The first cycle he discovered was the urea cycle, also known as the ornithine cycle, and is used in the body to remove waste ammonia by converting it to urea. It was the first biochemical cycle to be discovered.
His most famous discovery was the citric acid cycle, also known as the Krebs cycle. This is the set of reactions used to generate most of our energy from sugars; it also produces precursors used in biosynthesis. In animals it takes place in a specialised organelle called the mitochondrion. The third cycle was the glyoxylate cycle, a variant of the Krebs cycle found in plants and fungi.
Krebs received the Nobel Prize in Physiology or Medicine in 1953 for his discovery of the Krebs cycle, and was knighted in 1958. He was the first head of the Department of Biochemistry at Sheffield, now part of the Department of Molecular Biology and Biotechnology.