Back to the Basics: Part 2 Protein Digestion and Synthesis
- 08-11-2004, 05:02 AM
Back to the Basics: Part 2 Protein Digestion and Synthesis
As we know, proteins in foods do not become body proteins directly. Instead, they supply the amino acids form which the body makes it owns proteins. We will now look at the process of digestion and absorption of proteins.
The major event in the stomach is the partial breakdown (hydrolysis) of proteins. Hydrochloric acid uncoils (denatures) each proteinâ€™s tangled strands so that digestive enzymes can attack the peptide bonds. The hydrochloric acids also convert the inactive form of the enzyme pepsinogen to its active form, pepsin. Pepsin cleaves proteins- larger polypeptides- into smaller polypeptides and some amino acids. After this process then those polypeptides enter the small intestine where the majority of absorption occurs. When polypeptides enter the small intestine several pancreatic and intestinal proteases hydrolyze them further unto shirt peptide chains (oligopeptides), tripeptides, dipeptides and amino acids. Then peptidase enzymes on the membrane surfaces of the intestinal cells split most of the dipeptides and tripeptides into single amino acids. Only a few peptides escape digestion and enter the blood stream intact.
A number of specific carriers transport amino acids into the intestinal cells. Once inside the intestinal cells, amino acids may be used for energy or to synthesize needs compounds. Those not used by the intestinal cells are transports across the cell membranes into surrounding fluid where they enter the capillaries on their way to the liver.
Understanding these concepts helps understand some of the misconceptions spread through various publications. One misconception is that eating predigested proteins (amino acids supps, proteins powders) saves the body form having to digest proteins and keeps the digestive system from overworking. Such a belief is pure fiction and underestimates the bodyâ€™s abilities. The digestive system handles whole proteins and whole foods much better because it dismantles and absorbs amino acids at rates that are optimal for the bodyâ€™s use. In other words, consuming large amounts of â€œfast absorbingâ€ proteins will most likely disrupt your digestive system and only cause further problems. If we look overall at the process of absorption and digestion, the majority is done in the small intestine, not in the stomach as most believe.
Now we know how these proteins are digested and formed into new proteins we will look at a topic that is of particular interest to bodybuilders and that is protein synthesis.
Each human being is unique because of small differences in the bodyâ€™s proteins. These differences are determined by the amino acid sequences of proteins, which in turn, are determined by genes. The instructions for making every protein in a personâ€™s body are transmitted by way of the genetic information received at conception. This body of knowledge, which is filed in the DNA within the nucleus of every cell, never leaves the nucleus.
To inform a cell of the sequence of amino acids for a needed protein a stretch of DNA serves as a template for making a stand of RNA that carries a code. Known as messenger RNA, this molecule escapes through the nuclear membrane. Messenger RNA seeks out and attaches itself to one of the ribosomes (basically protein making machines). Thus situated, messenger RNA presents its list, specifying the sequence in which the amino acids are to line up to make a strand of protein. Other forms of RNA, called transfer RNA, collect amino acids from the cell fluid and bring them to the messenger. Each of the 20 amino acids has a specific transfer RNA. Thousands of transfer RNA, each carrying its amino acid, cluster around the ribosomes awaiting their turn to unload. When the messenger list calls for a specific amino acid, the transfer RNA carrying that amino acid moves into position.. Then the next loaded transfer RNA moves into place and then the next, then the next and so onâ€¦In this way the amino acids line up in the sequence that is called for and enzymes bind them together. Finally the completed protein strand is released, the messenger is degraded and the transfer RNA are freed to return for other loads of amino acids.
When a cell makes a protein as described earlier, scientists say that the gene for that protein has been â€œexpressedâ€. Cells can regulate gene expression to make a type of protein in the amounts and at the rate they need. Certain hormones, such as testosterone, can influence and increase the rate of gene expression. This is one reason for the increase in protein synthesis for steroid users. Nearly all of the bodyâ€™s cells possess the genes for making all human proteins but each type of cell makes only the proteins it needs. For example only cells of the pancreas express the gene for insulin; in other cells that gene is idle. Similarly the cells of the pancreas do not make the protein hemoglobin which is needed only by red blood cells. Recent research has unveiled some of the ways nutrients regulate gene expression and protein synthesis. These discoveries have begun to explain some of the relationships among nutrients, genes, and disease development. The benefits of polyunsaturated fatty acids in defending against heart disease are partially explained by their role in influencing gene expression for lipid enzymes.
Hopefully you have a better understanding how proteins are digested and how protein synthesis occurs. Next we will look at the roles of proteins such as building blocks, energy stores, enzymes and general protein metabolism.
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