Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Protein Metabolism

Dr Bushra Ahmed
1

Learning Objectives

By the end of this session and with some further study you
should be able to explain:
 Metabolic fates of Amino acid
 Transamination reaction
 Nitrogen Pool
 Nitrogen balance
 Glucose Alanin Cycle

Protein Metabolism
 Proteins make up the

structural tissue for
muscles and tendons,
transport oxygen or
hemoglobin, catalyze all
biochemical reactions as
enzymes, and regulate
reactions as hormones

 Protein metabolism is no

less important than
carbohydrate and lipid
metabolism

3

Protein Metabolism



Proteins in excess are used to supply energy or build reserves
of glucose, glycogen, or lipids



Our body must be able to synthesize proteins, amino acids,
and other non-protein nitrogen containing compounds needed
for growth, replacement, and repair

4

Amino Acid Structure
 All amino acids consist of C, N, H and O
 The polypeptide backbone is the same in all proteins

NCCNCCNCCNCCNCCNCCNCCNCCNCCNCCNCC

N
C
C

5

Dietary Proteins

Intracellular Proteins
Coenzymes

Signalling
molecules
Polyamines

Amino
acids

Porphyrins/
heme

Nucleic acids
Carbohydrates
Lipids

Degradation
6

Non-essential and Essential Amino Acids
Non-essential Essential (can not be synthesized by body,
must be supplied in the diet
Ala

Arg (semi-essential)

Asn

His can be obtained from phosphoribosyl phosphate and ATP

Asp

Ile

Cys

Leu

Glu

Lys

Gln

Met

required in large amounts to produce Cys if the latter is
not adequately supplied in the diet

Gly

Phe

Pro

Thr

Ser

Trp

Tyr

Val

required in large amounts to produce to form Tyr if the
latter is not adequately supplied in the diet

7

Digestion & Absorption of
Amino Acids

8

Digestion of Proteins to Single Amino Acids

9

Digestion of Proteins to Single Amino Acids
 Pancreatic proteinases secreted as inactive enzymes
 Activated in the gut lumen via triggered autocatalytic

cascade

 Trypsin hydrolyses peptide bonds with carbonyls

contributed by Arg and Lys

 Chymotrypsin hydrolyses on the C-terminus side of Phe,

Trp and Tyr

 Carboxypeptidase and aminopeptidase hydrolyse short

peptides to single amino acids

10

Digestion of Proteins to Single Amino Acids
 Free amino acids are transported into the epithelial cells of

the small intestine

 Small peptides and amino acids are transferred through

enterocytes to the portal circulation by diffusion,
facilitated diffusion, or active transport

 Efficiency:

 Most globular proteins from animals almost completely

hydrolysed (eg
...
g
...

 α-Ketoglutarate accepts the amino group,
becoming the amino acid glutamate
...


H2O

COO−
NAD(P)+
NAD(P)H

O
H2 H2
OOC C C C
α-ketoglutarate

−

COO− + NH4+

Glutamate Dehydrogenase

It is one of the few enzymes that can use NAD+ or
NADP+ as e− acceptor
...

22

Amino acid

α-keto acid

α-ketoglutarate

glutamate

Transaminase

+

NADH + NH4

+

NAD + H2O

Glutamate
Dehydrogenase

Summarized above:
The role of transaminases is to move amino N to
glutamate, which is deaminated via Glutamate
Dehydrogenase, producing NH4+
...
High [NH3] would drive Glutamine Synthase:
glutamate + ATP + NH3  glutamine + ADP + Pi

This would deplete glutamate - a neurotransmitter & precursor for
synthesis of the neurotransmitter Gamma-aminobutyric acid
(GABA)
2
...

amino acids catabolism produces energy and ammonia
...
e
...
g
...


29

Negative nitrogen balance
Plant proteins have a wide range of values from low---to--high
Plant proteins are less concentrated and less digestible
than animal proteins
...
Dephosphorylation gives serine

 Gly is formed from Ser by removal of a methylene group (–

CH2)

 Gln is formed by glutamine synthetase from glutamate at

the cost of an ATP

 Cys synthesised from homocysteine and Ser

33

Amino Acid Biosynthesis
 Glutamate and Aspartate
 Glutamate and aspartate are synthesized from α-keto

acid by 1-step transamination reactions

---