Allow me to present our recently accepted paper: Comparative analysis of the CDR loops of antigen receptors, to appear in Frontiers in Immunology [1]. In the blog post I will give a quick five-minute summary of the key messages in this work.
Continue readingCategory Archives: Protein Structure
A collection of prion factoids
It’s been several years since I last presented a talk on prions to OPIG, so I thought a neat way of getting up to date would be to read “The prion 2018 round tables“. What’s the current understanding and are we any closer to determining a structure of PrPSc?
Continue readingTwo Tools for Systematically Compiling Ensembles of Protein Structures
In order to know how a protein works, we generally want to know its 3-dimensional structure. We then can either try to solve it ourselves (which requires considerable time, skill, and resources), or look for it in the Protein Data Bank, in case it has already been solved. The vast majority of structures in the Protein Data Bank (PDB) are solved through protein crystallography, and represent a “snapshot” of the conformational space available to our protein of interest. Continue reading
How to Iterate in PyMOL
Sometimes pointing-and-clicking just doesn’t cut it. With PyMOL’s built-in Python interpreter, repetitive actions are made simple.
Continue readingWhich fragment first?
Crystallographic fragment based lead discovery is a now a routine technique, which can sample 1000’s of compounds per week. But how do we identify the most appropriate compounds to screen against our target of interest?
Continue readingModelling Conformational Flexibility of Kinases in Inactive States
I would like to shamelessly advertise my master thesis project which just got published in Proteins. Keep on reading if you are interested in kinases and/or systematic modelling of protein families.
Continue readingWhat is the hydrophobic-polar (HP) model?
Proteins are fascinating. They are ubiquitous in living organisms, carrying out all kinds of functions: from structural support to unbelievably powerful catalysis. And yet, despite their ubiquity, we are still bemused by their functioning, not to mention by how they came to be. As computational scientists, our research at OPIG is mostly about modelling proteins in different forms. We are a very heterogeneous group that leverages approaches of diverse scale: from modelling proteins as nodes in a complex interaction network, to full atomistic models that help us understand how they behave.
Continue readingMore Fun With 3D Printing
Recently the students of the Systems Approaches to Biomedical Science Centre for Doctoral Training took a 2-week module on our favourite subject: structural biology! As part of this, they were given the option to create their very own 3D printed model of a protein.
This year we had some great models created, some of which are shown in the picture above. The proteins are (clockwise from top left):
- Clathrin (PDB 1XI4) – a really interesting protein that forms cages around vesicles inside the cell. This one was mine; I wrote about clathrin as part of my undergraduate dissertation many years ago…
- GTPase (PDB 1YZN) – a protein that can bind and hydrolyse guanosine triphosphate (GTP), involved in membrane trafficking
- TAL effector (PDB 3UGM) – this bacterial protein binds to specific regions of DNA in a host plant to activate the expression of plant genes that aid bacterial infection. The DNA here is in blue, the orange wrapped around it is the protein.
- Mechanotransduction ion channel (PDB 5VKQ) – converts mechanical stimuli into electrical signals in specialized sensory cells.
- ATP synthase – this protein machine builds most of the energy storage molecule ATP, which powers our cellular processes.
- DNA (PDB 5F9I) – a double-helix strand of DNA, 20 base pairs long.
Protein Structure Classification: Order in the Chaos
The number of known protein structures has increased exponentially over the past decades; there are currently over 127,000 structures deposited in the PDB [1]. To bring order to this large volume of data, and to further our understanding of protein function and evolution, these structures are systematically classified according to sequence and structural similarity. Downloadable classification data can be used for annotating datasets, exploring the properties of proteins and for the training and benchmarking of new methods [2].
Yearly growth of structures in the PDB (adapted from [1])
Typically, proteins are grouped by structural similarity and organised using hierarchical clustering. Proteins are sorted into classes based on overall secondary structure composition, and grouped into related families and superfamilies. Although this process could originally be manually curated, as with Structural Classification of Proteins (SCOP) [3] (last updated in June 2009), the growing number of protein structures now requires semi- or fully-automated methods, such as SCOP-extended (SCOPe) [4] and Class, Architecture, Topology, Homology (CATH) [5]. These resources are comprehensive and widely used, particularly in computational protein research. There is a large proportion of agreement between these databases, but subjectivity of protein classification is to be expected. Variation in methods and hierarchical structure result in differences in classifications. For example, different criteria for defining and classifying domains results in inconsistencies between CATH and SCOPe.
