Nitrogen & Amino Acid Metabolism Flashcards
(28 cards)
Structure
Structure
Primary structure
▪ Chain of amino acids joined
together with peptide bonds
▪ Amino Acids = -NH2 (Amine)
+ -COOH (carboxyl) + R
group side chain
Amino Acids
Structure
Secondary Structure
▪ Hydrogen bonds between H
from NH2 and O from COOH
▪ Results in chain folding in
either α-helix or β-pleated
sheet
Structure
Tertiary Structure
▪ 3D structure, further folding
▪ Primarily due to interactions between the R
groups of the amino acids
▪ R groups with like charges repel one another,
while those with opposite charges can form
an ionic bond
▪ Disulfide bonds = covalent linkages between
the sulfur-containing side chains of cysteines
- stronger than the other types of bonds that
contribute to tertiary structure
Structure
Quaternary Structure
▪ Assembly of more than
one polypeptide chain
▪ E.g. DNA polymerase,
haemoglobin
Protein Digestion: Stomach
- Stomach is specialized for
protein digestion - Acid environment denatures
proteins - Pepsin digests proteins into
polypeptides
Protein Digestion: Small Intestine
- Enzymes released from the
pancreas as inactive forms
(zymogens) - Polypeptides degraded into smaller
polypeptides, di/tri-peptides and
amino acids - Transported into intestinal cells (e.g.
ATP-dependent Na+ symport) - Enter bloodstream
- Liver metabolism
- General circulation: Protein
synthesis NOT storage
Complete Protein
- Complete proteins contain
all nine essential amino
acids in consistent amounts - Animal proteins (meat, eggs,
milk) are complete - Complete Plant Proteins:
Quinoa, tofu, buckwheat,
spirulina, hemp seeds, chia
seeds, whey - May need to pair plant
proteins
Synthesising Amino Acids
Serine biosynthesis
* Cell cytoplasm.
* Utilizes 3-phosphoglycerate (a glycolytic
intermediate).
Glycine biosynthesis
* Mitochondria
* Two routes.
* Generation of glycine from serine (serine
hydroxymethyltransferase).
* Alternative route from folate intermediate (not
shown)
Synthesising Amino Acids
Cysteine biosynthesis
* Cysteine is formed from methionine and serine
in the cytoplasm.
* Deficiency of the enzyme cystathionine-β -
synthase results in accumulation of the
intermediate homocysteine.
Nitrogen Balance
- In one day, a typical adult will
- consume ~ 100 g protein
- breakdown ~ 400 g body protein
- resynthesize ~ 400 g protein
- excrete/catabolise ~ 100 g protein
Disposal of Nitrogen
Any surplus AA are degraded
Amine group NH2 forms ammonia (NH3) = toxic
Need to convert to a non toxic form = urea
2 routes to removing NH2
Transdeamination
Transamination
Hyperammonaemia
Tremors
Slurred speech
Blurred vision
Brain damage
Comma
Death
Urea
Small uncharged water soluble – Easily diffuses
across membranes and can be excreted
~50% of urea weight is N = very efficient N
carrier
Relatively little energy required; 1.5 ATP per
mole of urea formed
Transamination
2 transamination reactions
* 1) Transfer amino group to
α-ketoglutarate, forming
glutamate
* 2) Transfer amino group
from glutamate to
oxaloacetate, forming
aspartate
Transdeamination
- 1) Transamination in the cytosol
- 2) Oxidative deamination in the
mitochondria
Nitrogen Excretion
Fish: Need lots of water as ammonia
can be tolerated only at very low
concentrations
Mammals
* Not enough water to dilute toxic
ammonia.
* Result: produce urea – low
toxicity (from CO2 & NH3)
Birds
* Guano = mix od white uric acid
and brown faeces
* Semi solid paste, little water loss
Birds
The Urea Cycle
AKA ornithine cycle
Location: hepatocytes – mitochondria & cytosol
5 steps
Urea = 2 N
1 N from NH3 formed during transdeamination
1 N from aspartate
The Urea Cycle – Step 1 and 2
- Formation of carbamoyl phosphate
* Irreversible rate limiting step
* Consumes 2 ATP
* Entry of 1st amino group
* Carbamoyl phosphate synthase I - Formation of citrulline
* Carbamoyl group is transferred to
ornithine to form citrulline
* Ornithine transcarbamoylase
The Urea Cycle – Step 3, 4, and 5
- Synthesis of Argininosuccinate
* Entry of second amino group (aspartate)
* Condensation of citrulline with aspartate
* Requires ATP
* Arginosuccinate synthase - Cleavage of argininosuccinate
to fumarate and arginine
Arginosuccinate lyase
- Cleavage of arginine to
ornithine and urea
* Arginase (specific to liver)
* Urea transported in blood to kidneys
Producing energy
- When the pool of amino acids is
plentiful, the excess are metabolized
to compounds that can enter the TCA
cycle - Their amino groups are irretrievably
lost to urea, which then enters blood - The majority of amino acids form TCA
intermediates and pyruvate, and are
therefore glucogenic. - Others form acetyl-CoA and, thus, are
ketogenic. - Especially important for
gluconeogenesis are Ala (in the liver),
and glutamine (in the kidneys).
