Biochem 1: Biochemical Composition and Effects of GIT Secretions

How can dietary CHO, fats and proteins get absorbed?

Dietary CHO, fats and proteins are large complex molecules that cannot be absorbed. They must be digested to small simple molecules (monosaccharides, monoglycerides and amino acids) which are easily absorbed from the intestine.

What is the composition of salivary secretions?

Saliva is PH 6.2 - 7.6

Contains electrolytes (Na, K, Ca and Cl)

and organic material, like salivary enzymes (salivary amylase, lingual lipase, and mucus)

What are the effects of salivary secretions?

1) Starts digestion of carbs and fat

2) Salivary amylase partially digests cooked starch and glycogen into dextrin and few maltose

3) Lingual lipase is secreted by Ebner’s glands at dorsal tongue surface and helps fat digestion in infants

What is the composition of gastric secretions?

PH of gastric secretions is 1 - 3

Composed of inorganic HCL,

and organic gastric enzymes (lipase, pepsin, gelatinase, renin, mucus, and intrinsic factor)

What are the effects of gastric secretions on CHO?

No CHO digesting enzymes, only HCL can partially hydrolyze disaccharides and polysaccharides

What are the effects of gastric secretions on fats?

Triglyceride digestion.

Triglycerides are emulsified and then hydrolyzed by lipase enzymes at PH 3 - 6. Lipase only hydrolyze triglycerides with unsaturated FA at the third ester bond to form FFA and α, ꞵ (1,2)-diglycerides.

What is emulsification?

Breakdown of large fat globules into smaller ones. In the mouth (chewing), in the stomach (peristaltic contractions) and in intestine (peristaltic movement, bile salts and by lyso-phospholipids).

What are the effects of gastric secretions on protein?

Protein digestion begins in stomach.

Gatric HCL causes proteins to denature to the easily digested metaproteins, activates pepsinogen to pepsin, and makes PH in the stomach suitable for pepsin.

What is pepsin?

PH 1.5 - 2.2

An endo-peptidase acting on a central peptide bond where NH2 belongs to aromatic AA. Secreted as inactive pepsinogen and activated by HCL then autoactivation.

What is renin?

PH 4.

A milk-clotting enzyme in stomach of infants and young animals. Acts on casein converting it to soluble paracasein, that binds CA ions forming insoluble calcium paracasinate digested by pepsin.

What does gelatinase do?

Liquefies gelatin.

What does the digested food passing from the stomach to the duodenum contain?

Products of protein digestion: proteoses, peptones and large polypeptides.

Products of fat digestion: FFA, α,ꞵ (1,2)-diglycerides and

emulsified/nonemulsified undigested fat.

Products of CHO digestion: some dextrin, maltose, uncooked partially

hydrolyzed starch

What is the composition of bile secretions?

PH 7.5 - 8.8

Contains inorganic (Na, K, Cl, HCO3, Ca, and Mg)

and organic (bile salts, bile pigments, cholesterol, phospholipids and FA)

What are the effects of bile secretions?

Bile salts assist emulsification of fat by lowering surface tension and exposing large area of triglycerides to pancreatic lipase. They are required for the activity of pancreatic phospholipase A2 and cholesterol esterase.

What is the composition of pancreatic secretions?

PH 7.1 - 8.2

Composed of inorganic NAHCO3,

and organic pancreatic enzymes (amylase, lipase, colipase, trypsin and trypsin inhibitor)

What is the effect of pancreatic secretions on CHO?

Pancreatic amylase completely digests starch, glycogen, and dextrin into the disaccharide maltose and few glucose. Acts on uncooked and cooked starch

What is the effect of pancreatic secretions on fats? (triglycerides)

Pancreatic lipase and colipase hydrolyzes 1st and 3rd ester bonds producing two FFA and B 2RY monoglyceride, which may be converted to an a 1RY monoglyceride by isomerase

What is the effect of pancreatic secretions on fats? (phospholipids)

Phospholipase A2, activated by trypsin and require bile salt and CA for activity. It hydrolyzes glycero-phospholipids by removing FA in position 2 to form lyso-phospholipds

What is the effect of pancreatic secretions on fats? (cholesterol esters)

Cholesterol esterase hydrolyzes cholesterol ester into FA and free cholesterol. Requires bile salts for activity.

Enzymes and its types

1) Trypsin, PH 8, secreted as inactive trysinogen and activated by enterokinase enzyme then autoactivation.

2) Chymotrypsin, PH 8, secreted as inactive chymotrypsinogen, activated by trypsin

3) Elastase, PH 8, secreted as inactive proelastase, activated as trypsin

4) Carboxypeptidase, PH 7.4, secreted as inactive procarboxypeptidase, activated by trypsin

What are the actions of pancreatic secretions?

• Trypsin, tendopeptidase type, hydrolyzes central peptide bond in which the COOH belongs to basic amino acids e.g. arginine, lysine and histidine.

• Chymotrypsin, endopeptidase type, hydrolyzes central peptide bond in which the COOH belongs to aromatic amino acids.

• Elastase. endopeptidase type, acts on peptide bonds formed by glycine, alanine and serine. It digests elastin and collagen.

• Carboxypeptidase, exopeptidase type, hydrolyzes terminal (peripheral) peptide bond at the COOH end of the polypeptide chain.

What are the compositions of intestinal secretions?

PH 7 - 8

Contents: Alkaline fluid containing Na+ ,K+ ,CL- , HCO3 , Mucus and Enzymes

What are the effects of intestinal secretions on CHO?

Maltase, lactase and sucrase hydrolyze the corresponding disaccharides to produce glucose, fructose, and galactose.

What are the effects on intestinal secretions on fats?

1. Intestinal lipase:

Act within intestinal mucosal cells where it hydrolyze the absorbed 1ry monoglyceride forming glycerol and FFA.

2. Intestinal phospholipase:

Complete the hydrolysis of lysophospholipids to glycerol, FA, phosphoric acid and nitrogenous base.

What are the effects on intestinal secretions on proteins?

1. Aminopeptidase:

An exopeptidase acts on the terminal peptide bond at the NH2 terminus of the polypeptide chain. It releases a single amino acid.

2. Tri-peptidase: acts on tripeptides to release a single amino acid and a dipeptide.

3. Di-peptidase: acts on dipeptides to release 2 amino acids. The end products of protein digestion in the small intestine are amino acids.