BIOSINTESA AIR SUSU 1. Bahan Baku (prekusor)
Kandungan Nutrien Dalam Darah dan Susu Sapi
No 1. 2. 3.
4.
5.
Nutrien Air Glukosa Lactosa Asam amino Casein - Lactoglobulin - Lactalbumin Immunoglobulin Serum albumin Triasilgliserol Fosfolipida Asam sitrat Ca P K Na Cl
Darah
SUSU
91 0.05 0 0.02 0 0 0 2.6 3.2 0.06 0.25 trace 0.01 0.01
86 trace 4.6 trace 2.8 0.32 0.13 0.07 0.05 3.7 0.035 0.18 0.13 0.10
0.025 0.34 0.35
0.15 0.05 0.11
Bahan Baku menurut Komponen Susu 1. Glukosa : - Bahan baku laktosa susu - Sumber -gliserol - Dibutuhkan dalam perubahan NADP -> NADPH - Sumber ATP
2. Asam amino : - Bahan baku casein susu - Sumber glukosa
3. As. lemak (asetat, -hidroksi butirat, NEFA) : - Bahan baku asam lemak susu
4. Vitamin dan Mineral : - Diabsorbsi langsung dari darah
2. Biosintesis Lemak Susu • Terjadi di sitoplasma sel sekretori kelenjar ambing. • Kadarnya dalam susu berfluktuasi (2,5% – 6%); sangat dipengaruhi oleh jenis pakan, 2,5 – 6%. • 75 – 90% lemak susu berupa trigliserida yg mengandung asam lemak rantai pendek (C4-C14) dan rantai panjang (C > 14) • C4 – C14 disintesis dari asetat dan β-hidroksi butirat • C > 14 disentesis dari trigliserida plasma (NEFA) dan palmitat.
Sources of Fatty Acids The fatty acids used to synthesize the milk triglycerides may arise
from two sources:
1) From breakdown of blood lipids From 40 to 60% of the milk fatty acids come from the blood. These are primarily derived from very low density lipoproteins (VLDL), which are synthesized in the intestine or liver. VLDL are composed of 90 to 95% lipid (55-60% triglyceride) on the inner core and 5 to 10% protein at the outer urface. 2) From de novo synthesis within the mammary epithelial cells. Synthesis of short and medium chain fatty acids in the mammary gland occurs by de novo synthesis (synthesis from the start, or synthesis of new molecules of fatty acids from precursors absorbed from the blood) wich occurs in the cytoplasm of the mammary epithelial cell. In the ruminant, the carbon sources used for FA synthesis are acetate (the most important one) and b-hydroxybutyrate (BHBA). Glucose is a carbon source for FA synthesis in nonruminants. The reducing equivalents needed for FA synthesis come from NADPH2 (nicotinamide adenine dinucleotide phosphate, reduced form).
Origin of Milk Fat Fats NDF & NFC
Rumen Saturation + Hydrolysis
Intestine Saturated FA+ MG
“Glucose” Acetate Butyrate
VLDL
NEFA
TG
Adipose tissues
Acetate BOHB
TG Acetate BOHB
Liver
Fatty Acids
VLDL
Ketones FA = Fatty acids TG = Triglycerides MG = Monoglycerides BOHB = B-hydroxybutyrate
NEFA = Non-Esterified Fatty acids VLDL - very low density lipo-proteins
Acetate BOHB l
Glycerol
Fatty acids
Milk Fats
Mammary Gland
The two key enzymes involved in FA synthesis in the MG are: 1. Acetyl-CoA Carboxylase, which is the rate limiting enzyme for the fatty acid synthesis pathway. 2. Fatty Acid Synthetase, which is a large complex of enzyme activities responsible for the chain elongation of the fatty acid chains. The relative proportions of milk fatty acids derived from de novo synthesis or blood lipids is dependent on carbon chain length of the fatty acid. Typically, shorter chain fatty acids arise predominantly from de novo synthesis inthe epithelial cell, while longer chain fatty acids arise directly from blood lipids. An example is given in the table below for the cow. Proportional contribution of sources of fatty acids in cow milk
Fatty Acid C4 -C10 C12 C14 C16 C18
% of FA from De novo synthesis 100 80-90 30-40 20-30 0
% of FA from VLDL Fatty Acid 0 10-20 60-70 70-80 100
In ruminants, fermentation of dietary carbohydrate in the rumen results in formation of volatile fatty acids (VFA's): especially, acetate, propionate, and butyrate.
