Are all the essential amino acids for required for protein synthesis in humans?

Question

I came across this statement on a web page about protein. "If all the essential amino acids are not present it is not complete ( or usable as ) protein."

I did not believe the part specifically about "usable" to be true according to my understanding. I thought different proteins in the body required different amino acids, but this statement suggests they all require at least some amount of all the essential amino acids.

Could somebody clarify what is accurate?

By "essential amino acids" I am referring to amino acids that humans can only obtain through diet and are required by the body. There are 9 such amino acids. So, for example, in order for muscle tissue to be formed, does the body require that some degree of all these essential amino acids be present to create the tissue? And likewise for all other tissues, hormones, and anything else that requires protein, do they as well require availability of all these essential amino acids?

Answer 1

These two answers are pretty incorrect. First, you have to understand the mechanism of translation, how proteins are formed according to an mRNA template. mRNA has a codon format, such that three nucleotides codes for one amino acid (AA). The protein is synthesized at the ribosome, where tRNA carrying a specific AA binds to the mRNA codon. Anyway, that being said, you simply can't substitute AAs like they are car parts. If there is a deficiency in essential AAs then the protein won't be made, and there will be problems in the cell due to the lack of the protein being available.

So what is an essential AA? One that's obtained from the diet. See, we can only make some of the AAs we need; the rest we get from digestion of protein. And generally speaking, all proteins use all AAs. Proteins tend to be large, 100aa or more, so the probability that one of the 20 AAs isn't present is low. Of course there are proteins that are very simple structure such as actin, but by and large, all AAs are needed for most (99%) proteins. Each protein has a different sequence, so they use a different chain of AAs. For example, one protein might haev a sequence A-C-D-E-F-G, and another could have the sequence G-F-E-D-C-A. They have different 3D structure, so they would do different things, but the AAs used would be the same.

Hope this clears things up.

Update:

I understood the issue about what is 'essential' and what is not. Lets take a more or less random protein, b-actin. It's abundant in muscle tissue. The AA composition is:

Ala (A) 29 7.7%
Arg (R) 18 4.8%
Asn (N) 9 2.4%
Asp (D) 23 6.1%
Cys (C) 6 1.6%
Gln (Q) 12 3.2%
Glu (E) 26 6.9%
Gly (G) 28 7.5%
His (H) 9 2.4%
Ile (I) 28 7.5%
Leu (L) 27 7.2%
Lys (K) 19 5.1%
Met (M) 17 4.5%
Phe (F) 13 3.5%
Pro (P) 19 5.1%
Ser (S) 25 6.7%
Thr (T) 26 6.9%
Trp (W) 4 1.1%
Tyr (Y) 15 4.0%
Val (V) 22 5.9%

You see all possible AAs are used for this protein. Generally, since proteins are largish (this one is 375aa), the likelyhood is that all AAs are represented. So, an absence of any AA would cause issue. Some are able to be synthesized in the body (de novo), and are non-essential. Others are not, as you noted.

Bottom line/short answer: Generally speaking, all AAs are required for any protein. THe body also produces small peptide hormones, these are small so only represent a few AAs. If the protein is large, chances are it will require all AAs, essential and non-essential.


Let me know if you still have questions.

Answer 2

All the proteins of the body do not require all 20 amino acids. Many of the proteins your body makes can "substitute" a different amino acid if the intended one is not available. In some cases, the protein can function normally with the substitution; in others, it can't. An "essential" amino acid as used here means that the protein must have amino acid X in position Y in order to function.

Answer 3

From what I understand there are 20 different types of proteins. We require 9 of them for sustainable health. Also, for the protein that is found in meat, if we were vegetarian, we would have to consume 2 different types of vegetable protein to obtain the proper substitution. I hope this helps to answer your question.