The below mentioned article provides a note on fat respiration and protein respiration.
(i) Fat Respiration:
Fats are stored as triglycerides in cells, primarily of adipose tissue. They have a high energy content, and form a better fuel than the carbohydrates. They break up into fatty acids and glycerol in the cytoplasm before use in respiration.
Fatty acids are broken by a series of reactions into 2-carbon acetyl coenzyme A. The latter enters the Krebs cycle.
Glycerol combines with a phosphate group, forming phosphoglyceraldehyde. The latter enters glycolysis.
A molecule of 18-carbon stearic acid on complete oxidation produces 147 high-energy phosphates. A 6-carbon glucose molecule yields 36 or 38 ATP. With this rate, an 18-carbon molecule is expected to give 3 times more energy (36 or 38 x 3 = 108 or 114 ATP) but it provides about 4 times more energy (36 or 38 x 4 = 144 or 152 ATP) than 6-carbon glucose produces.
(ii) Protein Respiration:
The proteins split into amino acids in the cytoplasm for use in respiration. The amino acids enter respiratory routes in two ways: deamination and transamination.
In deamination, an amino acid loses its amino group (- NH2) and changes into a keto acid. The latter may further change into pyruvic acid or acetyl coenzyme A. Pyruvic acid is oxidized to acetyl coezyme A. The latter enters the Krebs cycle.
In transamination, an amino group of an amino acid is transferred to an appropriate keto acid, forming a new amino acid and a new keto acid. The keto acids so formed are normal participants of glycolysis or Krebs cycle.