2.10
Protein and Protein Structures
Proteins are one of the most abundant organic molecules in living systems and have the most diverse range of functions of all macromolecules. Proteins…
Proteins are polymers of amino acids linked together through peptide bonds. Most proteins fold into specific three-dimensional structures to perform their function.
Protein structure is classified into four categories: primary, secondary, tertiary, and quaternary.
The linear sequence of amino acids in the protein is its primary structure. These sequences are read in the polymer synthesis order, from the N-terminal, the free amino end, to the C-terminal, the free carboxyl end.
Secondary structures form through hydrogen bonding between the oxygen from the carbonyl group of one amino acid and the hydrogen from the amino group of another. Common secondary structures include α-helices and β-pleated sheets.
These and other connecting structures, such as β-turns, further interact to form the protein’s three-dimensional structure, known as the tertiary structure.
The formation of the tertiary structure occurs due to interactions between the R-groups of the amino acids.
R-groups with opposite charges can form ionic bonds, and sulfur in two cysteines can form a covalent disulfide bridge. Other contributors include hydrophobic interactions between hydrophobic side chains and hydrogen bonds between polar side chains.
Additionally, some proteins form quaternary structures, assemblies of two or more polypeptide chains. For example, hemoglobin is a protein made up of four subunits, two α and two β.
Scientists can use the structure of a protein to predict its function and location in a cell. Proteins in the cell cytoplasm have hydrophilic amino acids on their surfaces that interact with the water in the cytoplasm and have hydrophobic cores.
In contrast, proteins present in cell membranes often have hydrophobic amino acids on the surface that interact with glycerophospholipids in the membrane and have hydrophilic cores.
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Q1: What are the main components that make up a protein?
Proteins are polymers composed of amino acids linked together by peptide bonds. Amino acids contain a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a distinctive side chain. The sequence and arrangement of these amino acids determine the protein's structure and function.
Q2: How do amino acids bond together to form proteins?
Amino acids join through condensation reactions, where the carboxyl group of one amino acid reacts with the amino group of another, releasing water and forming a peptide bond. This process repeats to create long polypeptide chains. The resulting covalent bonds link amino acids in a specific sequence determined by genetic information.
Q3: What stabilizes the three-dimensional shape of a protein?
Protein structure is stabilized by noncovalent attractions in biomolecules, including hydrogen bonds, ionic interactions, and hydrophobic effects between amino acid side chains. These weak interactions work together to fold the polypeptide chain into its functional three-dimensional shape. Disulfide bonds between cysteine residues provide additional covalent stabilization.
Q4: What is the difference between primary and tertiary protein structure?
Primary structure is the linear sequence of amino acids in a polypeptide chain, determined by peptide bonds. Tertiary structure is the three-dimensional folding of that chain, created by interactions between distant amino acids through hydrogen bonds, ionic attractions, and hydrophobic effects. Tertiary structure determines how the protein functions in the cell.
Q5: How do multiple polypeptide chains form quaternary protein structure?
Quaternary structure forms when two or more polypeptide subunits associate through noncovalent interactions and sometimes disulfide bonds. Each subunit maintains its own tertiary structure while bonding to other subunits. This multi-subunit arrangement allows proteins to perform complex functions and respond to cellular signals.
Q6: Why is protein folding important for cellular function?
Proper protein folding creates the specific three-dimensional shape required for a protein to bind substrates, catalyze reactions, or interact with other molecules. Misfolded proteins cannot perform their intended functions and may accumulate, causing cellular damage. Cells use molecular chaperones to assist folding and maintain protein quality control.
Q7: What role do functional groups play in protein structure and function?
Amino acid side chains contain various functional groups that determine their chemical properties and interactions. Polar groups form hydrogen bonds, nonpolar groups cluster away from water, and charged groups participate in ionic attractions. These functional groups enable proteins to achieve specific shapes and perform diverse biochemical roles.