Protein simple /3-structural components

This discussion puts us in a position to define a set of structural and evolutionary objects the tracing of whose history via homology, as implied by sequence similarity, is the primary aim of sequence analyses. The simple view of protein folding produces a small set of structural components to consider. This is the set of regular secondary structures the amphipathic a helix, the transmembrane or hydrophobic a. helix, the... 

A vast number of hormones and neurotransmitters are synthesized from simple structural units including chains of amino acids. Among the amino acid based hormones are comparatively large protein molecules such as prolactin, growth hormone and insulin, and shorter chains of amino acids which may form peptide hormones, such as oxytocin and vasopressin. Protein based hormones are essential for metabolism, growth and some components of reproduction. However, evidence for specific influences of these compounds on human behavior is limited. 

In comparison to signaling pathways which utilize transmembrane receptors (see chapter 5, 8,11), signahng via nuclear receptors is of relatively simple structure. The pathways lead directly, with only a few participating protein components, from the extracellular space to the level of the DNA in the nucleus. Most important protein components of the signal pathway are known and well characterized. Nevertheless, we understand very little of the mechanism by which the activated receptors lead to a transcription initiation. This is due to the extreme complexity of transcription initiation in eucaryotes (see 1.2). Both the variety of proteins involved in the formation of a competent initiation complex, as well as the influence of chromatin structure, make it difficult to elucidate the exact function of nuclear receptors in transcription initiation. 

Conjugated proteins contain a nonprotein structural component in addition to the protein moiety, whereas simple pmteins contain only the polypeptide chain of amino acid units. Nucleoproteins arc conjugated proteins containing nucleic acids as structural components. Glycoproteins are car-... 

Proteins that have catalytic properties are called enzymes (i.e.. enzymes are biological catalysts of protein nature). Some enzymes have full catalytic reactivity per sc these are considered to be simple proteins because they do not hiive a nonprotein moiety. Other enzymes are conjugated proteins, and the nonprotcin structural components arc necc.ssary for reactivity. Occasionally, enzymes require metallic ions. Because enzymes are proteins or conjugated proteins, the general review of protein structural studies presented above in this chapter (e.g.. protein conformation and denaturation) is fundamental to the following topics. Conditions that affect denaturation of proteins usually have an adverse effect on the activity of the enzyme. 

In comparison to proteins, DNA is a fairly simple molecule. It consists of only four different bases, which are repeated throughout the whole structure and the double helix is its only structural component. 

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