The arrangements of secondary structure elements in space are known as folds. As a result of evolution, the number of folds that exist in nature is thought to be finite, predicted to be between 1000-10,000 [6]. Analysis of currently known structures appears to support this hypothesis, although solved structures in the PDB are likely to be a skewed sample of all protein structures space. Some folds are extremely commonly observed in protein structures.
In his ‘periodic table for protein structures’, William Taylor went one step further in his goal to find a comprehensive, non-hierarchical method of protein classification [7]. He attempted to identify a minimal set of building blocks, referred to as basic Forms, that can be used to assemble as many globular protein structures as possible. These basic Forms can be combined systematically in layers in a way analogous to the combination of electrons into valence shells to form the periodic table. An individual protein structure can then be described as the closest matching combination of these basic Forms. Related proteins can be identified by the largest combination of basic Forms they have in common.
The ‘basic Forms’ that make up Taylor’s ‘periodic table of proteins’. These secondary structure elements accounted for, on average, 80% of each protein in a set of 2,230 structures (all-alpha proteins were excluded from the dataset) [7]
- RCSB PDB Statistics
- Fox, N.K., Brenner, S.E., Chandonia, J.-M., 2015. The value of protein structure classification information-Surveying the scientific literature. Proteins Struct. Funct. Bioinforma. 83, 2025–2038.
- Murzin AG, Brenner SE, Hubbard T, Chothia C., 1995. SCOP: a structural classification of proteins database for the investigation of sequences and structures. J Mol Biol. 247, 536–540.
- Fox, N.K., Brenner, S.E., Chandonia, J.-M., 2014. SCOPe: Structural Classification of Proteins–extended, integrating SCOP and ASTRAL data and classification of new structures. Nucleic Acids Res. 42, 304-9.
- Dawson NL, Lewis TE, Das S, et al., 2017. CATH: an expanded resource to predict protein function through structure and sequence. Nucleic Acids Research. 45, 289-295.
- Derek N Woolfson, Gail J Bartlett, Antony J Burton, Jack W Heal, Ai Niitsu, Andrew R Thomson, Christopher W Wood,. 2015. De novo protein design: how do we expand into the universe of possible protein structures?, Current Opinion in Structural Biology, 33, 16-26.
- Taylor, W.R., 2002. A “periodic table” for protein structures. Nature. 416, 657–660.
- Ahnert, S.E., Marsh, J.A., Hernandez, H., Robinson, C. V., Teichmann, S.A., 2015. Principles of assembly reveal a periodic table of protein complexes. Science. 80, 350
Start2Fold: A database of protein folding and stability data
Hydrogen/deuterium exchange (HDX) experiments are used to probe the tertiary structures and folding pathways of proteins. The rate of proton exchange between a given residue’s backbone amide proton and the surrounding solvent depends on the solvent exposure of the residue. By refolding a protein under exchange conditions, these experiments can identify which regions quickly become solvent-inaccessible, and which regions undergo exchange for longer, providing information about the refolding pathway.
Although there are many examples of individual HDX experiments in the literature, the heterogeneous nature of the data has deterred comprehensive analyses. Start2Fold (Start2Fold.eu) [1] is a curated database that aims to present protein folding and stability data derived from solvent-exchange experiments in a comparable and accessible form. For each protein entry, residues are classified as early/intermediate/late based on folding data, or strong/medium/weak based on stability data. Each entry includes the PDB code, length, and sequence of the protein, as well as details of the experimental method. The database currently includes 57 entries, most of which have both folding and stability data. Hopefully, this database will grow as scientists add their own experimental data, and reveal useful information about how proteins refold.
The folding data available in Start2Fold is visualised in the figure below, with early, intermediate and late folding residues coloured light, medium and dark blue, respectively.
[1] Pancsa, R., Varadi, M., Tompa, P., Vranken, W.F., 2016. Start2Fold: a database of hydrogen/deuterium exchange data on protein folding and stability. Nucleic Acids Res. 44, D429-34.