Proteins 3-dimensional structure

Bdhm H-J 1994 The development of a simple empirical scoring function to estimate the binding constant for a protein-ligand complex of known three-dimensional structure J. Comp.-Aided Mol. Design 8 243-56... 

The protein folding problem is the task of understanding and predicting how the information coded in the amino acid sequence of proteins at the time of their formation translates into the 3-dimensional structure of the biologically active protein. A thorough recent survey of the problems involved from a mathematical point of view is given by Neumaier [22]. 

A particularly important application of molecular dynamics, often in conjunction with the simulated annealing method, is in the refinement of X-ray and NMR data to determine the three-dimensional structures of large biological molecules such as proteins. The aim of such refinement is to determine the conformation (or conformations) that best explain the experimental data. A modified form of molecular dynamics called restrained moleculai dynarrdcs is usually used in which additional terms, called penalty functions, are added tc the potential energy function. These extra terms have the effect of penalising conformations... 

Bioinformatics is a relatively new discipline that is concerned with the collection, organisatic and analysis of biological data. It is beyond our scope to provide a comprehensive overvie of this discipline a few textbooks and reviews that serve this purpose are now available (s the suggestions for further reading). However, we will discuss some of the main rnethoc that are particularly useful when trying to predict the three-dimensional structure and fum tion of a protein. To help with this. Appendix 10.1 contains a limited selection of some of tf common abbreviations and acronyms used in bioinformatics and Appendix 10.2 lists sorr of the most widely used databases and other resources. 

As more protein structures became available it was observed that some contained more that one distinct region, with each region often having a separate function. Each of these region is usually known as a domain, a domain being defined as a polypeptide chain that can folc independently into a stable three-dimensional structure. 

A number of structured databases have been developed to classify proteins according to the three-dimensional structures. Many of these are accessible via the World Wide Web, T1 protein databanlc (PDB [Bernstein d al. 1977]) is the primary source of data about the stru tures of biological macromolecules and contains a large number of structures, but many i these are of identical proteins (complexed with different ligands or determined at differet resolutions) or are of close homologues. 

A sequence alignment establishes the correspondences between the amino adds in th unknown protein and the template protein (or proteins) from wliich it will be built. Th three-dimensional structures of two or more related proteins are conveniently divided int structurally conserved regions (SCRs) and structurally variable regions (SVRs). Ihe structural conserved regions correspond to those stretches of maximum sequence identity or sequenc... 

Bowie J U, R Liithy and D Eisenberg 1991. A Method to Identify Protein Sequences that Fold into a Known Three-Dimensional Structure Science 253 164-170. 

I-J 1994. The Development of a Simple Empirical Scoring Fimction to Estimate the Binding istant for a Protein-ligand Complex of Known Three-Dimensional Structure. Journal of nputer-Aided Molecular Design 8 243-256. 

Charifson P S, J J Corkery, M A Murcko and W P Walters 1999. Consensus Scoring A Method fc Obtaining Improved Hit Rates from Docking Databases of Three-Dimensional Structures int Proteins. Journal of Medicinal Chemistry 42 5100-5109. 

Enzymes are excellent catalysts for two reasons great specificity and high turnover rates. With but few exceptions, all reac tions in biological systems are catalyzed by enzymes, and each enzyme usually catalyzes only one reaction. For most of the important enzymes and other proteins, the amino-acid sequences and three-dimensional structures have been determined. When the molecular struc ture of an enzyme is known, a precise molecular weight could be used to state concentration in molar units. However, the amount is usually expressed in terms of catalytic activity because some of the enzyme may be denatured or otherwise inactive. An international unit (lU) of an enzyme is defined as the amount capable of producing one micromole of its reaction product in one minute under its optimal (or some defined) reaction conditions. Specific activity, the activity per unit mass, is an index of enzyme purity. 

JU Bowie, R Liithy, D Eisenberg. A method to identify protein sequences that fold into a known three-dimensional structure. Science 253 164-170, 1991. 

To understand the biological function of proteins we would therefore like to be able to deduce or predict the three-dimensional structure from the amino acid sequence. This we cannot do. In spite of considerable efforts over the past 25 years, this folding problem is still unsolved and remains one of the most basic intellectual challenges in molecular biology. 

Two cysteine residues in different parts of the polypeptide chain but adjacent in the three-dimensional structure of a protein can be oxidized to form a disulfide bridge (Figure 1.4). The disulfide is usually the end product of air oxidation according to the following reaction scheme ... 

All protein molecules are polymers built up from 20 different amino acids linked end-to-end by peptide bonds. The function of every protein molecule depends on its three-dimensional structure, which in turn is determined by its amino acid sequence, which in turn is determined by the nucleotide sequence of the structural gene. 

Domains are formed by different combinations of secondary structure elements and motifs. The a helices and p strands of the motifs are adjacent to each other in the three-dimensional structure and connected by loop regions. Sequentially adjacent motifs, or motifs that are formed from consecutive regions of the primary structure of a polypeptide chain, are usually close together in the three-dimensional structure (Figure 2.20). Thus to a first approximation a polypeptide chain can be considered as a sequential arrangement of these simple motifs. The number of such combinations found in proteins is limited, and some combinations seem to be structurally favored. Thus similar domain structures frequently occur in different proteins with different functions and with completely different amino acid sequences. 

Figure 3.6 Four-helix bundles frequently occur as domains in a proteins. The arrangement of the a helices is such that adjacent helices in the amino acid sequence are also adjacent in the three-dimensional structure. Some side chains from all four helices are buried in the middle of the bundle, where they form a hydrophobic core, (a) Schematic representation of the path of the polypeptide chain in a four-helrx-bundle domain. Red cylinders are a helices, (b) Schematic view of a projection down the bundle axis. Large circles represent the main chain of the a helices small circles are side chains. Green circles are the buried hydrophobic side chains red circles are side chains that are exposed on the surface of the bundle, which are mainly hydrophilic.

Arthur Lesk and Cyrus Chothia at the MRC Laboratory of Molecular Biology in Cambridge, UK, compared the family of globin strucfures with the aim of answering two general questions How can amino acid sequences that are very different form proteins that are very similar in their three-dimensional structure What is the mechanism by which proteins adapt to mutations in the course of their evolution ... 

To answer the first question, Lesk and Chothia examined in detail residues at structurally equivalent positions that are involved in helix-heme contacts and in packing the a helices against each other. After comparing the nine globin structures then known, the 59 positions they found that fulfilled these criteria were divided into 31 positions buried in the interior of the protein and 28 in contact with the heme group. These positions are the principal determinants of both the function and the three-dimensional structure of the globin family. 

Kajava, A.V., Vassart, G., Wodak, S.J. Modelling of the three-dimensional structure of proteins with the typical leucine-rich repeats. Structure 3 867-877, 1995. 

There is a second family of small lipid-binding proteins, the P2 family, which include among others cellular retinol- and fatty acid-binding proteins as well as a protein, P2, from myelin in the peripheral nervous system. However, members of this second family have ten antiparallel p strands in their barrels compared with the eight strands found in the barrels of the RBP superfamily. Members of the P2 family show no amino acid sequence homology to members of the RBP superfamily. Nevertheless, their three-dimensional structures have similar architecture and topology, being up-and-down P barrels. 

Boumann, U., et al. Three-dimensional structure of the alkaline protease of Pseudomonas aeruginosa, a two-domain protein with a calcium binding parallel beta roll motif. EMBO J. 12 3357-3364, 1993. 

---