Rucete ✏ Lehninger Principles of Biochemistry In a Nutshell
4.3 Protein Tertiary and Quaternary Structures
This chapter introduces the principles of protein tertiary and quaternary structure, describing the three-dimensional folding of polypeptide chains, the interactions stabilizing these structures, and the classification and functional diversity of fibrous, globular, and intrinsically disordered proteins.
Defining Tertiary and Quaternary Structure
• Tertiary structure refers to the overall three-dimensional arrangement of all atoms in a single polypeptide, including interactions between distant segments stabilized by weak interactions and sometimes covalent bonds (e.g., disulfide bridges).
• Quaternary structure is the arrangement of multiple polypeptide subunits (identical or different) into a three-dimensional protein complex.
• Major classes of proteins by structure: fibrous (extended, strand-like), globular (compact, folded), membrane proteins (embedded in lipid bilayers), and intrinsically disordered proteins (lacking stable tertiary structure).
• Fibrous proteins have simple, repetitive structures, often dominated by one type of secondary structure; globular proteins combine multiple secondary structures; disordered proteins lack a defined structure and provide functional flexibility.
Fibrous Proteins: Keratin, Collagen, Silk Fibroin
• Fibrous proteins provide structural support and are insoluble in water due to high hydrophobic amino acid content.
• α-Keratin: Composed of right-handed α helices coiled into left-handed supertwisted coiled coils; strength is amplified by supertwisting and stabilized by hydrophobic side chains and covalent disulfide bonds. Forms hair, nails, claws, and more.
• Collagen: Most abundant protein in mammals, providing tensile strength in connective tissues. Made of three left-handed helices (Gly-X-Y repeats, where X is often Pro and Y is often 4-hydroxyproline) twisted into a right-handed superhelix. Strengthened by unique cross-links. Defects in collagen structure underlie diseases such as osteogenesis imperfecta and Ehlers-Danlos syndrome. Vitamin C is essential for hydroxyproline formation—deficiency causes scurvy.
• Silk fibroin: Produced by insects and spiders. Structure is stacks of antiparallel β sheets rich in Ala and Gly, enabling close packing and flexibility. Stabilized by hydrogen bonds and van der Waals forces, not covalent bonds.
Globular Proteins: Structure and Motifs
• Globular proteins fold into compact shapes with diverse functional roles (enzymes, transport, immunity, etc.). Hydrophobic residues cluster internally; polar residues are mostly exposed to water.
• The first globular protein structure solved was myoglobin, showing tightly packed α helices and hydrophobic core. The heme group is buried in a pocket for oxygen binding.
• Tertiary structure is built from motifs (folds)—recognizable combinations of secondary structures (e.g., β-α-β loops, β barrels).
• Domains are independently stable structural units within a polypeptide, often with distinct functions (e.g., binding calcium, small molecules, or other proteins). Large proteins typically have multiple domains.
• Protein folding is guided by: (1) the hydrophobic effect, (2) separation of α helices and β sheets into different layers, (3) adjacency of sequential segments, (4) right-handed twists in β sheets, and (5) construction of complex motifs like the α/β barrel from simple loops.
Intrinsically Disordered Proteins
• Many proteins or segments lack stable structure and function as intrinsically disordered proteins (IDPs). IDPs are rich in charged and Pro residues, lack a hydrophobic core, and are flexible.
• Structural disorder enables proteins to interact with multiple partners in different ways, as in regulatory proteins like p27 and p53. Disorder facilitates network "hub" or "scaffold" roles in signaling pathways.
• IDPs can adopt ordered structures upon binding to specific targets; their structural adaptability is crucial for function.
Structural Classification, Motifs, and Families
• Protein structural data are archived in databases like the Protein Data Bank (PDB) and organized into families and superfamilies by similarity of structure, not just sequence.
• Only ~1,400 unique folds are known among over 150,000 structures. Protein families share structural and functional features, while superfamilies link more distantly related proteins with similar motifs.
• Structural classification reveals evolutionary relationships and informs on function, as similar motifs are often found in widely divergent proteins.
Quaternary Structure: Multisubunit Proteins
• Many proteins are composed of multiple subunits (oligomers, multimers), which may be identical or different; repeating structural units are called protomers.
• Quaternary arrangements enable regulation, division of function (e.g., catalysis vs. regulation), structural strength, and assembly into large complexes (e.g., ribosomes, viral coats).
• Example: Hemoglobin is a tetramer of two α and two β chains, each with a heme group; its subunits interact to mediate oxygen binding and release.
In a Nutshell
• Tertiary structure is the complete three-dimensional folding of a polypeptide chain, while quaternary structure describes the assembly of multiple polypeptide subunits.
• Fibrous proteins provide strength and protection; globular proteins enable diverse functions; intrinsically disordered proteins offer structural and functional flexibility.
• Motifs and domains are building blocks of protein structure, and their classification reveals evolutionary and functional relationships.
• Multisubunit proteins achieve complexity and regulation through quaternary organization, essential for the full range of protein functions in biology.