interactions, and van der Waals interactions. These interactions are weak individually, but together they provide a significant stabilizing force for the protein.
The secondary structure of a protein is important because it contributes to the overall three-dimensional structure of the protein and can influence its function. For example, the alpha helices and beta strands of a protein can form hydrophobic cores that stabilize the protein’s structure, and they can also provide binding sites for ligands or other proteins.
The secondary structure of a protein can be predicted based on its primary structure using a variety of computational methods. These methods use statistical algorithms to predict the most likely conformation of the protein based on the properties of the amino acid sequence.
The secondary structure of a protein can also be experimentally determined using techniques such as X-ray crystallography or nuclear magnetic resonance (NMR) spectroscopy. These techniques allow researchers to visualize the three-dimensional structure of the protein and to identify the location and orientation of alpha helices, beta strands, and other secondary structures.
The types of secondary structure are the following :
2.2.1 The α-helix
The α-helix is a type of polypeptide helix that is commonly found in nature. It is a spiral structure made up of a tightly packed, coiled polypeptide backbone core with the side chains of the component amino acids extending outward from the central axis to avoid interfering with each other . α-helices are present in a diverse group of proteins. For instance, keratins are a family of closely related, fibrous proteins that have a nearly entirely α-helical structure. They are a major component of tissues like hair and skin, and their rigidity is determined by the number of disulfide bonds between the constituent polypeptide chains. On the other hand, myoglobin, whose structure is also highly α-helical, is a globular and flexible molecule .
- Hydrogen bonds play a crucial role in stabilizing an α-helix. The α-helix is stabilized by extensive hydrogen bonding between the peptide-bond carbonyl oxygens and amide hydrogens that are part of the polypeptide backbone . These hydrogen bonds extend up and are parallel to the spiral, from the carbonyl oxygen of one peptide bond to the – NH – group of a peptide linkage four residues ahead in the polypeptide. This ensures that all but the first and last peptide bond components are linked to each other through intrachain hydrogen bonds. Although hydrogen bonds are individually weak, they collectively serve to stabilize the helix.
- Each turn of an α-helix contains 3.6 amino acids. Hence, amino acid residues spaced three or four residues apart in the primary sequence are spatially close together when folded in the α-helix.
- Certain amino acids can disrupt an α-helix. For instance, proline disrupts an α-helix because its secondary amino group is not geometrically compatible with the right-handed spiral of the α-helix. Instead, it inserts a kink in the chain, which interferes with the smooth, helical structure. Large numbers of charged amino acids (e.g., glutamate, aspartate, histidine, lysine, and arginine) also disrupt the helix by forming ionic bonds or by electrostatically repelling each other. Finally, amino acids with bulky side chains, such as tryptophan, or amino acids like valine or isoleucine, that branch at the β-carbon (the first carbon in the R group, next to the α-carbon) can interfere with the formation of the α-helix if they are present in large numbers.
2.2.2 The β-sheet
The β-sheet is a type of secondary structure in proteins where all peptide bond components participate in hydrogen bonding, resulting in a “pleated” appearance on the surface , hence the name β-pleated sheets. β-strands, which are almost fully extended, make up two or more peptide chains or segments of polypeptide chains, in contrast to the α-helix. It is important to note that the hydrogen bonds are perpendicular to the polypeptide backbone in β-sheets.
A β-sheet can be formed from separate polypeptide chains or segments of polypeptide chains that are arranged in either an antiparallel or parallel manner. In the antiparallel arrangement, the N-terminal and C-terminal ends of the β-strands alternate, while in the parallel arrangement, all the N-termini of the β-strands are together . The hydrogen bonds between the polypeptide backbones of separate polypeptide chains are interchain bonds. In contrast, a single polypeptide chain can fold back on itself to form a β-sheet . In this case, the hydrogen bonds are intra-chain bonds.
β-sheets in globular proteins always have a right-handed twist when viewed along the polypeptide backbone. Twisted β-sheets often form the core of globular proteins.
2.2.3 Beta Bends
β-Bends, also known as reverse turns or β-turns, are structural features in proteins that facilitate the formation of a compact, globular shape by changing the direction of the polypeptide chain. These features are commonly located on the surface of protein molecules, where they often contain charged residues. β-Bends were named after their ability to connect adjacent strands of antiparallel β-sheets. Typically, a β-bend is comprised of four amino acids, including proline, which introduces a kink in the polypeptide chain, and glycine, which has the smallest R group among amino acids and is often present in β-bends. The stability of β-bends is due to the formation of both hydrogen and ionic bonds.
