Powering The Body

Blood Flow Physics

Circulation

Dehydration

Diabetes And Cardiovascular Disease

Energetics

Energy

Energy Fuels

Hunger

Muscle Protein

Nutrition

Shoes

Trans Fatty Acids

Vitamins

Vitamins - Overview

Weight Control

Weight Loss And Exercise

Weight Loss: Physiology And Psychology

Meats and Vegetables

These foods are rich in protein and nucleic acid. Potatoes and corn are rich in starchy carbohydrate (complex sugars) and fruits are naturally sweetened with simple sugar. Specialised enzymes (hydrolases) are secreted into the stomach and intestines. Proteins are broken down into ultimately the individual amino acids, nucleic acids into nucleotides and often into component phosphates, sugars and bases. Polysaccharides yield sweet, simple sugars and these may go toward making new protein, carbohydrate and nucleic acid: the raw material for new muscle, repair of injuries and the rejuvenation of blood. All the energy requirements are met from food.

The optimal temperature for enzymes to work efficiently is within a narrow range. Energy is consumed by enzymes. These drive each specific reaction at the right time to perform the proper chemical modification. Saliva contains amylases to break down carbohydrates into simple sugars. Hydrochloric acid is present in the stomach unfolding the tough knots of proteins making them more accessible to proteinases. Food passes into the intestine where the acid is neutralised by bicarbonate. The pancreas adds enzymes (lipases, nucleases and proteinases) continuing to cleave large fragments into smaller ones. [All -ases are cleavage enzymes.] Finally dozens of highly specific enzymes (tethered to intestine wall) complete the cleavage. The resulting sugars, nucleotides, amino acids and fatty acids are absorbed by the feathery lining of the intestine and delivered to bloodstream for body distribution.

Aspartyl Proteinases

Aspartyl proteinases (proteases) use a pair of aspartate amino acids to cleave proteins.They are typically active only in the acid stomach conditions. Pepsin is a major digestive enzyme. They are very stable and crystallised for purification. Chymosin (rennin) is a similar aspartyl proteinase found in cattle and is used in manufacture of cheese. A chymosin-rich extract of calf stomach (rennet) curdles the proteins of milk. Aspartyl proteinases from bacteria or fungi (penicillopepsin from Penicillium mould) are gaining wider usage. Since they are secreted by fungi, they are easier to isolate in pure form than the enzymes from livestock.

HIV proteinase is an aspartyl proteinase made by the AIDS virus. It is different to pepsin and chymosin, being smaller and composed of two subunits instead of one. Two flexible 'flaps' cover the active site, entrapping the target protein chain inside an active-site tunnel. HIV proteinase is not used for random digestion. Instead, it makes a few specific cuts necessary in the life cycle of the virus. HIV proteins are not made individually, like normal human proteins - they are made in the form of a long polyprotein chain. HIV protease clips these long chains in a few specific places, releasing the active proteins, which then assemble to form new, infectious viruses. HIV proteinase, since it performs an essential role in the reproduction of HIV is a major target for drug design in the ongoing fight against AIDS.

Peptidases

Proteinases cleave proteins into smaller peptides about a dozen amino acids in length, but not more. They become too small to grip them and are digested by peptidases (digestive enzymes). One amino acid is clipped off at a time from an end of the smaller peptide. Aminopeptidases start from the free amine end and carboxypeptidases from the free acid end. A carboxypeptidase requires an atom of zinc for its digestive action. The active site is located in a small pocket which is lined with reactive amino acids. Zinc is held on one side of the pocket where it is thought to weaken the bond being broken. The terminal amino acid of a peptide fits perfectly into this pocket and is snipped off. Like the proteinases, carboxypeptidase is a small, sturdy enzyme, which allows it to seek out and attack peptides under the harsh conditions of digestion. Aminopeptidases are often found tethered to the inner surface of the intestine, extending into the digestive tract. Unlike soluble proteinases such as pepsin and trypsin, the forest of peptidases does not need to be made for every meal. They have a long tail composed of carbon-rich amino acids which spans the cell membrane, locking the enzymes in place. The large globular heads contain the peptide-cutting active sites. Dipeptidyl peptidases also extend from the walls of the intestine, cleaving dipeptides (peptides containing only two amino acids) in half.