Most proteins are folded into a complex globular shape. Each protein molecule consists of one or more chains of amino acid monomers. The amino acids are linked by peptide bonds, so a protein polymer is often called a polypeptide. Because they are so complicated, proteins are usually described in terms of four levels of structure. Each protein has a unique primary structure - a particular number and sequence of amino acids making up the polypeptide chain. Twenty different amino acids are used to build proteins. Theoretically, the various amino acids could be linked in almost any sequence, forming an almost infinite variety of different proteins. This illustration shows some of the amino acids making up the primary structure of a protein. The structure of a single generalized amino acid is shown below. The main backbone of every amino acid is the same. This is what forms the backbone of the polypeptide chain. It is the R-group which projects out from the backbone that makes each of the twenty kinds of amino acids unique. Different amino acids have different properties that affect the folding of a protein. Thus, primary structure ultimately determines the shape of a protein, which determines its function. In most proteins, parts of the polypeptide chain are coiled or folded, forming twists and corrugations. This is secondary structure. The turns and folds of secondary structure contribute to the protein's overall shape. One kind of secondary structure is the alpha helix, where the chain twists. Another is the beta pleated sheet, where the chain folds back on itself or where two regions of the chain lie parallel to one another. Secondary structure results from hydrogen bonding between atoms along the polypeptide backbone. Oxygen and nitrogen atoms along the backbone are highly electronegative, and when bound to hydrogen atoms, O and N have partial negative charges while the H atoms have partial positive charges. These negatively and positively charged atoms are attracted to one another at regular intervals along the chain, causing parts of the protein to twist or fold back upon itself. Superimposed on primary and secondary structure is tertiary structure, irregular loops and folds that give the protein its overall three-dimensional shape. The irregular folding of the tertiary structure results from interactions among the R groups of amino acids. Acidic and basic R groups ionize, and these positively and negatively charged groups may form ionic bonds. Polar forces also contribute to tertiary structure. Hydrophilic or polar R groups may hydrogen bond with one another, or turn outward and hydrogen bond with the surrounding water. Hydrophobic, non-polar R groups cluster on the inside of the protein, away from water. Tertiary structure may be further stabilized by strong covalent bonds between sulfur atoms in certain R groups. Some proteins consist of two or more polypeptide chains. The fourth level of protein structure, quaternary structure, results from the combination of two or more polypeptide subunits. Quaternary structure is in turn stabilized by the same sorts of attractions that stabilize tertiary structure. Hemoglobin, the oxygen-carrying protein of blood, is an example of a protein with quaternary structure. It consists of two kinds of polypeptide chains. Two of each, a total of four chains, make up each hemoglobin molecule.
Table of contents
- 1. Introduction to Biology2h 40m
- 2. Chemistry3h 40m
- 3. Water1h 26m
- 4. Biomolecules2h 23m
- 5. Cell Components2h 26m
- 6. The Membrane2h 31m
- 7. Energy and Metabolism2h 0m
- 8. Respiration2h 40m
- 9. Photosynthesis2h 49m
- 10. Cell Signaling59m
- 11. Cell Division2h 47m
- 12. Meiosis2h 0m
- 13. Mendelian Genetics4h 41m
- Introduction to Mendel's Experiments7m
- Genotype vs. Phenotype17m
- Punnett Squares13m
- Mendel's Experiments26m
- Mendel's Laws18m
- Monohybrid Crosses16m
- Test Crosses14m
- Dihybrid Crosses20m
- Punnett Square Probability26m
- Incomplete Dominance vs. Codominance20m
- Epistasis7m
- Non-Mendelian Genetics12m
- Pedigrees6m
- Autosomal Inheritance21m
- Sex-Linked Inheritance43m
- X-Inactivation9m
- 14. DNA Synthesis2h 27m
- 15. Gene Expression3h 20m
- 16. Regulation of Expression3h 31m
- Introduction to Regulation of Gene Expression13m
- Prokaryotic Gene Regulation via Operons27m
- The Lac Operon21m
- Glucose's Impact on Lac Operon25m
- The Trp Operon20m
- Review of the Lac Operon & Trp Operon11m
- Introduction to Eukaryotic Gene Regulation9m
- Eukaryotic Chromatin Modifications16m
- Eukaryotic Transcriptional Control22m
- Eukaryotic Post-Transcriptional Regulation28m
- Eukaryotic Post-Translational Regulation13m
- 17. Viruses37m
- 18. Biotechnology2h 58m
- 19. Genomics17m
- 20. Development1h 5m
- 21. Evolution3h 1m
- 22. Evolution of Populations3h 52m
- 23. Speciation1h 37m
- 24. History of Life on Earth2h 6m
- 25. Phylogeny2h 31m
- 26. Prokaryotes4h 59m
- 27. Protists1h 12m
- 28. Plants1h 22m
- 29. Fungi36m
- 30. Overview of Animals34m
- 31. Invertebrates1h 2m
- 32. Vertebrates50m
- 33. Plant Anatomy1h 3m
- 34. Vascular Plant Transport2m
- 35. Soil37m
- 36. Plant Reproduction47m
- 37. Plant Sensation and Response1h 9m
- 38. Animal Form and Function1h 19m
- 39. Digestive System10m
- 40. Circulatory System1h 57m
- 41. Immune System1h 12m
- 42. Osmoregulation and Excretion50m
- 43. Endocrine System4m
- 44. Animal Reproduction2m
- 45. Nervous System55m
- 46. Sensory Systems46m
- 47. Muscle Systems23m
- 48. Ecology3h 11m
- Introduction to Ecology20m
- Biogeography14m
- Earth's Climate Patterns50m
- Introduction to Terrestrial Biomes10m
- Terrestrial Biomes: Near Equator13m
- Terrestrial Biomes: Temperate Regions10m
- Terrestrial Biomes: Northern Regions15m
- Introduction to Aquatic Biomes27m
- Freshwater Aquatic Biomes14m
- Marine Aquatic Biomes13m
- 49. Animal Behavior28m
- 50. Population Ecology3h 41m
- Introduction to Population Ecology28m
- Population Sampling Methods23m
- Life History12m
- Population Demography17m
- Factors Limiting Population Growth14m
- Introduction to Population Growth Models22m
- Linear Population Growth6m
- Exponential Population Growth29m
- Logistic Population Growth32m
- r/K Selection10m
- The Human Population22m
- 51. Community Ecology2h 46m
- Introduction to Community Ecology2m
- Introduction to Community Interactions9m
- Community Interactions: Competition (-/-)38m
- Community Interactions: Exploitation (+/-)23m
- Community Interactions: Mutualism (+/+) & Commensalism (+/0)9m
- Community Structure35m
- Community Dynamics26m
- Geographic Impact on Communities21m
- 52. Ecosystems2h 36m
- 53. Conservation Biology24m
4. Biomolecules
Proteins
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