Komposisi asam lemak dalam trigliserida air susu Asam lemak
Jenuh : Butirat Kaproat Kaprilat Kaprat Laurat Miristat Palmitat Stearat Tak jenuh : Oleat Linoleat lainnya
Panjang C
Manusia
Babi
Kambing
Sapi
4 6 8 10 12 14 16 18
2 8 9 23 9
2 2 2 2 2 2 29 6
7 5 4 13 7 12 24 51
10 3 1 2 3 9 21 11
18: 1 18: 2 -
34 7 8
35 14 12
17 3 3
31 5 4
Fatty Acid Synthesis Summary of Fatty Acid Synthesis Reactions: Each cycle through the malonyl-CoA pathway results in two carbons being added to the FA chain. The total reaction is (given here for synthesis of palmitate; C16):
Acetyl-CoA + 7 Malonyl-CoA + 14 NADPH2 are catalyzed by Fatty Acid Synthetase to yield :
Palmitate + 7 CO2 + 14 NADP + 8 CoA
The FA Synthesis Pathway involves the following steps : Activation - acetyl-CoA carboxylation Elongation - the malonyl-CoA pathway Condensation step
Reduction step Dehydration step another reduction step
The cycle is then repeated
Summary of FATTY ACID SYNTHESIS • it occurs in the cytoplasm • the intermediates are linked to ACP (acyl carrier protein) • the enzymes of FA synthesis are linked in a complex
• elongation occurs by 2 Carbons/cycle (source of 2-Carbon units is always acetyl-CoA at the beginning of each cycle) • the required reducing agent is NADPH2 • in general, elongation stops at C16 These two things are required for de novo FA synthesis: • a source of carbons, specifically acetyl-CoA • a source of reducing equivalents, specifically NADPH2 The origin of each of these varies among species, particularly in comparing ruminants and nonruminants.
Triglyceride Synthesis Synthesis of triglycerides from FAs (preformed from the blood or synthesized in the cell by de novo synthesis) occurs at the cytoplasmic surface of the smooth endoplasmic reticulum (SER). The FAs are esterified to the hydroxyl groups of the glycerol molecules. This occurs by a series of esterases. There is some specificity of which carbon on the glycerol is used to esterify particular chain length FAs. In the cow: * * * *
C16 is found predominantly on the #2 carbon of glycerol, C18 FAs mostly on the #3 carbon, C4-C8 mostly on the #3 carbon, and C10-C14 distributed among each of the glycerol carbons.
Lipid Droplet Formation Inside the mammary epithelial cell, as the triglycerides are synthesized at the outer surface of the SER (smooth endoplasmic reticulum), they start coalescing and form micro lipid droplets. These micro lipid droplets bud or bleb off from the SER surface into the cytoplasm. The micro lipid droplets may be secreted from the cells directly as very small milk fat globules (less than 0.5 mm), they may fuse with each other to form larger droplets (cytoplasmic lipid droplets), and they may fuse with cytoplasmic droplets, ultimately resulting in formation of larger milk lipid droplets.
Milk fat globules (in milk) range from less than 0.5 to greater than15 mm. The micro and cytoplasmic lipid droplets are not surrounded by a lipid bilayer membrane but apparently are surrounded by a nonbilayer coating made of protein and polar lipid (gangliosides). This surface coating prevents coalescence of the droplets with lipids in the cell, yet permits fusion between droplets. In fact, the protein and ganglioside of this coating, along with calcium, are involved in the fusion of the droplets.
Secretion of Milk Fat As the large milk lipid droplets migrate to the apical surface of the cell they continue to push outward and are enveloped by the apical membrane of the cell. This membrane eventually fuses at the base, releasing the membrane-bound milk fat globule, and closing the cell's apical membrane so ere is no hole in it. So, the milk fat globule is membrane-surrounded and has a number of membrane-associated proteins. These proteins and others trapped during the process of separating cream from whole milk (usually by some type of centrifugation) are important for the whipping properties of cream.
Overview of Milk Fat Synthesis The components of milk fat synthesis are summarized in the diagram below.
This fat globule membrane helps to stabilize the fat globules in an emulsion within the aqueous environment of milk (remember that cow milk is about 87% water). Lipid has a lower density than water, so when raw milk is centrifuged the fat rises to the top resulting in the cream layer (see image).
Milk fat is the primary component of cream. Below milk was centrifuged to separate the cream and the skim milk (also called the plasma phase of milk). Homogenization is the process by which fat globules in fluid milk are broken into sizes small enough that they will not rise in the milk to form cream under normal milk storage conditions. This is important for processing, storage and consumption of cow milk.
3. Biosintesis Laktosa Susu • Berlangsung di aparatus golgi sel sekretori kelenjar ambing. • Bahan baku sintesis laktosa adalah glukosa • Kadarnya dalam susu dipertahankan tetap yaitu 3,6 – 5,5% • 1 molekul laktosa tersusun dari 1 mol glukosa dan 1 mol galaktosa yang juga berasal dari glukosa. Jadi untuk sintesis 1
mol laktosa butuh 2 mol glukosa • 80% glukosa plasma digunakan untuk sintesis laktosa, 50-60%
diubah menjadi galaktosa dahulu.
Lactose is a disaccharide composed of D-galactose and D-glucose.
• Laktosa bertanggung jawab mempertahankan keseimbangan tekanan osmotik antara darah dan lumen susu. • Semakin tinggi produksi laktosa semakin banyak air yg dialirkan ke lumen susu produksi susu meningkat. • Ketersediaan glukosa dalam sel sekretoris kelenjar ambing sangat penting dalam peningkatan produksi susu. • Glukosa merupakan faktor pembatas pada sapi perah berproduksi tinggi.
