Proteins perform almost all of the work that keeps living things alive. They provide structure, bind to and carry molecules throughout the cell, and kick-start reactions as enzymes.
A single linear chain of amino acids – called a polypeptide – can take on various stable folding patterns, including alpha helices and beta sheets. These structures give proteins their distinctive globular shapes.단백질보충제
Secondary Structure
Secondary structure refers to local folded structures that form within a polypeptide chain due to interactions between the atoms of the backbone. It does not involve the R groups (amino hydrogen and carboxyl oxygen atoms). Two of the most common types of protein secondary structures are the a helix and the beta pleated sheet. Both are held in shape by hydrogen bonds, which form between the carbonyl O of one amino acid and the amino H of another molecule four residues further along the chain.
X-ray crystallography, NMR spectroscopy, and circular dichroism spectroscopy can all help determine a protein’s secondary structure. Often, proteins contain segments of helices and sheets that are connected by tight turns and loops.
Different amino acids have varying tendencies to form alpha helixes, beta pleated sheets, or turns. For example, methionine, cysteine, leucine, and valine prefer to adopt a helix conformation while aspartic acid, glutamic acid, and glutamine favor beta sheet formation. Moreover, proline and glycine tend to form beta hairpins, which connect two anti-parallel beta strands.
The combination of these secondary structures 단백질보충제 gives proteins their characteristic shapes. For instance, fibrous proteins, such as collagen and keratin, are important for giving tissues their overall structure while globular proteins, such as antibodies and hemoglobin, are the result of the interaction of a combination of helixes and beta sheets. Changes in protein secondary structure, such as denaturing the protein by high temperatures or certain chemical agents like urea and sodium dodecyl sulfate, can affect the protein’s ability to function.
Tertiary Structure
The tertiary structure of proteins is the overall three-dimensional shape of the polypeptide chain. It is determined by interactions between the amino acid residues and their R groups, which create a unique protein shape. The R groups have different chemical characteristics that give rise to a wide variety of tertiary structural patterns. Often, the hydrophobic R groups cluster together inside the protein to reduce interaction with water molecules, and the more polar R groups form hydrogen bonds and other dipole-dipole interactions with other amino acids and with the surrounding water molecules.
The complex tertiary structures of proteins provide the specific binding sites for other biological compounds, such as other proteins, small ions, and even large molecules such as DNA and RNA. For example, the enzymes that catalyze cellular reactions have active sites that are located in pockets formed by the three-dimensional folding of the proteins. Antibodies, which recognize and neutralize foreign substances in the body, have a specific tertiary structure that allows them to bind to their targets with high affinity. Hemoglobin, which carries oxygen in the blood, has a complex tertiary structure that enables it to change shape and carry oxygen at physiological pH levels.
The tertiary structure of proteins can be determined to an atomic level by X-ray and neutron diffraction studies of crystallized proteins, as well as by nuclear magnetic resonance (NMR) spectroscopy of proteins in solution. The Kinemage linked above shows an all-atom view of ribonuclease A, with the H atoms shown in blue and the main-chain and side-chain atoms in cyan.
Primary Structure
The primary structure of a protein is the unique sequence of amino acid residues that gives it its characteristic three-dimensional shape. The sequence is determined by the order of the nucleotide bases that code for it in the DNA molecule. The resulting amino acid residues are linked by hydrogen bonds to form a polypeptide chain, which then folds into a specific three-dimensional structure.
The next level of a protein’s structure is its secondary structure, which refers to any local folded structures that arise within a polypeptide chain from interactions between atoms in the backbone (the part of the chain apart from the R groups). The most common forms of secondary structure are alpha helixes and beta-pleated sheets. These are held in shape by hydrogen bonds between the carbonyl oxygen of one amino acid and the peptide bond amide hydrogen of an amino acid four residues farther along the chain.
Alpha helices, like the one in the Kinemage linked above, have a distinctive spiral shape, whereas beta-pleated sheets are flat bits of ribbon that can either be parallel or antiparallel to each other. The latter form when the peptide chains in different parts of the sheet line up, creating an overall dipole moment that can sometimes have functional significance.
A protein’s tertiary structure is its particular three-dimensional shape, which is stabilized by non-local interactions, such as hydrophobic forces and disulfide bridges. The latter, formed by a covalent linkage between two R groups of adjacent amino acids in the polypeptide chain, are particularly important for proteins that perform enzymatic reactions. Changes in a single amino acid can have significant effects on a protein’s structure and function. For example, sickle cell anemia is a consequence of just one amino acid substitution in the protein hemoglobin.
Quaternary Structure
Proteins that have a quaternary structure form structures that involve more than one polypeptide chain. The individual chains, which are called subunits in this context, are held together through weak interactions that depend on van der Waals forces between nonpolar side chains and hydrogen bonds between amide H atoms of the peptide bond backbone and carbonyl O atoms of different amino acid residues. This level of protein structure can be stabilized in a similar manner to secondary and tertiary structure, but also by a number of other methods including the formation of salt bridges and disulfide bonds, as well as by the folding into alpha-helixes or beta-sheets.
The quaternary structure of proteins is the overall shape of the protein as seen in three dimensions. This is a result of the secondary structure twisting and folding into the specific shapes known as alpha-helixes and beta-sheets.
In quaternary structure, the different parts of the protein are connected by multiple hydrogen bonds and disulfide bonds. This level of protein structure is found only in proteins with multiple polypeptide chains that assemble to form homodimers, homotrimers, or heteropolymers, or interact with other proteins to form oligomers. The formation of these complexes is often facilitated by the direct interaction of nascent proteins emerging from ribosomes. The quaternary structure of proteins is stable under most conditions, but can be disrupted by changes in temperature, pH, salt concentrations and chemical agents such as urea or sodium dodecyl sulfate.