Protein and Peptide Letters - Volume 19, Issue 7, 2012

Marine organisms are an immense source of new biologically active compounds. These compounds are unique because the aqueous environment requires a high demand of specific and potent bioactive molecules. Diverse peptides with a wide range of biological activities have been discovered, including antimicrobial, antitumoral, and antiviral activities and toxins amongst others. These proteins have been isolated from different phyla such as Porifera, Cnidaria, Nemertina, Crustacea, Mollusca, Echinodermata and Craniata. Purification techniques used to isolate these peptides include classical chromatographic methods such as gel filtration, ionic exchange and reverse-phase HPLC. Multiple in vivo and in vitro bioassays are coupled to the purification process to search for the biological activity of interest. The growing interest to study marine natural products results from the discovery of novel pharmacological tools including potent anticancer drugs now in clinical trials. This review presents examples of interesting peptides obtained from different marine organisms that have medical relevance. It also presents some of the common methods used to isolate and characterize them.

A lighthearted researcher and a disheartened student discuss the challenges of protein crystallization and how phase diagrams can be used to address these challenges. The student feels a little better afterwards, but many proteins remain uncrystallized.

The aim of this review is to provide biocrystallographers who intend to tackle protein-crystallization with theory and practical examples. Crystallization involves two separate processes, nucleation and growth, which are rarely completely unconnected. Here we give theoretical background and concrete examples illustrating protein crystallization. We describe the nucleation of a new phase, solid or liquid, and the growth and transformation of existing crystals obtained by primary or secondary nucleation or by seeding. Above all, we believe that a thorough knowledge of the phase diagram is vital to the selection of starting position and path for any crystallization experiment.

The outcome of protein crystallization attempts is often uncertain due to inherent features of the protein or to the crystallization process that are not fully under control of the experimentalist. The aim of this contribution is to propose user-friendly tools that can increase the success rate of a protein crytallization project. Different bioinformatic approaches to predict the crystallization feasibility (before any crystallization attempts are undertaken) are discussed and a novel approach to assess the nucleation process of a given protein is proposed. Practical examples illustrate these two points.

The limiting step in macromolecular crystallography is the preparation protein crystals suitable for X-ray diffraction studies. A strong prerequisite for the success of crystallization experiments is the ability to produce monodisperse and properly folded protein samples. Since the production of most protein is usually achieved using recombinant methods, it has become possible to engineer target proteins with increased propensities to form well diffracting crystals. Recent advances in bioinformatics, which takes advantage from an enhanced information in the protein databases, are of enormous help for the design of modified proteins. Based on bioinformatics analyses, the reduction of the structural complexity of proteins or their site-specific mutagenesis has proven to have a dramatic impact on both the yield of heterologous protein expression and its crystallizability. Therefore, protein engineering represents a valid tool which supports the classical crystallization screenings with a more rational approach. This review describes key methods of protein-engineering and provides a number of examples of their successful use in crystallization. Scope of Proposed Topic: This Topic is focused on state-of-art protein engineering techniques to increase the propensity of proteins to form crystals with suitable X-ray diffraction properties. Protein engineering methods have proven to be of great help for the crystallization of difficult targets. We herein review molecular biology and chemical methods to help protein crystallization.

To start systematically investigating the quality improvement of protein crystals, the elementary growth processes of protein crystals must be first clarified comprehensively. Atomic force microscopy (AFM) has made a tremendous contribution toward elucidating the elementary growth processes of protein crystals and has confirmed that protein crystals grow layer by layer utilizing kinks on steps, as in the case of inorganic and low-molecular-weight compound crystals. However, the scanning of the AFM cantilever greatly disturbs the concentration distribution and solution flow in the vicinity of growing protein crystals. AFM also cannot visualize the dynamic behavior of mobile solute and impurity molecules on protein crystal surfaces. To compensate for these disadvantages of AFM, in situ observation by two types of advanced optical microscopy has been recently performed. To observe the elementary steps of protein crystals noninvasively, laser confocal microscopy combined with differential interference contrast microscopy (LCM-DIM) was developed. To visualize individual mobile protein molecules, total internal reflection fluorescent (TIRF) microscopy, which is widely used in the field of biological physics, was applied to the visualization of protein crystal surfaces. In this review, recent progress in the noninvasive in situ observation of elementary steps and individual mobile protein molecules on protein crystal surfaces is outlined.

Synchrotron Radiation (SR) presents itself as a “play-ground” with a large range of methods and techniques suitable to unveil the mysteries of life. Here we attempt to present a few of these methods that complement those employed in the home laboratory. SR diffraction, spectroscopy and imaging methods relevant to the atomic structure determination and characterization of the properties and function of chemical compounds and macromolecules of biological relevance, are introduced.

Macromolecular crystallography has been, for the last few decades, the main source of structural information of biological macromolecular systems and it is one of the most powerful techniques for the analysis of enzyme mechanisms and macromolecular interactions at the atomic level. In addition, it is also an extremely powerful tool for drug design. Recent technological and methodological developments in macromolecular X-ray crystallography have allowed solving structures that until recently were considered difficult or even impossible, such as structures at atomic or subatomic resolution or large macromolecular complexes and assemblies at low resolution. These developments have also helped to solve the 3D-structure of macromolecules from twin crystals. Recently, this technique complemented with cryo-electron microscopy and neutron crystallography has provided the structure of large macromolecular machines with great precision allowing understanding of the mechanisms of their function.

High-throughput crystallisation requires the rapid and accurate dispensing of protein and precipitating agent solutions at nanovolumes, but does not end there. The choice of the initial screens is very important, especially with respect to the availability of protein material. Data from previous crystallisation experiments that are scattered in the literature and only partially available in databases have to be analysed in efficient ways that will maximise their utility for designing new screens. A larger portion of crystallisation parameter space should be made accessible to screening, through the use of nucleants and seeding. Observation, assessment and scaling up of the crystallisation trials should be efficiently performed and, finally yet importantly, optimisation of conditions must also be adapted to the high-throughput environment. The above requirements are briefly addressed in the following paper.

Of many factors affecting protein crystallization, randomness in proteins has been given less attention although highly structured proteins would be at low entropy state. The factors, which impact on protein crystallization, are almost exclusively related to non-random amino acid properties such as physiochemical properties of amino acids. In this study, we used logistic regression and neural network to model the success rate of crystallization of 420 proteins from Staphylococcus aureus with each of non-random and random amino acid properties in order to determine whether randomness in a protein plays a role in the crystallization process. The results show that randomness is indeed involved in the crystallization process, and this rationale would enrich our knowledge on crystallization process and enhance our ability to crystallize more important proteins.