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Description:
تتجمع السلاسل الثانوية ألفا وبيتا مع بعضها تاركة بينها نقاط انعطاف بواسطة مجموعة من الروابط، أهمها ثنائية الكبريت والكارهة للماء، لتعطي البنية الثالثية أو التحت وحدة. تتجمع التحت وحدات لتعطي بروتين ذو بنية رابعية وظيفي.
ولكل بروتين تسلسل للأحماض الأمينية خاص به فقط، وينتهي بطريقة ما، بالطيّة المميزة له. لكن مثل هذه السلسلة المعقدة يمكن أن تنطوي بعدد كبير جداً من الطرق المختلفة، ومن ثم كيف يتأتى للبروتين أن ينتهي بالشكل الصحيح له بالضبط? لا يمكن الإجابة عن هذا السؤال بالتخمين أو باتباع طريقة التجربة والخطأ. إن عمر الكون نفسه يعد قصيراً مقارنة بالوقت الذي يستغرقه بروتين صغير لتجربة بلايين من الطيّات الممكنة واحدة وراء أخرى، وصولاً إلى الطيّة الصحيحة!
إذ إن البروتينات تنثني وتلتوي وتلتف في شكل حلقات أو حلزونات، بينما تنضغط بعض البروتينات الأخرى في رقائق مطويّة تشبه الآلة الموسيقية (الأكورديون)، وكذلك في أشكال أخرى.
وهذه الطيّات تساعد على أداء البروتينات لوظائفها الجوهرية داخل الخلايا.
Tertiary structure
The overall three-dimensional structure of a polypeptide is called its tertiary structure. The tertiary structure is primarily due to interactions between the R groups of the amino acids that make up the protein.
R group interactions that contribute to the tertiary structure include hydrogen bonding, ionic bonding, dipole-dipole interactions, and London dispersion forces – basically, the whole gamut of non-covalent bonds. For example, R groups with like charges repel one another, while those with opposite charges can form an ionic bond. Similarly, polar R groups can form hydrogen bonds and other dipole-dipole interactions. Also important to a tertiary structure are hydrophobic interactions, in which amino acids with nonpolar, hydrophobic R groups cluster together on the inside of the protein, leaving hydrophilic amino acids on the outside to interact with surrounding water molecules.
Finally, there’s one special type of covalent bond that can contribute to tertiary structure: the disulfide bond. Disulfide bonds, covalent linkages between the sulfur-containing side chains of cysteines, are much stronger than the other types of bonds that contribute to tertiary structure. They act like molecular "safety pins," keeping parts of the polypeptide firmly attached to one another.
Quaternary structure
Many proteins are made up of a single polypeptide chain and have only three levels of structure (the ones we’ve just discussed). However, some proteins are made up of multiple polypeptide chains, also known as subunits. When these subunits come together, they give the protein its quaternary structure.
We’ve already encountered one example of a protein with a quaternary structure: hemoglobin. As mentioned earlier, hemoglobin carries oxygen in the blood and is made up of four subunits, two each of the α and β types. Another example is DNA polymerase, an enzyme that synthesizes new strands of DNA and is composed of ten subunits
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In general, the same types of interactions that contribute to tertiary structure (mostly weak interactions, such as hydrogen bonding and London dispersion forces) also hold the subunits together to give quaternary structure.
Denaturation and protein folding
Each protein has its own unique shape. If the temperature or pH of a protein's environment is changed, or if it is exposed to chemicals, these interactions may be disrupted, causing the protein to lose its three-dimensional structure and turn back into an unstructured string of amino acids. When a protein loses its higher-order structure, but not its primary sequence, it is said to be denatured. Denatured proteins are usually non-functional.
For some proteins, denaturation can be reversed. Since the primary structure of the polypeptide is still intact (the amino acids haven’t split up), it may be able to re-fold into its functional form if it's returned to its normal environment. Other times, however, denaturation is permanent
0:10 Revision
0:29 Super 2ndry structure
1:38 Domains
2:30 3ry structure
6:45 4ry structure
7:10 Bonus station


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