2.2.4 Nonrepetitive s econdary s tructure
Approximately fifty percent of an average globular protein is organized into repetitive structures, such as the α-helix and β-sheet. The other half of the polypeptide chain is characterized by a loop or coil conformation. These non-repetitive secondary structures are not stochastic in nature but simply possess a less regular structure than those previously described. It should be noted that the term “random coil” refers to the unordered structure that results when proteins are denatured.
2.2.5 Supers e condary s tructures (motifs )
Globular proteins are formed by combining secondary structural elements, such as α-helices, β-sheets, and coils, which create specific geometric patterns or motifs primarily in the protein’s core. These structural elements are linked by loop regions, such as β-bends, at the protein’s surface. The supersecondary structures, also known as motifs, are often formed by the close packing of side chains from adjacent secondary structural elements. Therefore, α-helices and β-sheets that are adjacent in the amino acid sequence are usually but not always adjacent in the final, folded protein.
2.3 Tertiary Structure
The tertiary structure of a protein refers to its three-dimensional conformation, which is determined by the interactions between amino acid side chains that are far apart in the primary structure. These side chain interactions include hydrogen bonds, electrostatic interactions, van der Waals forces, and hydrophobic interactions.
The tertiary structure of a protein is stabilized by a variety of non-covalent interactions between amino acid side chains. These interactions can occur between residues that are close together in the primary structure, such as neighboring residues in an alpha helix or beta sheet, or between residues that are far apart in the primary structure.
The tertiary structure of a protein is critical to its function because it determines the protein’s overall shape and the location of binding sites for other molecules. For example, enzymes rely on their tertiary structure to position reactive amino acid side chains in the correct orientation for catalysis, while antibodies rely on their tertiary structure to bind specifically to antigens.
The tertiary structure of a protein is a complex and dynamic arrangement of amino acid side chains that determines the protein’s overall conformation and stability. The interactions between these side chains can be classified into four broad categories: hydrogen bonds, electrostatic interactions, van der Waals forces, and hydrophobic interactions.
Hydrogen bonds are formed between a hydrogen atom and a pair of electronegative atoms, such as oxygen or nitrogen. In proteins, hydrogen bonds are often formed between the amide and carbonyl groups of neighboring amino acids, which can contribute to the stability of secondary structures such as alpha helices and beta sheets. Hydrogen bonds can also occur between amino acid side chains, helping to stabilize the protein’s tertiary structure.
Electrostatic interactions occur between charged amino acid side chains, such as negatively charged aspartic acid and glutamic acid, and positively charged lysine and arginine. These interactions can be attractive or repulsive, depending on the charge and distance between the interacting groups.
Van der Waals forces are weak attractions between nonpolar atoms or molecules that result from fluctuations in electron density. In proteins, van der Waals interactions occur between nonpolar amino acid side chains, such as those found in the hydrophobic core of the protein.
Hydrophobic interactions are the driving force behind the formation of the protein’s core. These interactions occur between nonpolar amino acid side chains that are excluded from water and prefer to interact with each other instead. The hydrophobic effect is one of the strongest forces driving protein folding and stability.
The tertiary structure of a protein can be visualized using a variety of techniques, including X-ray crystallography, NMR spectroscopy, and cryo-electron microscopy (cryo-EM). X-ray crystallography involves growing crystals of the protein and using X-rays to determine the positions of individual atoms within the crystal. NMR spectroscopy involves measuring the interactions between the protein’s nuclei and a magnetic field, which can provide information about the protein’s three-dimensional structure. Cryo-EM involves freezing the protein in solution and using electron microscopy to visualize its structure.
Computational methods can also be used to predict the tertiary structure of a protein based on its amino acid sequence. These methods use algorithms that simulate the protein’s folding process and calculate the most energetically favorable conformation.
2.4 Quaternary Structure
The quaternary structure of a protein refers to the arrangement of multiple protein subunits into a larger, functional protein complex. Many proteins exist as multimers, composed of two or more identical or different subunits that come together to form a functional complex.
The subunits in a protein complex are often held together by non-covalent interactions, such as hydrogen bonds, electrostatic interactions, van der Waals forces, and hydrophobic interactions. These interactions can occur between amino acid side chains within the subunits or between subunits.
The quaternary structure of a protein is critical to its function because it determines the protein’s biological activity, specificity, and regulation. For example, hemoglobin is a tetrameric protein complex composed of two alpha subunits and two beta subunits. The quaternary structure of hemoglobin allows it to bind and transport oxygen in the blood.
The quaternary structure of a protein can be determined experimentally using techniques such as X-ray crystallography or cryo-electron microscopy (cryo-EM). These techniques allow researchers to visualize the protein complex and to identify the location and orientation of each subunit.
The quaternary structure of a protein can also be predicted computationally using molecular modeling techniques. These methods use algorithms to calculate the most energetically favorable conformation of the protein complex based on the physical and chemical properties of the subunits and their interactions.