In the comments for my Meat Preservation post, Kreeli asked:
Can you tell me what the difference are between animal protein and plant protein, wheat specifically?
But I am still interested in knowing what makes some protein different from others. Is it the amino acids?
The simple answer is yes, it is the amino acids that make the proteins different but as the answer is a little more complicated I am going to revisit some more of my introductory food science lectures and try and explain why animal proteins and plant proteins are different. To make this even more scientific I am going to call animal proteins zooproteins and plant proteins will be referred to as phytoproteins.
Proteins are one of the three major nutritional components of food; lipids and carbohydrates are the other two. Water, of course is also a major component of food, but its nutritional value is questioned. I will write about lipids and carbohydrates in another post. All three nutritional compounds are polymer, proteins are made up of amino acids. More information on the biochemistry of proteins is around in lots of text books and on the web, so I am not going to repeat that information here. I am going to assume that you know (or have just read the Wikipedia pages linked to above) and know to what primary, secondary, tertiary and quaternary structure refers.
I am more interested in trying to answer Kreeli’s question – why are they different? I am going to answer it from a more devious route than the biochemistry, because all proteins are different because their structure varies, but zooproteins and phytoproteins have a different structure because of their different roles.
Animals, especially vertebrates, are protein. We would not be able to stand up without proteins as our skeletal system is made up of bone tissue which is mostly calcified or mineralized collagen. We could not move without the proteins that make up our muscles. Our organs are mostly made up of protein, unless you are eating Foie gras. We also need proteins for our digestive and nervous systems to work properly.
Plants do not need proteins for structure or movement. They use carbohydrates such are lignin and cellulose instead. In fact most plant proteins are found in the seeds, especially beans, where they are stored before germinating into the next plant. Wheat seeds or grains contain proteins that on mixing with water can be turned into gluten. This is an important structural component of bread. Seitan is made from gluten and has quite a meat – like texture. If you wash the water soluble starch away from a wheat flour dough, the gluten remains as it is in soluble. This can be used as chewing gum as gluten has viscoelastic properties. I’ve tried this in with a visiting high school class  and it has been confirmed by some undergrads who grew up on farms who told me that they used gluten as chewing gum. This website has lots of info on making your own seitan.
So most zooproteins need to be strong, be able to support weight and survive movement. Phytoproteins do not need to be strong, they need to be compact and fit in a small space.
Back in the day – when I did my undergraduate degree and before – zooproteins were called “first class” and phytoproteins were “second class”. We were given information on how to combine our phytoproteins so that we got a complete protein. This was because many plant proteins lack some of the human essential amino acids and need to be combined to give all essential amino acids. A good protein combination is that British staple of Baked Beans on Toast. I know that my diet contains more than enough protein that it really does not matter if I combine foods or not. I happen to like rice and beans, but as an adult I get enough of the essential amino acids just from the grains I eat.
I am sorry about all the Wikipedia links but, in this case as of today, they are quite informative. I also recommend any basic biochemistry text book for information on protein structure. Food Chemistry textbooks may also give some of the information given here.
I’m also here in North Carolina getting ready for the Science Blogging conference so I do not have my books and papers handy to use as a reference.
 You need lots of water and lots of time. Use bread flour if you want to try this at home – it has a lot more protein than general purpose flour.
 I almost can remember when my prof.s first told us about the wonders of soya, which is a complete protein, even better than some meat . They were so excited that at the time and even now, I thought they had just found out how great soy protein was. TVP had been around since the late 70s so they might not have realized until the mid1980s how great it was.