1. Dr. Ghufran Mohammed Hussein
Proteins
Dr. Ghufran Mohammed Hussein

2. Digestion and absorption of proteins
Proteins, like other dietary macromolecules, are broken down by enzymes termed peptidases (hydrolysis of specific peptide bonds).
These enzymes can either cleave internal peptide bonds (i.e. endopeptidases) or cleave off one amino acid at a time from either the –COOH or –NH2 terminal of the polypeptide (i.e. they are exopeptidases subclassified into carboxypeptidases , and aminopeptidases, respectively)

3. The endopeptidases cleave the large polypeptides to smaller oligopeptides, which can be acted upon by the exopeptidases to produce the final products of protein digestion, amino acids, di- and tripeptides, which are then absorbed by the enterocytes
Depending on the source of the peptidases, the protein digestive process can be divided into gastric, pancreatic and intestinal phases

4. Overview of protein digestion

5. A. Digestion of proteins by gastric secretion
The digestion of proteins begins in the stomach by hydrochloric acid and the proenzyme pepsinogen:
Hydrochloric acid:
Stomach acid (pH 2-3) is hydrolyze proteins; however, the acid functions to denature proteins, making them more susceptible to subsequent hydrolysis by proteases

6. Pepsin:
This acid stable endopeptidase is secreted by the serous cells of the stomach as an inactive zymogen (or proenzyme), pepsinogen
In general, zymogens contain extra amino acids in their sequences, which prevent them from being catalytically active. Pepsin releases peptides and a few free amino acids from dietary proteins

7. B. Digestion of proteins by pancreatic enzymes
On entering the small intestine, large polypeptides produced in the stomach by the action of pepsin are further cleaved to oligopeptides and amino acids by a group of pancreatic proteases
These enzymes, like pepsin, are synthesized and secreted as inactive zymogens

8. Cleavage of dietary proteins by pancreatic proteases

9. C. Digestion of proteins by intestinal enzymes
The final digestion of di- and oligopeptides is dependent on membrane bound small intestinal endopeptidases, dipeptidases and aminopeptidases. The end products of this surface enzyme activity are free amino acids, di- and tripeptides

10. Absorption of amino acids di-, and tri-peptides
Free amino acids, di- and tripeptide are absorbed across the enterocyte membrane by specific carrier mediated transport
Amino acids are transported by specific active transporters showing mechanisms which are similar to ones active in glucose transport

11. Classification of amino acids
• Structural classification : according to the chemical structure of the side chain (R) • Nutritional classification (essensial & non essential a.as) • Metabolic classification : according to the fate of amino acids inside the body (glucogenic, ketogenic and mixed a.as)

12. AMINO ACID CATABOLISM

13. α- keto acid (carbon skeleton)
A.A
Deamination products
NH3
α- keto acid (carbon skeleton)

14. AMINO ACID CATABOLISM
The catabolism of the amino acids involves the removal of α-amino groups, followed by the breakdown of the resulting α- keto acid (carbon skeletons).
These pathways converge to form seven intermediate products: pyruvate, oxaloacetate, α-ketoglutarate, fumarate, succinyl coenzyme A, acetyl CoA, and acetoacetate.
These products directly enter the pathways of intermediary metabolism, resulting either in the synthesis of glucose or in the production of energy through their oxidation to CO2 by the citric acid cycle.

15. GLUCOGENIC AND KETOGENIC AMINO ACIDS
1. Glucogenic amino acids
Amino acids whose catabolism yields pyruvate, oxaloacetate, α-ketoglutarate, fumarate and succinyl coenzyme A are termed glucogenic. These intermediates are substrates for gluconeogenesis.
2. Ketogenic amino acids
Amino acids whose catabolism yields either acetyl CoA or aceto acetyl CoA are termed ketogenic. Acetoacetate is one of the ketone bodies, which also include 3-hydroxybutyrate and acetone. Leucine and lysine are the only exclusively ketogenic amino acids found in proteins.

