ansc314 - proteins & AA

proteins

functions

• enzymes

• signals

• hormone synthesis

• storage

protein composition

complex organic compounds of high molecular weight

• carbon (51-55%)

• hydrogen (6-7%)

• oxygen (22-24%)

• nitrogen (16%)

  • CP = % N x 100/16

    • crude protein = N x 6.25

• most proteins have sulfur (0.5-2%)

• others have some Fe, P, and Co

crude protein

measure it by mass of N x 6.25

• little N in other things (DNA) but rumen bacteria can convert NPN → micorbial protein

protein structure

• primary - order of AA

• secondary - folding of polypeptide chain

• tertiary - 3D structure of polypeptide chain

• quaternary - protein macromolecule w/ subunits

primary protein

order of AA in polypeptide chain is unique & specific to a particular protein

• altering single AA causes gene mutation → non-functioning protein

secondary protein

coiling/folding of polypeptide chain to give protein 3D shape

• alpha helix & beta pleated sheets held together w/ H bonding

tertiary protein

comprehensive 3D structure of polypeptide chain

• hydrophobic interactions determine folding & shape of protein

• folding results in covalent bonding between R groups of cysteine AA

  • hydrophobic effect is driving force of self-assembly

quaternary protein

structure of protein macromolecule formed by interactions between multiple polypeptide chains

• subunit - each polypeptide chain

  • held together by covalent bonding, weak bonds, & disulfide bridges

pyruvate dehydrogenase

process of turning pyruvate → acetylCoA

protein types

comprised up to 25 diff AA

• simple

• conjugated

simple proteins

yield only AA or their derivatives when hydrolyzed in acid (HCl)

• primary structure of protein is defined by its AA sequence & AA composition

conjugated proteins

have non-protein moiety

• glycoprotein - glycosylated w/ sugar

• lipoproteins - w/ lipids

• phosphoproteins - phosphoric acid

• chromoproteins - pigment

  • metalloproteins

  • hemoglobin, cytochromes (Fe)

non-protein nitrogen

not all nitrogen in plants, feeds, bacteria & tissues is proteins

• amides - urea, uric acid

• alkaloids - many poisonous

• nucleic acids - purines, pyrimidines (RNA, DNA)

• ammonia (NH₂)

  • NPN converted → microbial protein by ruminants

    • add urea to ↑ CP in diet

AA parts

over 200 AA but only 21 encoded in proteins

• amino group + carboxylic acid group separated by 1 C (α-carbon)

• most α-AA: —COOH & —NH₂ group attached to same C

• exceptions

  • proline (imino or —NH group)

  • taurine (amino but no carboxylic acid, not in protein)

AA configurations

• L-amino acids

• D-amino acids

  • animals: enzymes to convert D- forms → L-forms

L-amino acids

naturally occurring

• AA in left side w/ carbox. acid group top & bottom carbon chain

• only L-form incorporated → animal proteins

D-amino acids

AA in right side

• some AA in bacterial walls

• also produced during acid hydrolysis of proteins

21 common AA

essential - PVT TIM HALL T

• Phenylalanine

• Valine

• Tryptophan

• Threonine

• Isoleucine

• Methionine

• Histidine

• Lysine

• Leucine

  • species

    • arginine (cats, birds, fish)

    • taurine (cats, infants)

semi-essential

• glycine

• glutamine

• proline

non-essential

• glycine (not birds & reptiles)

• glutamate

• bacterial & plants synthesize all AA

  • 5-13 enzymes

protein & AA species chart

• protein most costly aspect of feed

non-essential AA synthesis

• nonessential AA made from keto-acids apart of CH₂O metabolism

• other AA formed from these

  • glutamate & glutamine use NH₄+ others transaminate NH₂ from AA → keto acid

arginine

• only w/ ureotelic species

• uses ammonia & aspartate & produces arginine or urea

  • only in ureotelic species

glycine

semi-essential in uricotelic species

• no chiral center

• no L or D form

  • made from serine (serine → glycine)

• needed for uric acid synthesis

serine

semi-essential in uricotelic species

• serine → glycine

branched chain AA (BCAA)

leucine, isoleucine, valine

• BCAAs account for 35-40% dietary essential AA in body protein

  • 14-18% total AA in muscle proteins

• promotes protein synthesis

  • signals, regulates lipid metabolism aspects

leucine

isoleucine

valine

dicarboxylic AA

• aspartate → asparagine

• glutamate → glutamine

asparatate

→ asparagine (dicarboxylic AA)

