In Lehninger's Principles of Biochemistry, Pg. 116 states that "parts of proteins lack discernable structure." What exactly makes this protein not readable? Is it the complexity of the shape that makes it difficult for scientists to understand the structure?
There are two main methods for discerning a protein's structure; x-ray crystallography, and more recently cryo-electron microscopy. Both methods work by firing a high-energy beam through the sample and observing how the path of the beam has been changed by passing through the sample. The data obtained by measuring the beam includes "signal" information on the structure of the sample, but also a lot of random data, or "noise". If the signal from large numbers of copies of identical proteins can be averaged together coherently, it can be separated from the noise and the positions of the atoms that make up the protein can be determined. In crystallography, this is achieved by getting the proteins to arrange into a crystal, and in cryo-EM by a computational process after data has been collected. Either way, if the many copies of the protein are not quite identical, the signal averaging will not work, and the positions of the atoms cannot be determined as accurately, or at all. In some cases, the protein has two or a few distinct structures that it switches between, and these can be separated and determined accurately. In other cases, the protein only takes a specific shape when it is bound to another molecule, and just flops around randomly when nothing is bound. Here is the abstract of a review on this second type of protein:
Many gene sequences in eukaryotic genomes encode entire proteins or large segments of proteins that lack a well-structured three-dimensional fold. Disordered regions can be highly conserved between species in both composition and sequence and, contrary to the traditional view that protein function equates with a stable three-dimensional structure, disordered regions are often functional, in ways that we are only beginning to discover. Many disordered segments fold on binding to their biological targets (coupled folding and binding), whereas others constitute flexible linkers that have a role in the assembly of macromolecular arrays.
Finally, some proteins will not crystallize unless a portion of their sequence is removed. These portions may have a consistent structure, but it can't be determined by crystallography.