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> Its form is discovered:
> 
> https://phys.org/news/2022-10-three-dimensional-papp-a.html
> 
> "Danish researchers have determined the three-dimensional structure of the proteolytic enzyme PAPP-A. The results may allow us to better understand the basic biology that regulates linear growth of vertebrates. The same regulatory mechanisms are also involved in several age-related diseases, and thus, the research is an important step towards the development of novel types of drugs.
> 
> "The growth factor IGF plays a key role in human growth. In the absence of IGF signaling, we become dwarfs. Later in life, IGF is involved in age-related diseases, such as cancer and cardiovascular disease. In both cases, IGF must be converted from an inactive to an active form. This is what PAPP-A is able to do.
> 
> "'Seven years ago we discovered that the protein STC2 blocks the activity of PAPP-A, thus indirectly inhibiting the activity of the IGF growth factor. To block the activity, STC2 must form a complex with PAPP-A. We have studied this complex, and we now know its three-dimensional structure," Professor Claus Oxvig explains.
> 
> "'It is fascinating to see what a molecule, we know biochemically very well, actually looks like. PAPP-A is heart-shaped with an inner 'chamber'. But from a research point of view, the shape is not the most interesting feature. Rather, it is the interactions between the different elements of the molecule."
> 
> "There are still many unanswered questions about the molecular mechanisms, which regulate how much IGF is converted into the active form. It is likely that complex formation between PAPP-A and STC2 is highly regulated. Such a hypothesis is supported by earlier findings showing that natural human variants of STC2, in which just a single amino acid is substituted, form the complex with PAPP-A slightly slower. The consequence of this is that slightly more IGF can be activated by PAPP-A, resulting in an increase in height of up to 2.1 cm."
> 
> Comment: Same old point: this must be a designed mechanism because of the complexity of the enzyme molecule and the feed-back controls.