During protein metabolism, some protein is converted to glucose in a process called gluconeogenesis, the formation of glucose from non-carbohydrate sources.
The basic difference between protein and carbohydrate is that while carbohydrates are made out of simple sugars (carbon, hydrogen, and oxygen), protein is made from amino acids (carbon, hydrogen, oxygen, nitrogen, and sufur). The nitrogen is a basic component of the protein's amino acids and accounts for 13 to 20% of the total mass.
The first step in protein metabolism is to break it into its constituent amino acids. These are absorbed into the blood stream.
The second step is to break down the amino acids into their constituent parts--catabolism, if you want to get technical about it. This removes the nitrogen or amino group from the amino acids. The process is called deamination.
Deamination breaks the amino group down into ammonia and what is termed the carbon skeleton. Ammonia is converted to urea, filtered through the kidneys, and excreted in urine. The carbon skeleton--which is composed of carbon, hydrogen, and oxygen--can then by used either for protein synthesis, energy production (ATP), or converted to glucose by gluconeogenesis.
Most authorities believe that the amount of protein converted to glucose is quite small, except under conditions of intense exercise or metablic starvation. Under these conditions amino acids produce the major source of glucose for blood sugar maintenance.