Lactose Biosynthesis
OH OH CH2 OH
OH CH2
O O
OH
OH
Galactose
Liver
Blood
Secretory Cell
Propionate CH3 CH2
O C O-
O OH
Glucose
Golgi Apparatus
Water LACTOSE
Glucose
Glucose
Glucose A
Glucose-P
Glucogenic amino-acids (glutamte, aspartate serine, etc.)
Galactose
B Galactose
A = galactosyl transferase; B = alpha lactalbumin
OH
Ringkasan reaksi biosentesa laktosa ATP + glukosa Glukosa-6P
Heksokinase Phospoglucomutase
UTP + Glukosa-1P
Glukosa- 6P + ADP Glukosa-1P UDP-Glukosa + PP
UDP-Glucose Pyriphosporylase
UDP-Glukosa
UDP-Galactose 4-epimerase
UDP-Galaktosa+Glukosa
UDP-Galaktosa Laktosa + UDP
Lactose synthetase
Upaya meningkatkan produksi susu melalui peningkatan sintesis laktosa 1. Pemilihan bahan pakan yang tepat dipilih bahan pakan yang banyak memberikan pasokan glukosa pada sel kelenjar ambing. a. Bahan pakan yang menghasilkan propionat dalam rumen
b. Bahan pakan kaya pati namun lolos degradasi rumen, sehingga menghasilkan glukosa dalam usus.
2. Manipulasi Metabolisme Nutrien • Meningkatkan uptake glukosa oleh sel kelanjer ambing (manipulasi sistem trasport glukosa) • Pengambilan glukosa oleh sel kelenjar ambing membutuhkan IGF-I (pada sapi) dan hormon insulin (selain sapi)
• Potensi aktivitas insulin dan IGF-I dapat ditingkatkan oleh Glucoce Tolerance Factor (GTF) • GTF adalah suatu ligan yang terdiri dari Cr3+ dan asam amino glisin, sistein, asam glutamat dan niasin
Glucoce Tolerance Factor (GTF) - COOH
Glisin
N
Cr3+
Glutamat atau Glisin
N COOH -
Sistein
Difusi
Glukosa
Na+
Insulin Ligan GTF
Transport Glukosa Melalui Membran Sel
4. Biosintesis Protein Susu (Casein) • Terjadi di ribosom sekretoris kelenjar ambing, menggunakan bahan baku asam-asam amino • Kadarnya dipertahankan tetap yaitu 2,9 – 5%
• Kandungan asam amino esensial tertinggi dalam susu adalah leusin, lisin dan valin; sedangkan asam amino non esensial tertinggi adalah asam aspertat, asam glutamat dan prolin. • Sintesis protein susu membutuhkan energi dalam bentuk ATP.
Ringkasan Tahapan Reaksi Sintesis Protein Susu
1. Asam amino + ATP
-----------> Aminoasil-AMP +Pi
2. Aminoasil-AMP + t-RNA ----------- > Aminoasil-tRNA + AMP 3. (Aminoasil-tRNA)n + GTP
---------- > AA1–AA2–AA3 + GDP+tRNA
Kandungan AA dalam Protein air susu (g/100 g protein) AA esensial
Sapi
Babi
AAnon esensial
Sapi
Babi
Arginin Histidin Isoleusin Leusin Lisin Metionin Fenilalanin Treonin Triptofan Valin
3.6 2.7 5.6 9.7 7.9 2.5 5.2 4.6 1.3 6.6
5.6 2.5 5.1 8.2 6.0 1.5 4.2 4.1 1.3 5.2
Alanin As. Aspartat Sistin As. Glutamat Glisin Prolin Serin Tirosin
3.6 7.2 0.7 23.0 2.0 9.2 5.8 5.1
5.1 1.6 11.2 2.0 5.8 5.5
Biosintesi Komponen Susu Di Dalam Sel Sekretoris Kelenjar Ambing DARAH
MEMBRAN SEL
SEL KELENJAR AMBING
2-monogliserida Triasilgliserol (Kilomikron, LDL)
FFA
Asam lemak
Lemak susu
-gliserol-P
Gliserol NEFA Asetat
Asetat
-hidroksibutirat
-hidroksibutirat NADP +
Triose-P PPC Glukosa 6-P Glukosa
NADPH
Glukosa
Glukosa -1-P-UDP
Laktosa susu
Sitrat -ketoglutarat
Isositrat OP NAD +
NADH
ADP
ATP
Pi Asam amino
Asam amino
Mineral (Ca+, P+, Cl-, K+, Mg+, Na+)
(Ca +, P+, Cl-, K+, Mg+, Na+)
*NEFA = Asam lemak tak teresterifikasi OP = Fosforilasi oksidatif
;
Amino Acil-AMP
PPC = Siklus pentosa fosfat ;
Protein susu
Mineral susu