16. GLUCOGENIC AND KETOGENIC AMINO ACIDS

17. Catabolism of the a.a.s occurs by deamination (removal of the amino group):
A. Transamination
B. Oxidative deamination
C. Transdeamination

18. A. Transamination
►Definition: It is the transfer of amino group from one α- a.a. to α- keto acid to form a new α- a.a & a new α- keto acid (reversible reaction)
► Enzymes involved: Transaminases or aminotransferases Coenzyme: PLP (Pyridoxal phosphate)
R— C H — COOH l
NH2
R— C — COOH
l l O
α- a.a
α- keto acid ► ◄
Transaminase
PLP ► ◄
R— C — COOH
l l O
R— C H — COOH l
NH2
α- a.a
α- keto acid

19. A. Transamination
►Site: In the cytosol or both the cytosol & the mitochondria of most cells especially the liver ► All a.as except threonine, lysine, proline & hydroxy proline

20. Alanine transaminase (ALT) or Glutamate pyruvate transaminase (GPT)
B. Aspartate transaminase (AST) or
Glutamate oxaloacetate transaminase (GOT)
PLP
PLP

21. Value of transamination:
■Function:
1- Degradation of a.as to form α- keto acids.
2- Synthesis of non essential a.as.
■Diagnostic value:
** Increase level of both ALT & AST indicates possible damage to the liver cells.
** Increase level of AST alone suggest damage to heart muscle ,skeletal muscle or kidney.

22. B. Oxidative deamination
Oxidation deamination is the removal of the amino group which is liberated as free ammonia. Oxidation deamination (reversible reactions) give α- ketoacid (carbon skeleton) and ammonia

23. ► Site: In most tissues , mostly in the liver and kidney
► Enzymes involved:
L-glutamate dehydrogenase enzyme: Present in the cytosol & mitochondria of most tissues
HOOC–CH2–CH2–CH–COOH ◄ ► HOOC–CH2–CH2–C-COOH
NAD(P)
NAD(P)H+H
H2O
NH3
L-GLUTAMATE
DEHYDROGENASE ENZYME
l l O l
NH2
L-glutamic acid
α- ketoglutaric acid
Coenzyme is NAD or NADP

24. Glutamate transaminase
C. Transdeamination
It is the combination of transamination & oxidative deamination. It includes the transamination of most a.as with α– keto glutarate to form glutamate then the glutamate is oxidatively deaminated reforming α– keto glutarate and giving ammonia. This provides a pathway by which the amino group of most a.as is released in the form of ammonia. ►
α- keto glutarate ◄
α- a.a. ◄ ►
NH3 ►
NAD(P)H+H
Glutamate transaminase
L-GLUTAMATE
DEHYDROGENASE ENZYME
NAD(P) ► ►
H2O
α- keto acid
Glutamic acid ◄ ◄

25. Transport of ammonia to the liver
Two mechanisms are available in humans for the transport of ammonia from the peripheral tissues to the liver for its ultimate conversion to urea.
The first (glutamine), found in most tissues, uses glutamine synthetase to combine ammonia (NH3) with glutamate to form glutamine a nontoxic transport form of ammonia. The glutamine is transported in the blood to the liver where it is cleaved by glutaminase to produce glutamate and free ammonia.

26. Transport of ammonia to the liver
(glutamine)

27. Transport of ammonia to the liver
The second (alanine), used primarily by muscle, involves transamination of pyruvate (the end product of aerobic glycolysis) to form alanine. Alanine is transported by the blood to the liver, where it is converted to pyruvate, again by transamination.
In the liver, the pathway of gluconeogenesis can use the pyruvate to synthesize glucose, which can enter the blood and be used by muscle a pathway called the glucose-alanine cycle.

28. Transport of ammonia to the liver
(alanine)
glucose-alanine cycle.

29. UREA CYCLE

30. Urea cycle
The urea cycle (also known as the ornithine cycle) is a cycle of biochemical reactions occurring in many animals that produces urea from ammonia

31. Urea cycle Ammonia is toxic Urea is less toxic than ammonia.
The Urea Cycle occurs mainly in liver.
The 2 nitrogen atoms of urea enter the Urea Cycle as NH3 (produced mainly via Glutamate Dehydrogenase) and as the amino N of aspartate.
Urea excreted in urine

32. UREA CYCLE

33. Urea Cycle Enzymes in mitochondria:
1. Carbamoyl phosphate synthetase 1
2. Ornithine Transcarbamylase
Enzymes in cytosol:
3. Arginino Succinate Synthase
4. Arginino succinase
5. Arginase.

34. Regulation of Urea cycle
N-Acetylglutamate is an essential activator for carbamoyl phosphate synthetase I (the rate limiting step in the urea cycle).
N-Acetylglutamate is synthesized from acetyl coenzyme A and glutamate by N-acetylglutamate synthase, in a reaction for which arginine is an activator. Therefore, the intrahepatic concentration of N-acetylglutamate increases after ingestion of a protein rich meal, which provides both a substrate (glutamate) and the regulator of N-acetylglutamate synthesis. This leads to an increased rate of urea synthesis.

35. Regulation of Urea cycle