BUN
- In Europe, the whole urea
molecule is assayed,
whereas in the US only the
nitrogen component of urea
(the blood or serum urea
nitrogen, i.e., BUN or SUN)
is measured. - BUN: mg/dL.
- Urea: mmol/L whole urea
molecule is measured, not just
the nitrogen - Urea about twice as high as the
BUN measurement because
BUN only measures the
nitrogen part of the molecule
Serum Urea
Increased Production
* High protein
intake
* catabolic states,
* Absorption of
amino acids and
peptides after
gastrointestinal
haemorrhage.
Decreased Production
* Low protein
intake
* Liver disease.
- Urea is synthesized in the
liver, primarily as a by-
product of the deamination
of amino acids. - Urine is the major route for
nitrogen excretion. - Plasma urea concentration
is often used as an index of
renal glomerular function,
however plasma creatine is
more accurate
GFR
GFR declines with age
* In patients with renal
disease, there is loss of
function due to damage.
* To assess this function,
GFR can be analysed.
* This GFR decline must be
taken into account when
interpreting results.
* Black line = Median
* Green = 10th–25th and 75th–90th percentiles
* Red = 5th–10th and 90th–95th percentiles
GFR = Glomerular filtration rate
Glomerular Filtration Rate
Estimation of GFR can
be made by
measuring the urinary
excretion of a
substance that is
completely filtered
from the blood and is
not reabsorbed,
secreted or
metabolised by the
renal tubules.
* The most frequently
used clearance test is
based on creatinine.
* Endogenous substance
* Derived mainly from the
turnover of creatine in
muscle and daily
production is relatively
constant, being a
function of total muscle
mass.
* A small amount of
creatinine is derived
from meat in the diet.
Challenges
* Difficult with
outpatients, 24h
required
* Incontinence
* Tampering with sample
* 3 measurements
required
* CV can be as high as
10%
Therefore:
Assessment of kidney donors
Patients with minor abnormalities
Calculation of drug dose (drugs that
are excreted by kidneys)
So where does creatine fit in?
- Creatine is made internally and is found in protein rich foods.
- Supplies energy to your muscles (P + ADP -> ATP).
- Creatinine is a waste product of phosphocreatine.
- Creatinine is a by-product of muscle metabolism
- Continual production of creatinine and continual excretion in the urine.
- Typical 70-kg adult man produces ~2 g/day
Plasma Creatinine
- Most reliable simple biochemical test of
glomerular function - Ingestion of meat can increase plasma creatinine
by >30% up to 7 hours after - Collect fasting samples
- Strenuous exercise can also cause an increase
- Plasma creatinine is related to muscle bulk
- Reference range = 60-120 μmol/L
Serum creatinine
Serum creatinine
* Plasma creatinine is inversely related to
GFR
* GFR can decrease by 50% before plasma
creatinine concentration rises beyond the
normal range;
* Plasma creatinine doubles for each further
50% fall in GFR.
* Normal plasma creatinine does not
necessarily imply normal renal function,
although a raised creatinine does usually
indicate impaired renal function.
Reference range:
60-120 μmol/L
* Changes in plasma creatinine concentration
can occur, due to changes in muscle mass
* Decrease: starvation, wasting diseases,
immediately after surgery, corticosteroid
treatment
* Increase: can occur during re-feeding.
* Changes in creatinine concentration for these
reasons rarely lead to diagnostic confusion.
CKD-EPI
- The recommended formula for routine
use in the UK is the Chronic Kidney
Disease Epidemiology Collaboration
(CKD-EPI). - Takes into account the sex, age, serum
creatinine value and racial origin. - Tool for screening for CKD
Calculating eGFR
Calculating eGFR
* Calculate the GFR for a 30 year old male with a creatinine level of
90μmol/L
102mL/min/1.73m2
- Calculate the GFR for a 80 year old female with a creatinine level of
90μmol/L
56mL/min/1.73m2 - Leave the serum cystatin C blank
- Standardised assay: Yes
- Adjust for body surface area: No
Causes of an Abnormal Serum Urea : Creatinine ratio
Increase
High protein intake
GI bleeding
Hypercatabolic
state
Dehydration
Urinary Stasis
Decrease
low protein intake
Severe liver disease
Protein
- Glomerular filtrate is an ultrafiltrate of plasma
- e.g. it has a similar composition to plasma except it is almost free of
proteins - Proteins with molecular weights less than albumin (68kDa) are
filterable. Negatively charged molecules are less easily filtered than
positively charged ones. - Almost all protein in the glomerular filtrate is reabsorbed and
catabolised by proximal convoluted tubular cells
