Biochem Quiz 8

Intracellular Proteolysis

• removes mis folded, old, or damaged proteins

• Supplies essential amino acids when diet is not enough

• Controls cell-cycle transitions

Digestion of Dietary Proteins Provides

Nutritionally essential and nutritionally nonessential amino acids

De Novo Synthesis

• Provides nutritionally nonessential amino acids needed for synthesis

• Adjusts amino acid pools

• Adjusts energy metabolism

• Needed to make nucleotides, genes, hormones

9 Essential Amino Acids

1. Tryptophan

2. Phenylalanine

3. Leucine

4. Isoleucine

5. Lysine

6. Histidine

7. Threonine

8. Methionine

9. Valine

3 Conditionally Essential Amino Acids

1. Arginine

2. Tyrosine

3. Cysteine

Proteostasis

• Protein homeostasis

• Maintained by protein turnover

3 Protein Degradation Processes

1. Lysosome

2. Proteasome

3. Autophagic Path

Overview: Ubiquitin Proteasome System

• Selective degradation

• Highly selective

• In cytosol and nucleus

• ATP dependent

Autophagy Overview

• Bulk degradation

• Initiated in cytosol

• Requires energy

Overview Lysosomal Degradation

• Degradation of endocytosed protein, membrane proteins

• Less selective

• At lysosome

• Contributes to amino acid pool

Ubiquitination

• ATP → PPi + AMP

• Ub loaded to E1 (initiating enzyme)

• Ub to E2 (conjugating enzyme)

• Ub to E3 (protein ligase enzyme)

• Polyubiquitination possible

• Proteasome degrades

Zymogens

• enzymes released in this form

• Prevents self digestion

• Only activate in certain conditions

• Activation: proteolytic cleavage

Stomach (Initiation of Digestion)

• Pepsinogen → Pepsin

• Low pH

• Activation is auto catalytic

• Initial protein cleavage

Small Intestine (Amplification of Degradation))

• Trypsinogen → Trypsin

• Activated in intestinal lumen

• Trypsin activates other proteases

Absorption

• in small intestine across enterocytes

• via transporters

• in cell peptides → amino acids

• then amino acids → portal circulation → liver

Amino Acid Transport

• secondary active transport

• gradient via Na/K pump

When are amino acids oxidized?

• when during protein turnover they are released but not needed for synthesis

• when ingested amino acids exceeded needed amount

• when cellular proteins are used as fuel bc carbs are not available

Pyruvate is the keto-acid of ________ .

Alanine

Oxaloacetate is the keto-acid of ________ .

Aspartate

Alpha-Ketoglutarate is the alpha-keto-acid of ________ .

Glutamate

Transamination Overview

• Removal of amino group from amino acids

• Needed step in amino acid catabolism

• In cytosol

• Amino group transferred to alpha carbon of alpha keto acid

• Leaves behind alpha-keto acid analog of amino acid

Enzyme Type in Transamination (2)

1. Aminotransferases

2. Transaminases

PLP

• Needed in transamination reactions

• In first step amino group transferred onto PLP making it PMP

• Derivative of Vitamin B6

• Prevents release of amino group to environment

Alanine Transferase (ALT)

• Reversible

• Reactants: Alanine + a-KG

• Products: Pyruvate + Glutamate

Aspartate Transferase (AST)

• Reversible

• Reactants: Aspartate + a-KG

• Products: OAA + Glutamate

Proline and Hydroxyproline

• Secondary amines

• Cannot undergo transamination reactions

Lysine and Threonine

• Do not undergo transamination

• Form toxic nonmetabolites if they do

Glutamate Dehydrogenase (GDH) Overview

• Major route for oxidative deamination

  • Can also do reductive amination

• Regenerates amino acceptor (a-ketoglutarate)

• Provides ammonia (NH4)

• In mitochondrial matrix

• 2 step process

Oxidative Deamination

• Reactant: Glutamate

• Enzyme: GDH

• NAD+ reduced to NADH

• H2O added in second step

• Products: a-KG and NH4+

Reductive Amination

• Reactants: a-KG and NH4+

• Water produced

• NADPH oxidized to NADP+

• Product: Glutamate

True or False: No net deamination in transaminase reactions because a-kg becomes aminated.

true

Allosteric control of GHD dependes on?

Cellular energy state

GDH Regulation: HIgh ATP, GTP, NADH

• GDH inhibited

• Protein Synthesis initiated instead

GDH Regulation: High ADP, GDP, Free Amino Acids

• GDH activated

• Deamination initiated

Glutaminase

• Other deamination route

• Gultamine → Glutamate

• Ammonia formed is consumed by urea cycle

• Widely distributed in body

Asparaginase

• Other deamination route

• Asparagine → Aspartate

Reaction & Role of Glucose-Alanine Cycle

• Toxic ammonia transported to liver as alanine

• Alanine formed in muscle via transamination of pyruvate

• In liver alanine reconverted to pyruvate

  • Pyruvate used to create glycolysis via gluconeogenesis

Malate-Aspartate Shuttle & How it Supports Nitrogen Disposal

• Transfers reducing equivalents across mitochondrial membranes

• Regeneration of OAA

  • Can accept an amino group to form aspartate

• Aspartate to step 2 of urea cycle

• Maintains availability of aspartate

PLP deficiency ______ Transamination. Why?

• Decreases

• Required coenzyme for aminotransferases

PLP deficiency _____ Urea Cycle flux. Why?

• Decreases

• Fewer amino groups reach glutamate

• Decrease in NH4+ production via GDH

PLP deficiency ____ TCA Cycle flux. Why?

• Decreases

• a-KG, OAA, pyruvate production from amino acids decrease