PROTEIN METABOLISM

DEPT OF BIOCHEMISTRYACS MEDICAL COLLEGE

CHENNAI - 77

DIGESTION & ABSORPTION

DIETARY PROTEINS SERVE 3FUNCTIONS

1. THEIR CONSTITUTENT AMINOACIDS ARE USED FOR SYNTHESIS OF BODY PROTEINS

2. THE CARBON SKELETONS OF THE AMINO ACIDS ARE OXIDISED TO YIELD ENERGY

3. THEIR CARBON & NITROGEN ATOMS MAY BE USED TO SYNTHESISE OTHER ‘N’ CONTAINING CELLULAR CONSTITUENTS AS WELL AS MANY NON-NITROGEN CONTAINING METABOLITES

DIGESTION TAKES PLACE WITHTHE HELP OF ENZYMES

SECRETED BY

1. STOMACH2. PANCREAS3. SMALL INTESTINE

1.DIGESTION IN STOMACH

ENTERING PROTEINS (+) GASTRIN (+) GASTRIC JUICE (pH 1.5 – 2.5) CONTAINING

1. HCL2. PEPSINOGEN3. RENNIN (IN INFANTS)

HYDROCHLORIC ACID

• SECRETED BY PARIETAL CELLS• KILLS MICRO-ORGANISMS• DENATURES PROTEIN MAKING

INTERNAL PEPTIDE BONDS MORE SUSCEPTIBLE TO SUBSSEQUENT HYDROLYSIS BY PROTEOSES

• PROVIDES ACID ENVIRONMENT FOR PEPSIN ACTION

PEPSIN

• SECRETED BY CHIEF CELLS AS PEPSINOGEN – INACTIVE PRECURSOR

• CONVERTED TO ACTIVE PEPSIN BY AUTOCATALYSIS

• WORKS AT ACIDIC pH• CLEAVES PEPTIDE BONDS

INVOLVING AROMATIC & ACIDIC AA

PEPSINOGEN

PEPSINH+(pH 1.5 – 2.5)

PEPSIN

DIETARY PROTEIN SMALLER PEPTIDES + AA

RENNIN

• CHYMOSIN / RENNET• IMPORTANT IN INFANTS DIGESTIVE

PROCESS• CLOTTING OF MILK

R Ca++

CASEIN PARACASEIN Ca++PARACASEINATE

THE PURPOSE OF THIS REACTION IS TO CONVERT MILK INTO MORE SOLID FORM TO PREVENT THE RAPID PASSAGE OF MILK FROM STOMACH OF INFANTS

2.DIGESTION IN INTESTINEBY PANCREATIC ENZYMES

ACIDIC CONTENTS (CHYME) PASSING INTO SMALL INTESTINE TRIGGERS THE SECRETION OF

1. SECRETIN 2. CHOLECYSTOKININ

SECRETIN

• STIMULATES PANCREAS TO SECRETE BICARBONATE & RELEASE OF PANCREATIC JUICE INTO THE SMALL INTESTINE.

• NEUTRALISES GASTRIC HCL CHANGING pH FROM 1.5 – 7.0

CHOLECYSTOKININ

• STIMULATES SECRETION OF PANCREATIC ENDOPEPTIDASE & EXOPEPTIDASE

• ENDOPEPTIDASE INCLUDES CHYMOTRYPSIN & ELASTASE, CLEAVING INTERNAL PEPTIDE BONDS

• EXOPEPTIDASE CLEAVES 1 AA AT A TIME FROM EITHER ‘C’ OR ‘N’ END. INCLUDES CARBOXYPEPTIDASE & AMINOPEPTIDASE

ACTIVATION OF PANCREATIC PRO-ENZYMES

TRYPSINOGENENTEROPEPTIDASE

TRYPSINCHYMOTRYPSINOGEN CHYMOTRYPSIN

PRO-ELASTASE ELASTASE

PROCARBOXYPEPTIDASE CARBOXYPEPTIDASE

• TRYPSIN – HYDROLYSES PEPTIDE BONDS WHOSE CRBOXYL GROUPS ARE CONTRIBUTED NY LYS & ARG RESIDUES

• CHYMOTRYPSIN-CLEAVES PEPTIDE BONDS INVOLING CARBOXYL GROUP OF AROMATIC AA

• ELASTASE – HYDROLYSES THOSE PEPTIDE BONDS FORMED BY SMALL NON-POLAR AA RESIDUES SUCH AS ALA, SER & GLY

• THUS 3 ENZYMES HYDROLYSE POLYPEPTIDES TO SMALLER PEPTIDES

Degradation of short peptides in the small intestine is continued by an EXOPEPETIDASE- CARBOXYPEPTIDASEwhich removes the successive carboxy terminal aa residues from peptide

1. CARBOXYPEPTIDASE ‘A’ – RELEASES HYDROPHOBIC AA

2. CARBOXYPEPTIDASE ‘B’ – RELEASES BASIC AA

3.DIGESTION IN INTESTINEBY INTESTINAL PROTEOSES

• The digestion is completed by the PEPTIDASES of the mucosa of the small intestine namely:

1. AMINOPEPTIDASES2. GROUP OF DIPEPTIDASES

AMINO-PEPTIDASE- EXOPEPTIDASE- REQUIRE Mg/Mn - HYDROLYSE PEPTIDE BONDS

NEXT TO N-TERMINAL AA OF SHORT PEPTIDES

DIPEPTIDASES- COMPLETE THE DIGESTION OF

DIPEPTIDES TO FREE AMINO ACIDS

ABSORPTION OF AMINOACIDS

• MOSTLY THRU ACTIVE TRANSPORT

• 5 SEPARATE SYSTEMS – TRANSPORT L-AMINOACIDS FROM INTESTINAL LUMEN INTO INTESTINAL EPITHELIAL CELLS

• THUS AA RELEASED BY DIGESTION PASS FRM GUT THRU HEPATIC PORTAL VEIN TO THE LIVER

• TRANSPORT SYSTEM ALSO PRESENT IN RENAL SYSTEM

TRANSPORT SYSTEM

AMINOACID TRANSPORTD

DISORDER

Small neutral aminoacids

Ala Ser & Thr HARTNUP Disease

Large neutral aminoacids

Isoleu, Leu, Val,Tyr,Trp,Phe

Basic aa & Cysteine

Arg, Lys, Ornithine, Cys

CYSTINUTIA

Acidic aminoacids

Glutamic acid Aspartic acid

Iminoacid, Glycine

Pro, Hydroxy pro, Gly

GLYCINURIA

MEISTER CYCLEGAMMA GLUTAMYL CYCLE

AMINO ACID

GLUTATHIONE GLUTAMYL AMINOACID

CYSTEINYL GLYCINE AMINOACID

GLYCINE CYSTEINE

OXOPROLINEGLUTAMYL-CYSTEINE

GLUTAMATE