Protein Metabolism
From Stoker’s General, Organic, and Biological Chemistry 6th edition.
9.9 INTERRELATIONSHIPS AMONG METABOLIC PATHWAYS
Figure 1. Overview of Carbohydrate, Lipid, and Protein Metabolism. From Stoker
The figure on the left seems overwhelming, however, we only need to look at the purple (right-hand) section in order to understand what protein metabolism contributes to the interrelationship.
9.9.1 Protein and Digestion
There are plenty of dietary protein sources humans have access to. Examples are lean meat, eggs, dairy, nuts, and some vegetables. When eaten, digestion begins in the stomach where pepsin and stomach acid denature and proceed with the initial hydrolysis, turning protein complexes into large polypeptide chains. The resulting product then undergoes further hydrolysis in the small intestine catalyzed by multiple enzymes, which break down the polypeptides into amino acids. The amino acids then enter the intestinal lining in order to be dispersed into the bloodstream.
Figure 2. Summary of Protein Digestion. From Stoker.
Key enzymes:
Pepsin: Stomach
Trypsin (Proteolytic enzyme): Small intestine
Chymotrypsin (Proteolytic enzyme): Small intestine
Carboxypeptidase (Proteolytic enzyme): Small intestine
Aminopeptidase (Proteolytic enzyme): Small intestine
PRODUCT : AMINO ACID IN BLOODSTREAM
9.9.2 How Amino Acid is Used
The resulting amino acids in the bloodstream are then utilized in four ways:
Protein Synthesis: building or replacing old tissues
Synthesis of Nitrogen-Containing Compounds: like hormones, heme (in hemoglobin), nucleic acid bases, etc.
Synthesis of Nonessential Amino Acids: nonessential amino acids take 1-3 steps to make
Energy Production via ATP Formation: by making intermediates for Citric Acid Cycle
Figure 3. Amino Acid Utilization Mnemonics
Protein is Used in the Synthesis of:
P: Protein
Ni: Nitrogen-Containing Compounds
No: Nonessential Amino Acids
E: Energy
9.9.3 Transamination and Deamination Summary
Table 1. Summary of transamination/deamination role, product, and fate
9.9.4 Feast vs. Fast vs. Starve
Feasting is a term used when food is ingested beyond the body’s requirement of energy to operate, the excess energy is stored as fat (majority) and glycogen (minority).
Fasting is when no food is ingested and the body uses the stored glycogen and fat for energy.
Starvation is a prolonged state of fasting, after the glycogen stores are depleted and the body protein and fat is broken down to serve as fuel.
9.10. B Vitamins and Protein Metabolism
There are 7 B-Vitamins used in protein metabolism, given by Figure 4.
Figure 4. B-Vitamins Involved in Protein Metabolism.
Some notable "heavy-hitters" in protein metabolism are Pyridoxine, Cobalamin, and Folate.
Why:
Pyridoxine: Essentially is the main bridge for the transamination process; you need transamination to proceed with the major reactions needed for the production of Citric Acid Cycle intermediates from amino acids, as well as producing ammonium ion for Urea Cycle.
Cobalamin: Breaks down the "branched-chain" amino acids together with Biotin and prevents buildup of homocysteine.
Folate: Provides the single carbon atoms needed to build the structure of complex amino acids as well as Nucleic Acids for DNA.
Figure 5. B-Vitamins in Protein Metabolism Diagram. From Stoker.
Vitamin B6 acts as the central hub for transferring nitrogen between amino acids and keto acids via transamination. Other B-vitamins, including B1, B2, B3, and B5, collaborate to produce energy by processing the remaining carbon skeletons. Finally, B9 and B12 function as, specialized, transporters to move carbon units for amino acid synthesis or waste disposal.