Protein metabolism

During digestion, the proteins present in food are too large to pass directly through cell walls and membranes so they are broken down into the more simple amino acid units (by enzyme hydrolysis). These small amino acids are absorbed by the villi of the intestines and taken into the blood and the liver. Amino acids are not ‘stored’ for future use but are used very soon after their production to make cell material, peptides and proteins.

The synthetic process is stimulated by a ‘growth hormones’ and also by insulin.
Many different proteins have different jobs. Some act as enzyme catalysts for other chemical reactions in the fluids and cells of the body. Some go to make haemoglobin, which is synthesized in the bone marrow; others go to make material for muscles, hormones, collagen, elastin, etc.

Proteins are so valuable that they are not normally excreted from a healthy person but are converted into sugars (carbohydrates) by glycogenesis or into fats (lipids) by lipogenesis. Unused material is broken down again into small amino acid units ready to be remade into new protein material. The body has a great way of recycling the ‘worn out’ but valuable protein material for remaking amino acid units. Amino acid residues that are not needed are usually converted into urea, CO(NH2)2, before being excreted in the urine.
There are approximately 20 amino acids in use in our body, but 10 of these are the ‘essential’ amino acids and these must be taken in from food as our bodies are not able to synthesize them. The 10 nonessential, but still important, amino acids can be synthesized from proteins. These amino acids are synthesized by the cells of the body by a process called ‘trans-amination’, i.e. the body uses the amino groups from unwanted proteins to make the ones it needs.