Protein Tertiary and Quaternary Structures: -Protein Motifs Are the Basis for Protein Structural Classification

As we have seen, the complexities of tertiary structure are decreased by considering substructures. Taking this idea further, re searchers have organized the complete contents of databases according to hierarchical levels of structure. The Structural Classification of Proteins (SCOP) data base offers a good example of this very important trend in biochemistry. At the highest level of classification, the SCOP database (http://scop.mrc-lmb.cam.ac.uk/scop) borrows a scheme already in common use, in which protein structures are divided into four classes: allα, allβ, α/β (in which the and segments are interspersed or alternate), and α+ β (in which the α and β regions are somewhat segregated) (Fig. 4–22). Within each class are tens to hundreds of different folding arrangements, built up from increasingly identifiable substructures. Some of the substructure arrangements are very common, others have been found in just one protein. Figure 4–22 dis plays a variety of motifs arrayed among the four classes of protein structure. Those illustrated are just a minute sample of the hundreds of known motifs. The number of folding patterns is not infinite, however. As the rate at which new protein structures are elucidated has in creased, the fraction of those structures containing a new motif has steadily declined. Fewer than 1,000 different folds or motifs may exist in all proteins. Figure 4–22 also shows how proteins can be organized based on the presence of the various motifs. The top two levels of organization, class and fold, are purely structural. Below the fold level, categorization is based on evolutionary relationships. Many examples of recurring domain or motif structures are available, and these reveal that protein tertiary structure is more reliably conserved than primary sequence. The comparison of protein structures can thus provide much information about evolution. Proteins with significant primary sequence similarity, and/or with demonstrably similar structure and function, are said to be in the same protein family. A strong evolutionary relationship is usually evident within a protein family. For example, the globin family has many different proteins with both structural and sequence similarity to myoglobin (as seen in the proteins used as examples in Box 4–4 and again in the next chapter). Two or more families with little primary sequence similarity sometimes make use of the same major structural

motif and have functional similarities; these families are grouped as superfamilies. An evolutionary relationship between the families in a superfamily is considered probable, even though time and functional distinctions—hence different adaptive pressures—may have erased many of the telltale sequence relationships. A protein family may be widespread in all three domains of cellular life, the Bacteria, Archaea, and Eukarya, suggesting a very ancient origin. Other families may be present in only a small group of organisms, indicating that the structure arose more recently. Tracing the natural history of structural motifs, using structural classifications in databases such as SCOP, provides a powerful complement to sequence analyses in tracing many evolutionary relationships. The SCOP database is curated manually, with the objective of placing proteins in the correct evolutionary framework based on conserved structural features. Two similar enterprises, the CATH (class, architecture, topology, and homologous superfamily) and FSSP ( fold classification based on structure-structure alignment of proteins) databases, make use of more automated methods and can provide additional information. Structural motifs become especially important in defining protein families and superfamilies. Improved classification and comparison systems for proteins lead inevitably to the elucidation of new functional relationships. Given the central role of proteins in living systems, these structural comparisons can help illuminate every aspect of biochemistry, from the evolution of in dividual proteins to the evolutionary history of complete metabolic pathways.

اخر الاخبار

اخبار العتبة العباسية المقدسة

مؤسّسة الوافي تُهدي مكتبةَ مسجد الكوفة نسخةً من موسوعة فتوى الدفاع الكفائي

المجمع العلمي يقيم برنامج (سراج المنتظرين) في محافظة المثنى

قسم الشؤون الفكرية ينظم ندوة فكرية عن أبعاد نهضة الإمام الصادق (عليه السلام)

العتبة العباسية المقدسة تناقش تكثيف جهودها الإغاثية لدعم الشعبين الإيراني واللبناني

المزيد

الآخبار الصحية

دواء جديد يحمي القلب ويعزز علاج السرطان في آن واحد

سر غذائي بسيط لحماية القلب مع التقدم في العمر

علامات تشير إلى اختلال عمل الغدة الدرقية

السمنة واللقاحات.. دراسة تكشف تأثيراً سلبياً

المزيد

الآخبار التكنلوجية

البعوض في آيسلندا إشارة مبكرة على تحولات بيئية في القطب الشمالي

صاروخ يطلق قمرا اصطناعيا في مدار خاطئ

"أوبن إيه آي" تطلق نموذجا للعلوم الحيوية واكتشاف الأدوية

تفوق على البشر.. روبوت يحطم الرقم القياسي لسباق نصف ماراثون !

المزيد

مواضيع ذات صلة

Complementary Interactions between Proteins and Ligands: The Immune System and Immunoglobulins: -The Immune Response Features a Specialized Array of Cells and Proteins

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Sickle-Cell Anemia Is a Molecular Disease of Hemoglobin

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Oxygen Binding to Hemoglobin Is Regulated by 2,3-Bisphosphoglycerate

Protein Denaturation and Folding: - Loss of Protein Structure Results in Loss of Function

Protein Tertiary and Quaternary Structures: -There Are Limits to the Size of Proteins

Protein Tertiary and Quaternary Structures: -Protein Quaternary Structures Range from Simple Dimers to Large Complexes

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins: -Protein-Ligand Interactions Can Be Described Quantitatively

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Myoglobin Has a Single Binding Site for Oxygen

Reversible Binding of a Protein to a Ligand: Oxygen-Binding Proteins:- Oxygen Can Be Bound to a Heme Prosthetic Group

Protein Denaturation and Folding:- Some Proteins Undergo Assisted Folding

Protein Denaturation and Folding:- Polypeptides Fold Rapidly by a Stepwise Process

Protein Denaturation and Folding: - Amino Acid Sequence Determines Tertiary Structure