Been trying phrase an answer to this for a few minutes and keep coming up with something that sounds like it's been written while half dunk on Christmas Day. which it has so with that disclaimer. structural proteins are ones where their function is related to 'structural roles'. things like actin or tubulin which form parts of the cells cytoskeleton, or collagen or fibronectin in the extra cellular matrix. the shapes off these monomers madness them suitable for building complex 3d structures. this is different from things like enzymes where their shape is critical too their ability to catalyse reactions. there are other biochemicals that can have structural roles such as large sugars things like hyaluronic acid but most structural elements are proteins, probably because of the diversity of different possible structures and the fact that cells already have systems for making, processing and transporting them.

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Your question's very weird honestly. It's completely unfocused and therefore really hard to answer satisfactory.

First, There are very many different proteins with different functions. They look nothing alike. So yes, some are stretchy or bendable, some are easy to manufacture. Others, not so much.

You learned DNA/RNA, those are like a prescription/recipe on how to form a protein. They're "just a strain of repeated units than can be read by ribosomes" (for simplicity's sake we stick to the highschool explanation here).

The ribosomes "read" the recipe and built the protein strand, which starts out as a long chain of amino acids. Each amino acid has its own unique characteristics like pH or sulpher atoms. When connected into a strand, those characteristics influence nearby amino acids to bend away or towards them. Polarity and Hydrogen and sulpher bridges shape the strand into a 3D structure. This is the "why": interactions make it that way. That kinda answers your question as well.

The last answer is "because it works". Proteins which are useless and cost energy to make or have a cool function but cannot be broken down afterwards affect the cell negatively. Efficient cells on average do better than inefficient ones, so good traits survive in the long run (yes, again, I simplify). Structures that work well are conserved throughout evolution, hence why we can make whole trees based on similarity. Which is also a reason why they're structural (and even more so than based on sequence similarity alone)

The amino acids interact with each other. The individual interactions are often very simple and involve such things as hydrogen bonding, acid/base salt bridges, hydrophobic exclusion, Van der Waals, sometimes covalent bonds such as disulfides.

While the interactions are simple, the sum of these interactions is very complex and significant. By changing the order that the amino acids are connected (and some other factors that are a bit more complex), the shape of the protein can be controlled.

The end result is a very large and bulky molecule with a very specific shape and that can interact with other molecules in very specific ways.

It's kinda like Legos. The individual bricks connect to each other in very simple ways, but a skilled builder with a plan can build a large creation with a very specific shape and function.

It's kind of like "entropy is always trying to destroy structures, these are just the ones that continue to work" or whatever? Like, "nature is in fact constantly trying other ways of continuing these structures--they just get destroyed faster than they can be built" or whatever?

Also to add, for proteins “structure dictates function”

Without the correct structure, which arises due to the things listed above (and hydrophobic/philic interactions I may add) proteins cannot perform their intended function which can lead to various consequences.

Some proteins even work almost like tiny machines in the cell and can perform work using ATP

SOME proteins are used as structural elements of the cell, but overall “structure” of proteins is important for their functional roles

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Not to mention the proteins have different forms in different chemical solutions. So pH, temperature and catalysts can all modify the proteins in different ways. Not to mention proteins can inherently interact with RNA allowing for complex bio machinery basically attached to written code.

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Entropy just is, it's not trying anything. Same for nature.

Cells actively remove proteins which arem't needed anymore. E.g. Ubiquitin gets attached so the proteasome can recognise them and break them down. Protein decay happens over time as well but that's also a good thing for cells. It's part of the waste management to keep cells from getting stuffed with old proteins that can aggregate together.

So no, they are not destroyed faster than they can be built. There's a whole system in place to determine what needs to go when.

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>I know proteins are important as structural elements, my question (as ever) is just why?

A couple things. First, a lot of "why" questions in biology are unanswerable, and the explanation you get will come down to quirks of life's evolutionary history.

Second, if I may, I'd like to reframe your question as "Why do we see proteins playing structural roles in the places that we do?" Proteins are just one type of structural molecule, and nature uses many.

Arguably lipids are the most critical structural component for all life since they can form membranes. Cellulose and chitin are polysaccharides (i.e. many sugars linked together) that have structural functions in plants and fungi/arthropods respectively. Lignin is another plant cell wall component, but it is actually a polyphenol. Some organisms even use inorganic compounds to provide structure - bones are obvious, but for a microscopic example look at the silica frustules of diatoms. Even in these examples, it gets messy. You can have several different types of molecules that together form some structural component.

You can think of evolution as a process of blindly stumbling into "solutions" for "problems". In some cases, evolution reaches different solutions for similar problems. Things like hair and fingernails are made of keratin (a structural protein) while insect exoskeletons are made of chitin (a structural polysaccharide). In other cases like the cytoskeleton, you only see protein being used. That brings me back to my reformulation of your question. You can make hypotheses about why this is the case, but they are very hard to test.

I think the structural value of proteins comes from the diversity of amino acids (although possibly causality went the other way there). Lets consider all our possible structural elements. They need to be something that's big....basically, they need to be a chain of repeating units that's arbitrarily long. Fats don't really do this.

That leaves sugars, nucleic acid, and protein. Long sugar chains are sometimes structural (see cellulose and chitin) but there are only a few kinds of sugars used in these structures and they don't have a lot of complexity in chemical nature. So the number of things you can make from them is limited. Nucleic acid has more options, with four bases it can and does bind to itself to form special molecules that use special structures that have catalytic function. A lot of people think this used to be what life originally used, before proteins really got going (the RNA-world hypothesis).

And finally we have proteins. Proteins have 20 types of amino acids to work with, which allows for a ton of variety in chemistry of each individual part. Which in turn means proteins can do all sorts of things, because the different arrangement of parts let proteins fold in all sorts of ways and interact with their environment and each other in all sorts of ways. With so many different amino acids, you can make a ton of different proteins that do a ton of different things...including structural proteins.

To make an analogy, building with sugars is like if you have one kind of lego block, building with nucleic acid is like if you had 4 kinds, and building with protein is like having a 20 different widely varying lego pieces to work with. It's just more versatile.

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