system
Given the following post, respond as if you're commenting on Reddit:
human
Does the body store protein for muscle building, like it stores energy as fat for later use? If so, how does it do this?
gpt
Amino acids are 'tied up' in protein structures in the body. It should be noted that proteins in the body refer to a wide variety of different biological structures with varying functions - not just components muscle cells themselves. Proteins contribute as structural components, biocatalysts (enzymes), antibodies, hormones, lubricants, carriers etc. All of these may contribute to skeletal muscle protein synthesis in some way. The basic functions of proteins can be divided into: growth (pr synthesis), maintenance (pr turnover), regulation (hormones, acid/base balance, enzymes), energy (contribute to glycogenic or ketogenic processes).

There are basically three fates for an AA in the body: 

1. protein synthesis
2. as metabolic intermediaries for gluconeogenesis or fatty acid synthesis
3. carbon skeletons used in energy production

They can provide a significant percentage of energy requirements - sometimes as high as 50% in high protein diets. Also the BCAAs can be directly used by skeletal muscle as an energy source, and glutamine and alanine by the digestive tract.

There is a small free amino acid pool located in the intra/extracellular spaces that is not bound up in protein structures. This contributes to around 0.5-1% of the total amino acids in the body and plays a large role in the balance of muscle protein. It is thus in constant flux and can vary significantly. 

For a net increase in protein synthesis in the muscle, you have to have:
1. increase in rate of synthesis
2. decrease in rate of breakdown
or
3. both

To rephrase this: Your protein balance is a function of input versus output and this is in a constant state of flux. Muscle tissue is highly plastic and can definitely function as a 'reserve' within the body. Less active muscles actually see more use as a supply of AAs when needed. For example, the amount of leucine oxidised during training can be as much as 25x the amount available in the free AA pool, so endogenous breakdown can be a major source of AAs.

there are at least four ways cell protein concentration could change:

1. increased transcription
2. increased translation
3. decreased rate of mRNA degradation
4. decreased rate of degradation of the protein

So the rate of protein synthesis in skeletal muscle taking place during or after exercise depends to a large extent on having a complete complement of precursor AAs. It can also be affected by the availability and efficiency of various enzymes, tRNA, ATP etc.

Long story short, the amount of intracellular AAs can be an indicator of the level of protein synthesis in muscle, but there is no particular 'storage' capability equivalent to glycogen or fat.
 Amino acids are either used in metabolism or to form protein structures with a function beyond simple storage. A very small amount is kept as a free AA pool.