Small-Angle X-Ray Scattering (SAXS) is a method used to quantify nanoscale differences in the density of a sample. This method is gaining importance in the field of metabolomics, for the assessment of metabolic state and related disorders.
.jpg "SAXS can be used to study the structure of proteins, including apolipoproteins")
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What is Small-Angle X-Ray Scattering?
SAXS is an X-ray scattering technique that measure the elastic scattering of X-rays when it is traveling through a material. Although it is fundamentally similar to X-ray diffraction, there are also some distinct differences.
Both methods employ an intense X-ray beam, but in SAXS the sample molecule is in solution. This is in contrast to X-ray diffraction, where the sample is embedded in a crystal. X-ray diffraction often yields higher resolution and better signal-to-noise ratio but there are certain proteins that are flexible and do not crystallize easily. For such molecules, small-angle X-Ray scattering may be applied.
Mechanism of Small-Angle X-Ray Scattering
SAXS is based on differences in contrast. The signal is derived from the difference in the average electron density of the solvent and solute or the protein. This difference in the density is based on the composition of the solvent and the concentration of the sample. The data collection is based on subtracting the data obtained from the “blank” buffer and the sample.
It is important to measure the blank precisely, as even small differences in the composition of the buffer can affect the small-angle X-Ray scattering data. Also, low angle scattering data can be contaminated by unscattered X-rays. Thus, the experimental station should be carefully set-up to ensure minimum contamination.
Using Small-Angle X-Ray Scattering to reveal the structure of lipoproteins
Apolipoproteins have been shown to emulsify lipids and have protective roles against cardiovascular diseases and obesity. They act as structural scaffolds and can activate lipoprotein-modeling factors. They can also interact with cell surface proteins to modulate lipoprotein metabolism.
As they have great importance in lipid biology, it is important to know the structure of apolipoproteins. However, many of the apolipoproteins transition between self-associated states, such as dimers, trimers, tetramers etc. Thus, it is difficult to characterize their structure.
One way is to stabilize one particular oligomeric form using site-directed mutagenesis. However, this method has drawn criticism as it has the potential to alter the protein conformation along with the oligomeric state.
A recent study performed SAXS on a full-length apolipoprotein, apoA-IV. Using this method, the authors were able to visualize the full length of the protein along with its truncation variants. They were also able to analyze how the termini affect the conformation of the structure as a whole. Understanding the structure also provides a clue to how the protein functions in various metabolic processes.
How can SAXS be used to understand the interactions among digested food?
A recent study used SAXS to investigate how caseinophosphopeptides (CPP), a major digested product of milk, and chitosan interact in a simulated gastrointestinal condition. There is a change in pH value as the food passes from stomach to small intestine, and this pH change induces the formation of a complex between caseinophosphopeptides and chitosan. This complex formation is driven by electrostatic interactions between these two molecules. SAXS was used to characterize the structure of these complexes.
SAXS was also used to study how polyphenols, a class of micronutrients, interact with CRPP and chitosan. Using this method, it is possible to analyze how digested food complexes interact with each other and with other molecules, giving critical insights into digestion and metabolism.
Further Reading