• made from oxaloacetate w/ NH₂ from glutamate

  • amino group from glutamine

glutamate

→ glutamine (dicarboxylic AA)

• made from NH₄⁺ + 𝛼-ketoglutarate

  • amino group from NH₄⁺

• synthesis detoxifies & recycles NH₄⁺

  • nonessential

basic AA

basic side chains w/ neutral pH

• lysine

• histidine

• arginine

  • side chains have N & resemble ammonia (base)

  • high pKa

limiting AA

if diet is inadequate in any essential AA, protein synthesis can’t proceed beyond rate at which AA is available

lysine

1st or 2nd limiting AA in pig diets, poultry, dairy cows, & humans

• NH₂ reacts (N-glycosidic linkage

histidine

arginine

• chickens, turkeys, & some fish/reptiles can’t synthesize arginine

  • no urea cycle

sulfur-containing AA

• methionine → cysteine:

  • → cystine

  • → taurine

cysteine

sulfur-containing AA

• → cystine

• → taurine

  • dietary requirement met by methionine

cystine

• made from cysteine

• contributes to structure of proteins

  • oxidized form

methionine

1st/2nd limiting AA in pig, poultry, human & dairy cow diet

• → cysteine

taurine

• only essential in cats & infants

• don’t make it naturally from methionine/cysteine

• not part of protein, part of bile & CNS, eyesight

aromatic & heterocyclic AA

• phenylalanine

• tyrosine

• tryptophan

• proline (imino group)

phenylalanine

½ phenylalanine dietary requirement spared by adding tyrosine

• phenyl → tyrosine

  • aerobic environment: accept OH

tyrosine

• made from phenylalanine

• aerobic environment: doesn’t accept H₂

proline

non-essential

• except laying hens

tryptophan

precursor of niacin (B vitamin)

selenocytsteine

latest AA to be discovered

• made from cysteine & selenium on specialized tRNA

  • not freely present in cells

• stronger reducing compound than cysteine

• part antioxidant, reducing enzymes

protein digestion

secretin

digestive hormone secreted by wall of upper part of small intestine (duodenum) - -

• regulates gastric acid secretion & pH levels in duodenum

proventriculus protein digestion

gastric protein breakdown

chymosin - ruminant animals (prenatal to weaning)

• milk-clotting / coagulation

• prochymosin is precursor (pro-enzyme)

• activity only 2% of pepsin

  • helps preserve colostrum & antibodies for absorption

• pepsin activity low 1st 2 weeks of life

chymosin

pepsin equivalent in ruminants

acid/base AA

• ↑ NH₂ → ↑ basic

• ↑ COOH → ↑ acidic

small intestine

duodenum & jejunum

• enterokinase - cleaves trypsinogen → trypsin

• trypsin - active form cleaves pronezymes → active enzymes

• Food in stomach and SI ↑ secretin & cholesystoinin secretion

  • stimulates release of enzymes from pancreas

  • Pancreas secretes pro-enzymes

    • Trypsinogen is cleaved & activated by enterokinase secreted by intestinal mucosal cells

    • Trypsin then activates other pro-enzymes

      • Each cleaves at different residues

        • leaves peptides

        • are fully degraded by carboxypeptidases (carboxyl terminus) or aminopeptidases at amino terminus

AA absorpbtion

rumen protein degradation

food → rumen (microbial fermentation)

• un-degraded protein important b/c needs to be absorbed in s. intestine

ruminant small intestine proteins

3 major proteins in s. intestine to absorb protein & turn → AA

• rumen undegraded protein (RUP)

  • digestion in lower tract

  • indigestible

• microbial protein - made in rumen

• endogenous protein - everything except what’s coming from food

protein & energy metabolism

• AA needed for protein synthesis

  • protein → build muscle

• glycogen break down → energy

  • then lipids → energy

  • then proteins → AA → muscle protein → energy (emergency)

• carbohydrates build fat

Maillard reaction

non-enzymatic browning

• amino groups

• sugars + heart → brown product (Maillard product)

  • e.g. steak

Maillard reaction

• Between free amino groups & sugars

  • Lysine, Arginine

• glycation - random non-enzymatic glycosylation

• not reversible in animals

• turns steak brown (seared) or burnt, toast

  • turns hay or silage brown & ↓ digestibility

• ↓ protein digestion in rumen

• ↓ protein & carbohydrate digestion in animal

• products associated w/ cancer

• occurs slowly in body, aging process

  • occurs faster in body w/ diabetes (glucose)

    • causes retinopathy, nerve damage

    • ↑ A1C (HbA1c) (glycated hemoglobin - indicator of diabetes)