Biochemistry

TLO: Describe the metabolism of proteins

During digestion, proteins are broken down into amino acids. Unlike carbohydrates and triglycerides, which are stored, proteins are not warehoused for future use. Instead, amino acids are either oxidized to produce ATP or used to synthesize new proteins for body growth and repair. Excess dietary amino acids are not excreted in the urine or faeces but instead are converted into glucose (gluconeogenesis) or triglycerides (lipogenesis).

 Protein Catabolism

A certain amount of protein catabolism occurs in the body each day, stimulated mainly by cortisol from the adrenal cortex. Proteins from worn-out cells (such as red blood cells) are broken down into amino acids. Some amino acids are converted into other amino acids, peptide bonds are re-formed, and new proteins are synthesized as part of the recycling process. Hepatocytes convert some amino acids to fatty acids, ketone bodies, or glucose. Cells throughout the body oxidize a small amount of amino acids to generate ATP via the Krebs cycle and the electron transport chain. However, before amino acids can be oxidized, they must first be converted to molecules that are part of the Krebs cycle or can enter the Krebs cycle, such as acetyl CoA.

Before amino acids can enter the Krebs cycle, their amino group (NH2) must first be removed—a process called deamination. Deamination occurs in hepatocytes and produces ammonia (NH3). The liver cells then convert the highly toxic ammonia to urea, a relatively harmless substance that is excreted in the urine.

Figure 1 Deamination process refers to the process of removing the amine group of the amino acid

The conversion of amino acids into glucose (gluconeogenesis) and into fatty acids (lipogenesis) or ketone bodies (ketogenesis) may be reviewed in the figure below. Gluconeogenesis is the formation of glucose from non-carbohydrate sources that usually happens during fasting, low-carb diets, intense exercise and starvation which utilize amino acids (mainly alanine and glutamine), lactate or glycerol and converts it into glucose molecule. The gluconeogenesis pathway mainly is reversal of the steps in glycolysis (the process to breakdown glucose).

Figure 2 The process of gluconeogenesis is the reversal with the glycolysis process. (source: https://step1.medbullets.com/biochemistry/102052/gluconeogenesis)

Watch this video to understand amino acids metabolism