Deanna Page, Mike Lin, Mali Bowers, Gayle Seales Digestion of Proteins Deanna Page, Mike Lin, Mali Bowers, Gayle Seales

FUNCTION OF PROTEINS Building blocks of our bodies (“you are what you eat”) Source of amino acids for body’s metabolic functioning Enzymes/catalysts Hormones Cell membrane transporters ATP production (glucogenic and ketogenic AAs)

PROTEIN STRUCTURE Proteins are an organic molecule composed of amino acids linked by peptide bonds There are 20 amino acids which contribute to all different proteins important in the human body All amino acids have a similar structure containing a: Hydrogen Atom Alkaline Amino Group Acidic Carboxyl Group Variable Group (Side chain/R-group)

PROTEIN STRUCTURE Form follows function - Function follows form Primary Secondary Tertiary Quartanary (not as common) Form follows function - Function follows form Denaturing proteins influences the Tertiary, then Secondary structure

ESSENTIAL/NON-ESSENTIAL AMINO ACIDS - Essential Amino Acids must be obtained through your diet, cannot be synthesized by the human body - Non-Essential Amino Acids are synthesized by the human body - Conditionally Non-Essential Amino Acids may not be produced when the body is under stress or suffering from illness

DIETARY SOURCES OF PROTEIN Amino Acids are found in animal sources and also found in plant based sources: Meat (9 essential amino acids) Greek yogurt, cottage cheese, milk (9 essential + some nonessential) Nuts, seeds (different nuts/seeds have very different levels of AAs--variety is the spice of life!) Eggs (9 essential AAs, 3 of 4 nonessential) Plant sources: pumpkin, avocado, leafy greens (9-11 servings/day) Beans, quinoa (varying degrees of essentials + non essentials)

BREAK-DOWN OF PROTEINS Mouth, Pharynx, Esophagus: just mechanical breakdown and transport of proteins occur. No chemical breakdown or absorption yet. Starts in Stomach: HCl is released by parietal cells, creating a pH of 1.5-3.5 which is good for protein denaturation. Pepsinogen is released by chief cells, activated by HCl to form Pepsin, which breaks proteins into smaller polypeptides. Mechanical mixing forms chyme, increasing surface area for further digestive juices to access & break peptide bonds

BREAK-DOWN OF PROTEINS CONT. Pancreas: Pancreatic enzymes are made and released in their in-active zymogen precursor forms Trypsinogen and Chymotrypsinogen. Bicarbonate is also relased to buffer chyme from stomach acid as digestive enzymes require a higher pH to function. Small Intestine: Activated by Enterokinases, Trypsinogen is converted to Trypsin, which then converts Chymotrypsinogen to Chymotrypsin. Both active enzymes result in Proteolysis (breaking down of polypeptides). Brush Border Cells in small intestine secrete other proteases aminopeptidase & carboxylpeptidase to create free amino acids for absorption.

MAJOR DIGESTIVE ENZYMES INVOLVED DIGESTIVE ENZYMES target a specific type of/component of an amino acid for proteolysis Endopeptidases: cut inner peptide bonds in amino acid chains Pepsin - from chief cells within gastric pits of stomach Trypsin - from pancreatic acinar cells Chymotrypsin - from pancreatic acinar cells Elastase - from pancreatic acinar cells Exopeptidases: cut outer peptide bonds in amino acid chains Aminopeptidase - secreted from and in brush border cells Carboxylpeptidase - secreted from and in brush border cells **Inhibitors prevent over active peptidases from unwanted proteolysis

ABSORPTION OF PEPTIDES AND AMINO ACIDS All peptides and amino acids are absorbed in the small intestines from the lumen through the apical side of the brush border cells, to the basal surface, and then into the bloodstream. Dipeptides & tripeptides are brought into epithelial cells with cotransport hydrogen ions. Then aminopeptidases break them down further to individual amino acids. Free amino acids are brought into epithelial cells with cotransport sodium ions Very few peptides enter the bloodstream directly, but do so through transcystosis.

OVERVIEW OF BREAKDOWN AND ABSORPTION

PROTEIN METABOLISM AA→ ATP accounts for 10-15% all ATP production Body metabolises AAs via TRANSDEAMINATION Different from metabolism of glucose and fatty acids, because of the amine group Ketogenic vs Glucogenic AAs Amino acids can be classified as being “glucogenic” or “ketogenic” based on the type of intermediates that are formed during their breakdown or catabolism. The catabolism of glucogenic amino acids produces either pyruvate or one of the intermediates in the Krebs Cycle. The catabolism of ketogenic acids produces acetyl CoA or acetoacetyl CoA. Amino acids are building blocks of bodily tissue, but how? How do we make these proteins from broken down AAs? How do we get energy from protein for metabolic function? Keto makes acetyl coa and acetoacetyl coa, glucogenic makes pyruvate, other intermediates and glucose

TRANSAMINATION, DEAMINATION & UREA CYCLE TRANSAMINATION: the transfer of an amino group from an amino acid to a keto acid, with the formation of a new amino acid and a new keto acid, catalyzed by a group of enzymes called transaminases, important for redistribution of amino groups and production of non-essential amino acids, diverts excess amino acids towards the energy generation, takes place in the cytoplasm. DEAMINATION: the process by which amino acids are broken down if there is an excess protein intake. The amino group is removed from the amino acid and converted to ammonia. The rest of the amino acid is made up of mostly carbon and hydrogen, and is recycled or oxidized for energy. UREA CYCLE: converts excess ammonia into urea in the mitochondria of liver cells. The urea forms then enters the bloodstream, is filtered by the kidneys and is ultimately excreted in the urine.

TRANSAMINATION, DEAMINATION & UREA CYCLE Reversible, requires transaminase or aminotransferase enzymes Glutamic acid DEAMINATION in liver hepatocyes → UREA CYCLE Transamination for use in krebs cycle and building proteins, and then excess amino acids go through Deamination, mostly happens in liver, some deamination of glutamate in kidneys, results in ammonia production.

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