1404- Biochem- catabolism 1

https://www.youtube.com/watch?v=0OMNyVzLnVc

What is catabolism?

The sequence of enzyme catalyzed reactions by which relatively large molecules in living cells are broken down , or degraded.

General pathway for Protein breakdown

Protein → Amino acid → ammonia (excreted as urea ) + carbon skeleton used ( recycled in varyinging biosynthetic pathways

Overview of processes in the breakdown of protein

Transamination → deamination → urea cycle

How are amino acid building blocks generated

By the digestion of proteins in the intestine

Degradation of proteins in the cell

Primary uses of amino acids

Building blocks for protein and peptide synthesis

As a source of nitrogen for the synthesis of other amino acids and other nitrogenous compounds.

What happens to excess nitrogen

Cannot be stored.

Used as metabolic fuel.

converted into urea while the carbon skeleton transforms into a metabolic intermediate via the citrate acid cycle.

Types of metabolic intermediates

Glucogenic

ketogenic

Glucogenic metabolic intermediates

Degraded to alpha ketoglutarate, succinyl- CoA, fumerate or oxaloacetate.

Glucose precursors

Ketogenic Intermediates

Degraded into acetyl- CoA or acetoacetate

Can be converted into fatty acids or ketone bodies.

Glucogenic non- essential amino acids

Alanine

Argine

Asparagine

Glucogenic essential amino acids

Histidine, Methione, Threonine, Valine

Glucogenic and ketogenic non essential amino acid

Tyrosine

Glucogenic and ketogenic essential amino acid

Isoleucine, Phenylalanine, Tryptophan

Ketogenic essential amino acids

Lysine and Leucine

Transamination

Combines reversible amination and deamination

Mediates redistribution of amino acid groups

Does not occur at the alpha position of an amino group

Which amino acids undergo transamination?

All except lysine, threonine, proline and hydroxyproline

Role of alpha keto-glutarate/L- glutarate in transaminase reactions

Acts as an amino group donor/ acceptor pair

Transamination and essential amino acids

Usually unidirectional b/c the body can’t synthesize the equivalent alpha keto acid

Enzymes and substrate involved in transamination

Pyroxidal phosphate

Another amino acid/alpha-keto acid pair

What is the function of glutamate and Glutamine for most amino acids?

They act as donors

Why does glutamate acts like a donor for most amino groups?

This is because the alpha amino group of most amino acids originate from glutamate.

What is the importance of glutamine in amino acids?

They contribute to side chain N in the biosynthesis of tryptophan and histidine.

Carries NH3 from peripheral tissues to the kidneys, where amide N is hydrolysed by the enzyme glutaminase; this process regenerates glutamate and free ammonium ion, which is excreted in the urine.

What are aminotransferases

enzymes that catalyze the transfer of amino groups from amino acids to alpha-keto acids derived from pyroxidoxal phosphate.

Location of pyroxidoxal phosphate

Found at the active site of the ‘resting’ aminotransferase, covalently attached to alpha amino acid group of a lysine residue of the enzyme.

How is pyroxidal phosphate linked to the enzyme?

By formation of a schiff base between its aldehyde group and the e amino group of a specific lysyl residue at the active site and held covalently through its positive charged nitrogen atom and the negatively charged phosphate group.

Mechanism of transamination

'Ping pong’ double displacement

Enzymes involved in the conversion of inorganic ammonium into alpha amino nitrogen of amino acids

include glutamate dehydrogenase, glutamine synthases and aminotransferases.

Nitrogen atom conversion to free ammonium ion

The nitrogen atom that is transfered to alpha keto- glutamate in the transamination reaction via oxidative deamination.

What is the oxidative deamination process catalysed by?

glutamate dehydrogenase, which converts glutamate to alpha-ketoglutarate and ammonia.

How does the oxidative deamination reaction proceed?

by the removal of an amino group from glutamate via dehydrogenation of the C-N bond, followed by hydrolysis, resulting of the Schiff base producing alpha-ketoglutarate and releasing ammonia.

Which product does the oxidative deamination equilibrium favors and why?

It favors the formation of alpha-ketoglutarate→ glutamate due to the release of ammonia, which drives the reaction forward.

Where is glutamate dehydrogenase located?

in the mitochondrial matrix of cells.

Why is glutamate dehydrogenase located in the mitochondria of cells?

It sequesters free amonnia, which is toxic

What is glutamate dehydrogenase activity regulated by in most vertebrates?

• availability of substrates like alpha-ketoglutarate and ammonia

• allosteric inhibitors - GTP and ATP

• Allosteric activators GDP and ADP

What are the two principle forms in which nitrogen can be transported?

1. Alanine via the alanine cycle

2. Glutamine

The alanine cycle

1. Glutamate formed by transamination reactions→the nitrogen is transferred to pyruvate to form alanine→ alanine is transported by blood to the liver for urea cycle.

2. Liver takes up the alanine→ converts back to pyruvate via transamination.

The pyruvate can be used for gluconeogenesis and the amino acid group appear as urea.

How is nitrogen transported as glutamine

through the action of glutamine synthetase. This synthesizes glutamine from NH4+ and glutamate in an ATP-dependent reaction.This allows for safe transport of ammonia in the bloodstream to the liver, where it can enter the urea cycle.

The Ns of the glutamine can be converted into urea via urea cycle